Fast Superfine Components and Sound Packets in Phenomena Induced by the
Impact of a Drop on a Target Fluid in Quiescent Conditions

The structure of the flow and the acoustic signals generated by the impact of a freely falling drop of water with an underlying quiescent fluid have been investigated for droplets having diameter 0.5 cm and a contact velocity in the range 1.5 < U < 5 m/s. The experimental study has been supported by high-resolution videos of the flow (as seen from above and from the side). The evolution of ejecta, spikes, droplets spray, cavity, splash, secondary cavity, streamer, secondary droplets and sequence of capillary waves is reported accordingly. In particular, perturbations of the smoothed free surface with transverse dimensions from 0.03 cm to 0.15 cm have been observed in the center of the cavity and at the head of a growing splash. The total duration of the fine structure’s existence has been found to range from 30 to 50 ms. Related acoustic packets have been recorded by means of a measuring hydrophone and a sensitive microphone. Registered acoustic signals have revealed a high-frequency pulse associated with the primary contact and a delayed main lower-frequency packet (with a time shift ranging between 50 ms and 210 ms). The content of the acoustic signals indicates that from a large number of observed gas bubbles, only one was actually sounding. For such a case (the formation of the bubble resulting from the break of a thin air bridge), the shock generated by the fast-moving remnant of the bridge produces volumetric oscillations of the bubble. The sound stops as soon as the bubble takes a smooth elliptical or spherical shape. Several details are reported about the relationship between the generation of capillary and acoustic waves and the features of hydrodynamic flows relating to the cavity, or the formation of a splash and the flows caused by the interaction of secondary drops with the perturbed surface of the target fluid. The formation of fine flows components is associated with the release of available potential energy and the conservation of perturbations in a thin “double layer” located in proximity to the original contact surfaces.


Introduction
The public interest in the physical processes caused by the impact of drops on liquid surface and forms of fluid flows, which reflected in literature, music, painting, jewelry production even in ancient times, has survived to the present day. Scientific studies of drop impact flow began relatively recently, but the scope of work and the number of applications began to grow rapidly. The processes of the formation of cascades of vortex rings initiated by a drop of a colored liquid fell in the water, discovered in the first experiments [1], stimulated the study of other types of spatial structures of flows and the mechanisms of their evolution. Firstly, the formation of vortex rings and the subsequent cascades of vortices was visualized, and the influence of the properties of contacting liquids was studied [2]. Later, the topics of work began to expand rapidly due to the influence of a number of factors. The invention of bright spark sources of light, the improvement of the photographing technique [3], the development of optical observation methods, in particular, the creation of microscopes with a distant observation plane had a positive effect. Another valuable factor was practical importance of new physical results. During experiments on tracking the motion of microdrops in a flat capacitor, the value of the discrete electron charge was determined [4] and along the way, the well-known formula for calculating the drag on a sphere [5] was corrected.
Rayleigh's papers on the stability of liquid jets and the calculation of the frequencies of natural oscillations of drops started new way of theoretical research [6]. Slow oscillations of drops of the liquid and compound drop of immiscible liquids in gravity [7] and electrostatic fields [8] as well as highfrequency oscillations of gas bubbles emitting acoustic waves were actively studied. The sound generated by gas bubbles was firstly recorded by a microphone [9] and with the appearance of hydrophones-in a liquid bulk [10,11]. The developed early theory of gas bubbles oscillations [12] was many times confirmed in experiments. Later it was noticed, that flashes of light accompany intense forced oscillations of bubbles in an external acoustic field. The effects of sonoluminescence, both emitted by group of bubbles discovered back in the first half of the last century, and by a singular bubble, have been actively studied in recent years [13,14].
An analysis of the decay process of a charged oscillating droplet led L. Meitner and O. Frisch to the idea of constructing a "drop model of decay of the nucleus", the oscillations of which were excited by an absorbed neutron [15]. The decay of the nucleus results in the formation of large (new nucleus) and small (new neutrons) fragments that are analogues of small droplets-satellites from the remnants of the neck between separated fragments [16]. The nuclear liquid-drop model applied to radioactive decay, containing a number of empirical coefficients, successfully describes the energy and decay processes of the nuclei of a large number of elements [16] although, before the invention of high-speed video recorders [17], many of the subtle details of actual flow patterns of the breakneck remained unknown. Experiments clarifying the geometry of the processes of separation and disintegration of the drops, the dynamics of the satellites continue to be carried out rather actively and at present [18].
The list of technological applications of drip technologies has been expanding quite actively in recent years. Here, an important place is occupied by studies of the interaction of micro-drops with a solid surface in order to investigate hydrodynamic of a jet and drop contact [19] and to optimize the operation of drip printers [20]. Now the technology of drip printing is widely used in microelectronics for soldering contacts. Spray coating [21], metallurgical processes of high-intensity melting of metals and gas injection during smelting [22,23]; injection of solid particles into metal melts; gas drainage from the molten mass [24] are among fast developing technologies. Physics of a drop impact is studied to combat icing of aircraft [25].
In recent years, drip technologies applied for enhancing heat transfer and cooling processes by liquid jets and films [26] as well as by spraying the coolant at normal [27] and reduced gravity at space conditions [28]. Techniques of bubble boiling intensification was actively discussed [29,30]. Experimental results were used to develop recommendations for the prevention of fires [31,32] and improvement of volumetric explosions technologies [33]. Drip technologies are used to activate catalytic reactions by spraying the liquid phase of the reagent onto the surface of metal powders [34], for optimization of fuel injection in internal combustion engines [35].
Drops impact in the liquid is accompanied by the formation of gas bubbles [36]. The experiments clarify the spatio-temporal characteristics of the structural components of the flows, both fine (thin ejecta and lamina at the initial contact of the drop with the surface of the target liquid, sprays), and large ones that are cavities, crowns, capillary waves. In recent years, much attention has been paid to detailed analysis of the droplet matter transport in a target liquid [37]. From the review follows, that more and more attention is paid to study the fine flow structure in all phases of the process of a freely falling drop immersion and spreading in the target fluid, which is of both scientific and practical interest.
In the given paper, the data of systematical studying the fine structure evolution of the flow pattern produced by a free falling drop on the surface of a target fluid in quiescent conditions, registered by high-resolution video register and emitted acoustic packets measured by hydrophone and microphone are presented.

The Main Parameters of Droplet Impact Flows
Theoretical studies and parametrization of the pinch-off, motion, and immersion of a freely falling drop processes, as well as a series of subsequent rapidly changing flow patterns in a target liquid, are conventionally carried out on the basis of the fundamental equations of fluid mechanics. The basic system includes the equations of continuity, momentum and energy transfer in approximations of an ideal fluid (system of Euler equations) or taking into account the action of dissipative factors. For only viscosity effect the system of conventional Navier-Stokes equations [38,39], which are not written here for brevity, are used.
The boundary conditions on the free surface fðr; tÞ, which are derived from the integral formulations of the laws of conservation of the mass of the liquid and the transfer of momentum and energy, are divided into three types that are kinematic, dynamic, and energy.
The set of basic dimensional parameters of the problem includes densities of all the media involved in the processes under study (air r a , drop r d and the target r t liquids, below noted r a;d;t ), kinematic m a;d;t and dynamic m a;d;t viscosities of media, basic s a d , s a t and normalized to the density of liquids c a d ¼ s a d =r d ; c a t ¼ s a t =r t cm 3 =s 2 surface tension coefficients on the liquid-air contact surface, diffusion coefficient k [38,39]. The description of experimental conditions includes the gravity acceleration g, equivalent diameter of the droplet D, height of its free fall H or drop velocity U at the time of initial contact.
The physical parameters of the environments and experimental conditions determine sets of intrinsic length and time scales, which characterize the properties of the studied problem. They are collected into several groups. In one of them, the scales depend on the physical parameters of the media, in others they depend on the size D or velocity U of the falling drop.
One group of length scales contains a capillary-gravitational parameter d c g ¼ ffiffiffiffiffiffiffi ffi c=g p included in the dispersion equation of short surface waves [38], and dissipative-capillary scales d m In low-viscosity media, the range of variability of scales is quite wide [37]. In particular, for water, the largest and smallest length scales equal d max ¼ d c g ¼ ffiffiffiffiffiffiffi ffi c=g p = 0.24 cm and d min ¼ d m c ¼ m 2 =c ¼ 1:37 Á 10 À6 cm, respectively, and the time scales are t max ¼ t d c ¼ 0:032 s и t min ¼ t n g ¼ 1:9 Á 10 À10 s. Ratios of different spatial or temporal base scales determine a set of dimensionless combinationsnumbers of Reynolds Re d ¼ UD=m d characterizing the motion of a drop and Re t ¼ u t L t =m t for flows inside the bulk of the target fluid with scales of velocity u t and length L t ; Froude Fr d ¼ U 2 gD; Weber The values of scales define the requirements for the experimental technique: the choice of the size of the observation field, the resolution of the optical register and the exposition time, parameters of hydrophone and microphone. The large scales flow components are easily resolved by modern experimental techniques, while small scales components of a type d m c , whose sizes are approaching the size of a molecular cluster, cannot be resolved.

Technique of Experiment
The experiments performed at the Stand for the study of surface and underwater processes (ESP) as part of the USF "HPC IPMech RAS" [40]. The basis of the stand designed to study the fine structure of the surface and underwater processes by consistent optical, contact, and acoustic instruments, is a transparent Plexiglas's basin 1 with dimensions of 1.5 m × 0.4 m × 0.6 m. High-quality optical windows are inserted into side walls of the basin, which was filled with degassed tap water. A photograph of the stand is shown in Fig. 1.
Sloping light rays of two multi-element sources of constant light MultiLED illuminated the observation area. The system of caustics in the pattern of reflected and scattered light from a multipath collimated source, forming on the free surface of a transparent liquid, visualizes capillary waves of small amplitude and steepness. The method represents the simplified realization of "the synthetic schlieren method" [41] with a multipoint source of light replacing the dot mask.
Optronis CR3000 × 2 high-speed video cameras 2, 5 (shooting frequency is up to 20 000 frames per second, shutter speed is 1/10000 s) registered the flow pattern.
A broadband microphone 3 was placed above the basin. Measuring hydrophone 6 was installed inside the basin. Controller 4 on the quartz resonator, the signals from which were transferred to a computer, carried out the synchronization and collection of measuring systems signals, cameras, the hydrophone, and the microphone. FDMP, 2020, vol. 16, no.4 Above the basin at a height of 20 cm-120 cm from the surface of the target liquid, a drop dispenser was installed, which could form droplets with diameters from 0.2 cm to 0.6 cm with a prescribed frequency. Separated drops fell freely due to gravity. The drop trajectory crossed the line of sight of the optical shutter signal, which fixed its passage. The signal from the shutter fed to the input of the adjustable delay controller 4 to start the video cameras.
Digital processing of optical images and data analysis performed in a Matlab environment. The image was scaled on a grid with a step of 1.0 cm. The experimental technique and data processing were developed taking into account the requirements for recording fine details of the flow pattern (up to 12 μm) and resolution of short-lived components up to microseconds.

Hydrodynamics of a Drop Impact
The first presented frame from the video (Fig. 2), demonstrates that drop, freely falling from the dispenser installed at a height of H ¼ 70 cm, has the almost perfect shape and is slightly elongated vertically (the width and the height of the drop are D d h ¼ 0.61 cm and D d v ¼ 0.62 cm, respectively) right before the contact with target liquid. The dark spot in the lower right corner of the frame is the image of the hydrophone head.
In the initial phase of immersion of the droplet at t ¼ 0.5 ms around the dark rim, which is the edge of the immersed drop, a thin gray translucent plane ring ("ejecta") with width DR e ¼ 0.76 mm is expanded. The outer edge of fast-expanding ejecta formed by sharp narrow cusps, separated by smooth deep troughs. Thin translucent, radially oriented strokes separated by relatively wide clean spaces fly out from the tips of cusps. At this stage, the temporal resolution of the instruments does not allow to unambiguously indicate whether individual strokes are thin water jets-spikes, or a blurry image of rapidly flying droplets. Some radial spikes, which are formed by the merger of two touching sides of the cusps are clearly visible in subsequent frames.
In the next frame, when the wall of the cavity is separated from the drop edge (at t ¼ 1.0 ms the width of the light ring is DR R ¼ 0.6 mm), the flow velocity decreases and the identification of the structural components becomes more reliable. In the center of the image, the remnant of the impacting drop is immersing (D d h ¼ 0:60 cm, D d v ¼ 0:58 cm). A light ring surrounding the drop remnant is the visible part of the cavern bottom (with an outer diameter of D c v ¼ 0.8 cm). A dark adjoining ring displays the inclined wall of the cavity and the upper edge of the growing crown. The ring contacts with an expanding translucent flat ring-lamina, with a pronounced thickening along the edge rim. On the rugged outer edge of the lamina (its width DR l lies in the range from 0.78 cm to 1.51 cm) pointed cusps are expressed (there are more than 20 here, then their number decreases with time), separated by smooth deep troughs.
Thin jets flying out from the tops of cusps on the lamina edge are spikes breaking up into separate droplets. Some droplets continued spikes; some are shifted and inclined, which indicates the multiplicity of mechanisms of their generation.
Detailed examination of the enlarged image of the frame at t ¼ 1 ms helps to distinguish two groups of droplets and two subsequent rings on this image. The outer lamina, where the liquid enters from the edge of the crown, is thicker and more contrasting. The primary internal ejecta, which is closer to the surface of the target fluid, is thinner and less contrast. Liquid enters into primary ejecta directly from the contact line of the falling drop and the target medium. Similar structures were visualized earlier when recording the flow induced by a drop of silicone oil or glycerol-water mixture impact in the deep layer of the same fluid [42] and reproduced numerically in [43].
A complication of the flow pattern is observed in the next frame at t ¼ 1.5 ms, where the width of the cavity is D c h ¼ 0.80 cm. The thickness of the dark ring, visualizing the inclined wall of the cavity and the crest of the growing crown, is DR w ¼ 2 mm. its outer edge transformed into a translucent lamina which width is DR l ¼ 0.78 mm. The cusps on the outer edge of the lamina become more pronounced with time.
Thin radial jets are spikes with length from 4.1 mm to 4.8 mm, which are ejected from the tops of the cusps. Small droplets periodically fly out from the outer ends of the spikes in the radial direction. Droplets are concentrated in several thin circular zones separated by wide empty rings. The manifestation of zonal structures in the pattern of the spatial position of droplets that fly out from different separated spikes synchronously and at close velocities indicates the existence of fine trickles in the walls of the cavity and crown, and general oscillatory processes.
Over time, separate protrusions appear on the walls of the cavity with a diameter of D c ¼ 0.85 cm at t ¼ 2.5 ms. They represent crests of three-dimensional capillary waves emerging from the upper edge of the by crown and going to the remnant of the primary drop with a major axis equal to D d h ¼ 0.53 cm and a minor The distances between the peaks of the protrusions are 1 mm and 1.25 mm, their height is 0.7 mm. The wave pattern on the crown was studied in [44]. At the same time, short capillary waves of small amplitude begin to run away from the outer wall of the crown, as evidenced by the appearance of caustics (a regular set of light spots) between crown and hydrophone.
Although in the frame at t ¼ 2.5 ms the remnants of a completely immersed drop are not visible, dimensions the cavity, as well as the width of the crown, are growing due to inertia. Capillary waves (similar to observed in [44]) cover the walls of the cavity, whose diameter D c ¼ 0.96 cm at t ¼ 2.5 ms. Thin jets fly out from the thickened tips of the spikes with length at the range from 0.5 mm to 3 mm at the cusps of the lamina edge. With time the length of spikes decreases as separated droplets take off some matter. In the spatial distribution of the secondary droplets that have escaped from the tips of the spikes, the zonal structure is expressed. Circular slots containing droplets are separated by empty rings. With time, the strokes which lengths here are 1. The systems of ring short capillary waves propagating on the surface of target liquid from the outer edge of the crown are visualized by a system of caustics that are bright light spots on a dark image of the hydrophone. Comparisons with neighbor images show that the number and size of spots, the lengths of the lightened domain, which are determined by the number, length, amplitude, and velocity of propagating waves, grow with time. The length of the shortest waves in this group is 0.5 mm, and increases to 2 mm.
The image of the hydrophone is split, the convex part of its head is located inside the image of the cavity, and the concave remainder is on the outside. The complication of the image shape is due to an increase in the amplitude of the concavity of the liquid surface outside the crown, as well as growing the lengths and amplitudes of the propagating ring capillary waves. However, their amplitudes are still so small that they do not appear in surface images and visualized by the motions of caustic points only.
At t ¼ 12 ms, only twelve irregularly spaced spikes remain above the crown crest with an external diameter of D c ¼ 23.9 mm, and rather large drops fly out from the tops some of them. The successive distances between the annular rows of droplets are With the expansion of the crown, the diameter of which reaches D c ¼ 29.7 mm at t ¼ 26.2 ms, the capillary waves cover both the inner and the outer sidewalls of the crown, while the bottom of the cavity remains smooth. The upper edge of the crown is partially smoothed out, only a few protrusions with a width of 4.05 mm, 3.93 mm, 3.07 mm remain on it, the distances between their centers are 1.94 mm and 1.37 mm. The outer sidewall of the crown covered with non-uniform ridges. With an increase in their amplitudes, surface perturbations of crests of external annular capillary waves around the crown also become noticeable.
The position of the caustics in the lower right corner of the image shows that the first group of external capillary waves has reached the frame boundary. The distorted image of the leading edge of the hydrophone indicates a large steepness of the wave and the presence of amplitude modulation of the ridges with a scale of 1.2 mm. The increase in the size of the shaded area in the cavity indicates that the entire hollow around the crown continues to deepen. Only separate large droplets ejected from the tops of crown edge crests remain in the flow pattern.
With the decay of the fine components of the flows caused by the immersion of a drop, the flow pattern becomes smoother as well. The crown with an external diameter of D c ¼ 32.1 mm at t ¼ 36.5 ms begins to settle down, the protrusions on its upper edge settle as well, and capillary waves become one of the main visible structure components of the flow. Now they fill all of the visible areas including the bottom and walls of the cavity, the inner and outer walls of the crown, the area around it. All wave fields, with a common ring structure, are modulated with a pitch of Dlφ ¼ 0.41, 2.4, 1.91, 1.56 mm on the outer wall of the crown, and with step from Dlφ ¼ 1.15 mm inside the cavity. A clear light line delineating the crown is evidence of the formation of a narrow trough with a pointed bottom on its outer border. With distance away from the rim of the crown, the modulation scale increases and inhomogeneities with length Dlφ ¼ 10.1 mm observed on the crest of the second external ring of capillary waves.
The complex image of the hydrophone inside the cavity is a consequence of the deep multiscale distortion of the free surface. It complements wave patterns, known from the first observations, by new details. The rugged leading edge of the hydrophone image indicates the three-dimensional nature of the near-surface flows.
Further spreading of the crown (external diameter D c ¼ 35 mm and width DR ¼ 4.7 mm at t ¼ 43.2 ms) generating outgoing ring capillary waves as well as the collapse of the cavity are supplemented by the formation of a new structural component that is the ascending splash (Worthington's jet or cumulative jet) with its own set of small-scale surface perturbations. Simultaneously a system of fine spikes arose in the center of the upward flow. Protruding spikes (diameter of the central is d s ¼ 0.65 mm) surrounded by deep potholes, separated by ribs with sharp edges. The pattern of the flow is rapidly evolving and seems "bubbling"; subtle perturbations "emerge" from the depths and sink. The domain of their location is limited. They are placed to the central part of the flow on the head of the growing splash. Several groups of smooth capillary waves with length c ¼ 1.12, 2.07, 1.85, 1.89 mm remain between the central region and the rim of the crown with smooth ridges with width in a range from 7.0 mm to 10.1 mm.
The paradoxical appearance of new fine structural components against the background of the surface with smoothed initial perturbations indicates that for a long time there is a layer inside the liquid with intense thin flows. The layer is carried to the surface by regular gravitational-capillary inward flow supplemented outward spreading the crown.
The complexity of the picture of the hydrophone image underlines the increase in the amplitude and steepness of capillary waves. The magnitude of the modulation of the external field Dl φ ¼ 7.01, 10.13 mm reflects the heterogeneity of the shape and velocity of displacement of the outer wall of the crown.
In the next frame at t ¼ 45.2 ms, when the outer diameter of the crown is D c ¼ 36.92 mm and the width is DR ¼ 6.28 mm, the head of the growing splash with a diameter of D s ¼ 12.24 mm is clearly defined. The splash covered by a system of annular perturbations with pointed ridges and deep depressions, both long and short, with dimensions d ¼ 2.02, 1.95, 1.8 mm. In the center of the angular region measuring 5.06 mm by 5.2 mm, outlined by ribbed ridges, there is a spike with diameter d s ¼ 1.2 mm. The original video shows that individual spikes randomly appear to grow and disappear, the depressions fill and form again, and the overall picture is constantly changing.
At t ¼ 47.7 ms, when the outer diameter of the crown is D c ¼ 40.47 mm, and the width of the ridge is DR ¼ 9.12 mm, the fine-structure head of the growing splash with a side length of 6.75 mm is even more pronounced, while the perturbations in the surrounding trough are smoothed out. In the center of the angular region, measuring 6.75 mm and 6.5 mm, there is a spike with a diameter of d s ¼ 0.9 mm. The top of the growing splash is covered by a system of annular perturbations with pointed ridges and deep depressions with length d ¼ 2.02 mm, 1.95 mm, 1.8 mm and separated from the crown by an annular depression with smooth surface perturbations.
On the edge of the crown, there are only pointed depressions. Small-scale perturbations are preserved here only at the leading edge of the hydrophone image, which falls on the boundary of the growing splash.
Then, the monotonous evolution of the formed structural components continues, and at t ¼ 51.5 ms the outer diameter of the crown is D c ¼ 5.23 cm and its width is DR ¼ 9.91 mm. Continuing to grow in the center of the trough, the splash takes a cylindrical shape with a diameter of D s ¼ 9.44 mm. The scales of pronounced perturbations on its surface are d ¼ 0.63, 0.66, 1.04 mm. The diameter of the central spike is d s ¼ 0.6 mm is smaller than in the previous frame, despite the smoothing effect of viscosity. The sharpening of structure can be explained by the emergence on the surface of a layer with an even finer structure of flows from the depth of the liquid. The size of the annular cavity outlining the base of the splash is d s ¼ 0.94 cm, the diameter of the region of the maximum depth of the trough between the splash and the remnant of the crown is d d ¼ 2.04 cm.
At 52.7 ms, the diameter of the crown, the outer edge of which went beyond the frame, is 51.31 mm, the diameter of the inner edge is 19.09 mm, and the brightly lit annular depression is 11.13 mm. The head of the splash has grown and slightly thinned; its diameter is now equal to 0.9 cm. The structure of perturbations on the splash surface has changed. Now they have the form of steps with a size of 0.67, 0.94, 1.34, 1.62 mm. A conical pedestal formed between the structured head and the bottom of the central trough. The diameter of its base that is a brightly lit annular depression is 1.9 cm. The bottom of the trough is smooth, on the surface of the remainder of the crown, there are gentle protrusions (length in azimuthal direction is 10.4 and 10.8 cm) and narrow depressions.
In the interval 0.78 ms between the previous and analyzed frames (60.5 ms), small-scale perturbations disappeared in the flow pattern. The surface and the peak of the splash become smooth, the dark base of which has a diameter of 12.5 mm, and a spreading crown with an outer diameter of 6.01 cm and an inner diameter of 4.79 cm. The bottom of the trough covered with wide protrusions of arbitrary shape and narrow depressions, some of which are oriented in radial directions, some troughs are located at an angle to the local radius vector.
At t ¼ 84.5 ms, when the inner diameter of the crown is D c ¼ 6.1 cm, the splash becomes the main structural element of the flow. The jet consists of a smooth spherical head with a diameter of D d s ¼ 0.62 mm, turning into a cylindrical section, which rests on a conical pedestal with a base diameter of d d ¼ 1.4 cm. A system of troughs of a spiral shape with a characteristic scale of l t ¼ 4.68 mm is suitable for the base. The shape of the hydrophone shadow indicates an increase in large-scale deformations of the liquid surface and the appearance of intermediate troughs.
Over time, the height of the conical base, cylindrical section, and the diameter of the spherical tip of the splash increase uniformly (t ¼ 97.5 s) and fine structure features decay in all components of the flows. Past the splash, there is a small circular hole-the annular trail left by the fallen droplet, which was previously ejected by one of the streamers on the outline of the crown. At the last stage of crown development, the orientation of the streamers, which ejected droplets, was close to vertical. By the moment t ¼ 145.7, a significant part of the splash had flowed into the central drop, the diameter of which is now equal to D d s ¼ 9.74 mm. As the fluid inflow velocity decreases, the conical base begins to thin (here its diameter in the area of contact with the target fluid is only 2.28 mm). Next, the remnant of the splash begins to sink, not having the energy to completely separated away from the target fluid. A change in the direction of motion of the splash is accompanied by the generation of a new group of short capillary waves of small amplitude. They are not visualized in surface displacements directly but are manifested in the structures of ring caustics on the right and left, as well as in the change in the entire light field around the splash.
When a splash is immersed, ring short capillary waves form both in the surrounding trough and directly on its lateral surface (t ¼ 162 ms). A droplet with a diameter of D d s ¼ 0.97 cm falls more slowly than a splash. The thickness of their contact area and the radius of curvature of the liquid surface in the zone begins to decrease rapidly, which leads to the appearance of a bright caustic under the droplet. Some dark spots in these images are parts of the hydrophone image caused by the deflection of light rays due to large three-dimensional variations of the surface slopes and height. These perturbations are not visible with applied visualization technique in given and several subsequent frames (t ¼ 196 s and t ¼ 209 s), when the conical cavity closes and the groups of capillary waves propagate. Gas bubbles of conical shapes with sizes 3:65 mm Â 2:66 mm and 2:63 mm Â 1:87 mm at t ¼ 196 ms are located in the center of a cavity with a diameter of D c ¼ 8.53 mm. The cavity actively collapses, and at t ¼ 209 ms its diameter is 4.46 mm. In the center of the cavity, there is a spherical bubble with a diameter of 2.28 mm, to which a smaller bubble adjoins to the right.
By the time t ¼ 237.2, when the cavity is completely collapsed, a fast, thin, vertically oriented jet that is a streamer is ejected. A droplet with a diameter d st ¼ 3.2 mm is ready to fly out into the air from the streamer top. The streamer rests on a conical pedestal with a base diameter of 7.6 mm. From the whole set of gas bubbles, only two close-together bubbles of dimensions 2.1 mm × 0.9 mm and 2.9 mm × 1.35 mm right to the bottom at a distance of R ¼ 6.3 mm from the streamer center are preserved in the field of view.
By the time of t ¼ 245.9 ms, the upper droplet with a diameter of d st ¼ 3.64 mm breaks off the top of the streamer and a new one begins to form in its place. Gas bubbles are separated by a distance of DL g ¼ 0.5 mm and are somewhat removed from the center of the flow (the distance between the centers of flow and bubbles is R b ¼ 7.5 mm and 7.7 mm).
Gradually, the base of the pedestal begins to expand and cover by short capillary waves. The drop-off droplet actively oscillates, which is manifested in the variability of its diameter equal to d st On a smoothed observable surface completely freed from capillary waves at t ¼ 419.9 ms, a cavity with a diameter of D c ¼ 4.34 mm and two gas bubbles of a spherical shape with a diameter of d b ¼ 2.33 mm and 1.93 mm are observed.
As the droplet sinks, a new complex structure formed by deformed gas bubbles is observed in the cavity with a diameter of D c ¼ 4.76 mm inside of the rim with diameter D r ¼ 6.7 mm in the center of flow at t ¼ 434.4 ms. The large gas fragment of complex form is placed in the central part of the cavity. An elongated gas hollow with dimensions of 2:34 mm Â 1:57 mm adjoins the lift side of the cavity. The diameters of spherical gas bubbles near the hydrophone are d b ¼ 2.14 mm and 1.92 mm. With time, bubbles on the periphery of the flow begin to converge. During the cavity collapse, the new groups of annular capillary waves are actively generated.
At the last stage of the active evolution of the flow, one of the gas bubbles, transporting by remnants of flows remains in the center of the flow. Bubbles on the periphery also merged into one with a diameter of An analysis of the images shows the active role of the fine components of the flows at the initial stage of the evolution of the flows, accompanied by the rapid spreading of the immersing primary drop in the stationary target fluid at t < 5 ms, then at the beginning of the splash formation at 43:2 ms < t < 60:5 ms and at the final stage of the flow evolution at t > 260:2 ms, when they manifest itself in the formation of groups of short capillary waves, as well as severe deformation and intense transport of gas bubbles. The processes of contact, fusion, and pinch-off of gas cavities are accompanied by the emission of sound wave packets.
For a more complete description of the geometry of the three-dimensional flow, Fig. 3 shows the results of lateral video recording of a three-dimensional cavity formed by a drop contacting with a target fluid. The resolution here is lower than in case of the free surface observation from above, due to losses during the propagation of light through the contact surfaces of the tank optical windows and on the optical path in water, the refraction in the regions of the capillary raising of the water surface near the tank wall. These images do not contain fine details like videograms, given in Fig. 2, nevertheless the obtained data allow clarifying important features of the subsurface flows.
Due to the shadowing of the image caused by capillary uplift, the first stages of the drop immersion in target fluid are invisible in this experiment. In the first image in Fig. 3 at 4 ms, a growing cavity with a flat bottom with depth h c ¼ 3.7 mm and diameter D c ¼ 9.8 mm is shown. As follows from data of similar experiments performed in other projections the drop by this moment has already immersed. The flat bottom of the cavity does not reproduce the initial distribution of the pressure created by the spherical drop submerging, which might be caused by the unperturbed transfer of the drop momentum at the target liquid. The flat shape of the bottom indicates the need to search for a mechanism for the accelerated distribution of drop substance and the momentum associated with the matter along the bottom of the cavity.
The bottom of the cavity with and a diameter of D c ¼ 16.4 mm and the depth of h c ¼ 10.4 mm at t ¼ 18.5 ms accepts already a spherical shape but its form is not smooth. At the bottom of the cavity, there are appearing individual protrusions up to Dh p ¼ 0.53 mm high. They were previously identified as the crests of the high capillary waves [44]. Above the cavity, there is a crown with a height of h cr ¼ 3.1 mm in this projection.
By inertia, after immersion of a drop, the cavity continues to increase in size and at t ¼ 24.5 ms its diameter is D c ¼ 17.6 mm and depth is h c ¼ 11.7 mm. Dark and light bands on the contour of the cavity corresponding to the crests and troughs of the waves, cover the entire surface of the cavity. The external contour of the cavity is perturbed as well.
In course of time, under the action of buoyancy forces and surface tension, the cavity begins to collapse and at t ¼ 31.  From the analysis of the geometric features of the given sample of frames presented in Fig. 3, it follows that the "correct" shape of the primary cavity with a spherical bottom exists for a short time in the microsecond interval. At the very beginning of the drop submerging, the bottom of the cavity is flat. At large times, its shape is distorted by general flow, capillary waves escaping from the crown and by smallscale intense trickles, expressed even at the end of the phase of active flow evolution, when a secondary cavity close to conical shape collapses with the formation of a streamer a thin fast jet flying out in the air.
The main features of the flow structure preserved when a drop of water free falls into deep water from a height of H ¼ 1.2 m are given in Fig. 4. Under the conditions of the experiment, the available potential surface energy E s ¼ sS d ¼ 56.5 erg remains unchanged, but the kinetic energy of the approaching drop E D ¼ 1:54 Á 10 4 erg increases noticeably (E s =E D ¼ 0:36%).
As in the previous case, the bottom of the cavity in the phase of immersion of the drop is flat. The shape of the cavity at the initial stages of the spreading process of the droplet is maintained until it is completely immersed. In this case, the dimensions of the cavity, the radius of the local rounding of the transition of the bottom into cylindrical walls, and the height of the crown increase monotonously. At and t ¼ 3.0 ms, the depth of the cavity is h ca ¼ 4.5 mm, its diameter is D c ¼ 11.5 mm, and the height of the crown is h cr ¼ 4 mm (Fig. 4).
The inner diameters of the crown with a height of h cr ¼ 11.1 mm and the cavity coincide, the thickening of the crown is provided by viscous entrainment of the target fluid. Under the action of capillary forces, the crown contracts, the diameter of its serrated edge becomes smaller than the size of the cavity. Further, the cavity width continues to increase by inertia (at t ¼ 39.6 ms, D c ¼ 28.5 mm, depth is h ca ¼ 11.6 mm) and the crown height is only h cr ¼ 4.6 mm. It should be noted, that for all frames of this videogram, perturbations of the walls of the cavity remain shadowed and do not appear on the presented images.
As the cavity collapses, its shape changes again, in particular, the bottom becomes flattened (the visible width is D c ¼ 25.11 mm and the depth is only h ca ¼ 5.0 mm at t ¼ 57.5 ms). Subtle perturbations, which appeared in the flow pattern and on the inner surface of the cavity, clearly visible at high magnification of the image. The crown here is collapsed almost completely, with the exception of individual troughs. On the free As the splash grows (at t ¼ 82 ms, its height is h s ¼ 14.9 mm and its diameter is d s ¼ 8.7 mm), the uneven nature of the surface is preserved only in its upper part, the conical lower one is smoother. The cavern bottom became smooth; it has four trenches (on the left, 5.6 mm deep and 8.7 mm wide, then 6.4 mm and 4.5 mm wide, the central one, 4.1 mm deep and 13.8 mm wide, and 5.4 mm and 10.5 mm wide on the right). The activity of subsurface flows is also evidenced by the variability of the positions and sizes of gas bubbles, the diameter of the largest of them being d b ¼ 0.45 mm.
With an increase in the height of the splash, at t ¼ 96.4 ms the diameter of its central cylindrical part is d s ¼ 6.8 mm and the depth of the underwater part of the perturbations, which include seven pointed trenches with a depth starting from the left h t ¼ 3.8, 4.2, 3.57, 2.78, 4.03, 3.6 mm with relatively smooth sides, decreases. A new group of gas bubbles appears under the right trench, as attached to the surface of the liquid as drifting inside its bulk.
During the interval between consecutive frames, by the time t ¼ 198.8 ms, the splash managed get completely immersed, and a droplet with a diameter of d d ¼ 10.2 mm and a height of h d ¼ 6.4 mm approached the surface of the liquid, in the bulk of which only individual gas bubbles are presented. An immersed drop pushes through the surface of the target liquid and, by the time of t ¼ 234.6 ms, a cavity accepts a form of a truncated cone of an irregular shape with a base diameter of d c ¼ 21 mm and a depth of h c ¼ 10.7 mm. Small-scale disturbances cover surface of the cavity. A gas bubble is observed on the left sidewall of the cavity at a depth of h b ¼ 7.1 mm.
Under the influence of fast flows, capillary tension, and buoyancy forces, the cavity contracts into a cone with flat piecewise smooth walls and at t ¼ 248.4 ms its diameter is D c 2 ¼ 28.2 mm and its depth is h c ¼ 9.5 mm. The bubble also approached the free surface; its center is located at a depth of 6.7 mm. At the final collapse of a cavity of this shape, a thin energetic jet-streamer ejected into the air. The streamer shown in Fig. 2 existed at the time interval 237:2 < t < 266:2 ms.
Conducted experiments with a high spatial and temporal resolution confirmed the known basic forms of the structural components of the flows caused by the drop impact on the motionless liquid and reliably showed the existence of fine components at all stages of flow evolution. Small radial flying droplets were observed in the phase of the primary contact of the drop with the target liquid surrounds the thin disc ejecta with cusps on the outer edge. At the drop immersion stage, the bottom of the deepening cavity contoured by a protruding crown is flat at the given regime. On the upper edge of the crown, crests were formed, ending in thin jets-streamers. New droplets fly out from the tops of streamers, the diameter of which grows with time and the flight velocity decreases. The walls of the cavity and the crown covered by a family of three-dimensional capillary waves running down from the upper edge of the crown to the center of the cavity bottom. In the phase of the formation of a cumulative splash in the center of the trough, the surface of the liquid was covered with thin spikes and depressions. In the splash immersion phase, the bottom of the cavity takes a complex shape, including large depressions, protrusions, and small perturbations. The secondary cavity also has a complex shape.
At all stages of evolution, there existed small-scale components of the flows-short capillary waves on the surface and fast thin trickles that change the shape of the secondary cavity, the position and shape of gas bubbles in the bulk of the target fluid are observed.
The variety and complexity of the processes reflected in the graphs of changes in the diameter (curve I) and depth (curve II) of the cavity formed by free fallen drop impact on a motionless fluid from time, which are shown in Fig. 5.
At the initial stage of the drop immersion, the horizontal and vertical dimensions of the cavity [L] = mm grow linearly with time where the coefficients A 1 x ¼ 4:11; B 1 x ¼ 3:35 at 0.61 ms < t < 1.75 ms and A 1 z ¼ -1.32; B 1 z = 2.63 at 0.61 ms < t < 3.2 ms. After immersion of a half drop, the form of the curves changes and, over a long transition interval, the experimental points are approximated by fourth-order curves L ð2Þ x x ¼ À1:179 Á 10 À5 for horizontal size of the cavity over the interval 1.8 ms < t < 20.2 ms and A ð2Þ for cavity depth in the interval 3.4 ms < t < 32.4 ms.
Then, over the interval 37.8 ms < t < 51.9 ms the horizontal size of the cavity increases linearly with time L ð3Þ 005141 over the interval 32.5 ms < t < 60.0 ms.
At the last stage of the cavity evolution at 52:0 < t < 55:6 ms the horizontal size decreases linearly L ð4Þ  Neglecting the presence of linear sections, both sets of experimental data are well described by fourthorder curves (with a 95% confidence interval) L ð5Þ where the approximation coefficients of the horizontal size are L ð5Þ Important features of the applied experimental technique that are bright "multipoint" light sources, fast video recording, careful tuning of lenses helped to visualize and register fine short-lived but stably reproduced structural components of flow, which were not previously studied. Spikes in the phase of the onset of grows of the splash after smoothing of the liquid surface, thin fast trickles near cavities, which pinch-off, deform, and actively transport gas bubbles in the bulk of the liquid and short capillary waves are among them. Besides the ring capillary waves around the crown, there are several systems of short capillary waves formed in the center of the flow when the splash, streamer, and previously ejected secondary drops immersed. Moreover, the measurements show that besides the capillary waves packets of high frequency acoustic waves were generated as well.

Acoustic Wave Packets Accompanying Drop Impact
Synchronized outputs of hydrophone (Section A) and microphone (Section B) are shown in Fig. 7. At the time of the falling drop contacts with the undisturbed water surface, simultaneously with the ejection of thin circular ejecta with spikes and secondary droplets, the acoustical pulse I with a sharp positive leading edge is emitted. The amplitude of acoustic pressure is P a = 9.3 Pa, the front duration (at the level of 0.7) is 7 μs, and the rising rate is dP a =dt ¼ 1.15 Pa/μs. An expanded image of the leading edge is shown in the Inset a.
The initial jump is followed by a twofold decrease in the pressure amplitude, which then oscillates with a high frequency. In the given hydrophone signal, along with the short high-frequency component of f ¼ 84 kHz (it is represented by the first two oscillations), there is a longer component of f ¼ 45 kHz lying on the boundary of the microphone working band. The ratio of the spectral peaks of these components is 0.25. According to signal processing data, the law of decay of the observed oscillations frequency is close to an exponential with a time constant of t d ¼ 550 ms. The oscillation frequencies of the primary hydrophone signal vary from experiment to experiment: oscillations with frequencies in the range 90 < f < 140 kHz adjoin the front, and then oscillations in the range 40 < f < 80 kHz follow.
The primary signal at the microphone, the stretched image of which is shown in Inset e, appears with the delay of 250 μs after the initial front of the hydrophone signal. The delay equals exactly to the difference in the propagation times of the sound from the contact point of the drop to the microphone and hydrophone respectively, taking into account the velocity of sound propagation in water and air. Due to the limited After the intense initial pulse, acoustic signals from the sensors do not arrive; all observed transient signals are caused by variations of the free surface level.
The main packet II, which is appeared with a delay of t ¼ 0.179 s is present in signals from both the hydrophone and the microphone. Stretched fragments of the main signal from the hydrophone and microphone are shown in insets b and d in Fig. 6. Here the sound pressure rises rapidly (during two oscillations) and then fades. The unevenness of the acoustic pressure is more pronounced. The spectrum of the signal from the hydrophone (inset c in Fig. 7) shows oscillations with a frequency of f ¼ 7.6 kHz, the signal duration to complete attenuation is Dt ¼ 30 ms, and the exponential decay time constant is t d ¼ 900 μs. The signal consists of a group of packets, which can also be interpreted as modulation, the average frequency of which, estimated from the side frequencies in the signal spectrum, is 600 Hz. From experiment to experiment, the frequency of the main signal varied in the range of 4 < f < 30 kHz. With an increase in the frequency of the observed oscillations, the duration of the full signal is significantly reduced, and modulation sharply decreases.
The initial section of the main microphone II signal also contains the main packet of oscillations with a frequency of f ¼ 7.6 kHz, which decays exponentially with a time constant of t d ¼ 720 μs for Dt ¼ 1.5 ms. Further, new increases and decreases in amplitude were observed, which can be characterized as modulation with an average frequency of f m ¼ 600 Hz.
A small number of packets in the acoustic signal presented in Fig. 7 indicates that most of the observed gas bubbles in Figs. 2-4 are "silent bubbles", which do not emit acoustic waves. The observation is confirmed by a set of phonograms obtained in repeating the experiment under constant conditions, shown in Fig. 8.
All signals, except for the first (Fig. 8a), contain a short shock pulse arising at the moment of contact of the immersing drop with the surface of the target water, and main acoustic packets. After a steep front pulse follows a depression reflecting the formation of a cavity, which is replaced by a rise in level due to the formation of a splash, followed by irregular low-frequency pressure oscillations of hydrodynamic nature. Against their background, short high-frequency packets are observed in the phase of the splash formation (Figs. 8b and 8c), or in the immersion of the first secondary drop and the formation of a thin streamer (Figs. 8b and 8d) or in the phase of immersion of the splash head (Fig. 8c).
Detailed observations made earlier [45,46] showed that acoustic oscillations occur when the gas cavity is separated with the formation of a thin neck, which breaks with the producing of pointed conical residues. The conical remnants of the neck quickly drawn into the new gas cavities. The shock produced by collapse of pointed conical residue of the torn neck excites the volumetric oscillations of the formed closed gas fragment. The main frequency of the sound is well described by the known formula for equivalent spherical volume oscillations [12]. An analysis of these experiments and a large number of phonograms of the process showed that bubble detachments with the formation of a long and thin neck can occur at any phase of the evolution of the flow, but most often with a delay from 0.17 s to 0.22 s. The time corresponds the interval when the prime droplet, which was previously ejected from the tip of the splash, came back and contacted the perturbed surface of the target fluid.
In a number of experiments, additional delayed acoustic packets were observed at time intervals corresponding to the fall of smaller droplets ejected from the top of the central streamer onto the surface of the target liquid. Separations of gas fragments from the deformed cavity, as well as ruptures of gas bubbles in the bulk of a liquid, are caused by the action of fast fine trickles in subsurface flows on running capillary waves.
The impact of trickles that are presented by thin and fast filaments in flow images is clearly illustrated by the fibers patterns of the coloured drop substance distribution in the target fluid. A substance of a uniformly colored drop (in the given experiment a dilute solution of alizarin ink) is concentrated in fine fibers (Fig. 9).
In the compose photograph on the left side of Fig. 9a, in the lower part of which an inverted image of the lower side of the free surface is shown, the dye is concentrated in thin fiber loops. The dyed loops form a complex pattern, which is deformed by a large component of the flows, and is rapidly changed. In a side view in the same phase of the flow in Fig. 9b around the base of the pedestal, which hides the loops visible in Fig. 9a, there are two groups of paired filaments radially directed that are short and long length. At the tops of long filaments, there are small vortex rings.
The simultaneous existence of fibers of different length confirms the existence of slower and faster thin components of the flows in the bulk of the liquid, analogues of spikes in the air at the initial stage of immersion of a drop. In more detail, the pattern of the flow and transport of a colored substance of the primary drop was analyzed in [47].

Mechanism of Fine Flow Structure Formation
The theoretical basis of fluid mechanics is a system of fundamental equations for the transfer of matter, momentum, and energy [38,39]. In general, all components of the energy are taken into account that are potential energy, the kinetic energy of the flow and the internal energy, which is described by scalar potentials. The set of potentials includes the internal energy, enthalpy (thermal function), Helmholtz free energy and Gibbs potential (free enthalpy). Differentials of potentials determine the traditional thermodynamic parameters of the medium, in particular, the differential of the Gibbs potential determines entropy and density, energy-temperature and pressure, Helmholtz potential-entropy and pressure. The modern definitions of conventional physical quantities that are density, pressure, temperature, and others based on potentials. Functional relationships between thermodynamic quantities forming a set of state equations, are given in a series of works International Association for the Properties of Water and Steam (IAPWS) [48].
The differential of the Gibbs potential dG s ¼ ÀsdT þ VdP, which depends on variations of temperature T and pressure P, defines entropy s ¼ À @G s @T P and q ¼ 1 T explicitly and implicitly-other thermodynamic parameters.
Traditionally, the additional surface pressure proposed by P.-S. Laplace characterizes the surface tension of the interface between contacting media. Later Gibbs [49] described the boundary between media as a new surface phase and inset additional term into internal energy, in a supplement to the potential of the bulk phase. This term is now explained as a manifestation of anisotropy of atomic-molecular interactions on the contact border, which are differed from isotropic interactions in the bulk of the liquid. Upon deformation of the boundary with a change in the area of the contact surface dS b , additional work is produced dA ¼ sdS b , where s is the surface tension coefficient.
In modern physicochemical hydrodynamics by the methods of optical and X-ray reflectivity and refractometry, spectroscopy and atomic force microscopy [50] was established that the density, dielectric constant, dipole moment are differed in the bulk of the liquid and in the structurally separated layer with a thickness of the order of 250 nm. The thickness of the liquid boundary with gas or solid, that is the order of the molecular cluster size [51].
In this layer, the Gibbs potential has a form dG s ¼ ÀsdT þ VdP À S b ds, where the last term describes available surface potential energy, which can be transformed in other forms. Elimination of the free surface of immersing drop occurs very fast (Dt r ¼ d c =U $ 10 À8 s for conditions of given experiments) due to the small thickness of the subsurface layer d c $ 10 À6 cm. The transfer of the falling drop kinetic energy takes more time as the duration of complete immersion of the drop is longer than 1 ms.
The released surface energy, which is "frozen" in a thin "double layer" near the disappeared contact surfaces, creates large perturbations of temperature and pressure, accelerating local flows. Over time, the layer thickness increases due to the viscous diffusion of the momentum and the involvement of a thicker layer of the adhering fluid into the motion. The released energy transported away by produced flows as well as emitted capillary and sound waves. With time, the energy of fluid motions is disappeared due to the viscous dissipation.
The initial release of surface energy occurs at the contact patch of the surfaces of the drop and the target fluid. The actual size of the contacting surfaces area determines the amount of released energy. The action of the mechanism of available potential surface energy release continuing during the fusion of contact surfaces. The duration of phase of fusion, which is determined by the time of the whole drop immersion, that is, by the drop size and contact velocity Dt i ¼ D=U $ 10 À3 s, provides long-term maintaining the large spreading velocity of the fluid in the double layer and fast growing of the cavity diameter in this observations. Processes in a thin layer with a thickness d c determine the geometry of the pattern of the distribution of the droplet substance in the target liquid and on the inner surface of the "double-layer" of the growing crown as well. Formed individual "fast fibers" support process of cusps formation and elongation of spikes on the edge of the growing crown, as well as the ejection of small droplets from their tips. The dissipative increase of the thickness of the energy-saturated double layer explains the decrease in speed and the increase in the diameter of the spray formed by small droplets flying from the tips of the spikes with time. The zonal distribution structure of droplets indicates the existence of radial periodic motions that are uniform in the horizontal plane.
High-energy "double-layers", which are formed upon the immersion of each new droplet, including that ejected from the top of the splash or streamer, support fine fast flows in the target liquid. Small trickles transport bubbles from the near-surface layer into the bulk of the fluid, deforms the cavity, as well as pinch-off gas cavities from its walls, deforming gas fragments and dividing gas bubbles, accompanied by radiation of sound pulses.
It should also be underlined that thin jets (ligaments or trickles) are an important part not only drop impact flows but all fluid flows and described by singular perturbed solutions of the system of fundamental equations both in linear and non-linear approximations [52].

Conclusion
Consistent investigations of hydrodynamic and acoustic processes induced by impact of the free-falling drop on a deep fluid at rest were performed in the laboratory basin. A high-resolution top and side video registration of flow patterns revealed fine trickles formed during the drop immersion. Evolution of ejecta, cavity, crown with cusps on its the upper edge with attached spikes as well as a set of spikes at the stage of the splash formation and degradation was traced. New fine jets are formed during the impact of secondary drops. Formation of secondary cavities and a streamer is supplemented by generation of short capillary waves and intensive subsurface flows.
Localized regions of small-scale fluctuations, distorting the shape of the free surface at the phase of splash formation, reflect the complex pattern of energetic fine flows under the target fluid surface.
In the experiments performed, a complex structure of acoustic signals was traced by a hydrophone in water and microphone in air. Initial a shock sound packet was generated at the contact of a drop with target liquid. Resonating gas cavities emitted the main sound packets. The spectral composition of acoustic signals includes both high-frequency (about 100 kHz) and low-frequency (about 1 kHz) components. In the studied range of experimental conditions, the shock pulse repeated stably. The reproducibility and amplitude of resonant sound packets non-monotonously depend on the drop height.
The fact of the existence of fine flows in different phases of the drop immersion, spreading, the formation of cavity, splash and secondary cavities with streamer reflects the multiscale exchange processes between different components of the total energy that are the kinetic energy of the flows, potential and internal energy, including available surface potential energy.
The formation of fine flow components is associated with the fast release of available potential energy in the process of contacting fluids fusion due to the elimination of the free surface and the conservation of perturbations in a thin "double-layer".