Manipulation of Ferrofluid‐Wrapped Drops: Translation, Coalescence, and Release

Magnetic manipulation of droplets has emerged as a promising strategy to achieve several complex tasks ranging from targeted drug delivery and micro‐robotics to controlled chemical synthesis. Hence, proper control in creating magnetically responsive droplets is indispensable for successful implementation. Here, an impact‐based encapsulation technique is employed to create stable ferrofluid‐wrapped single‐liquid and compound droplets inside a water bath. Thereafter, a permanent magnet is used to manipulate the resulting encapsulated cargo and demonstrate its feasibility for various applications. The manipulations reported herein are underwater magnet‐assisted drop translation and coalescence of compound droplets. The release of the innermost cargo in the compound droplet is also experimentally demonstrated via magnetic actuation. Importantly, for the first time, magnetically controlled coalescence of ferrofluid encapsulated compound droplets containing water‐soluble species in a water pool is demonstrated. The non‐contact manipulation technique presented in this work promises significant implications for magnet‐assisted actuation technologies.


Introduction
Manipulation of droplets has recently gained immense attention from the research community owing to its application in microfluidics systems, [1][2][3][4][5][6][7][8] drug delivery, [9][10][11][12] controlled reactions, [1,[13][14][15][16] Deoxyribonucleic Acid (DNA) analysis, [17,18] and microfabrication, [19,20] among others. The breadth of applications has led to significant developments in droplet manipulation/actuation techniques using active stimulus. The various active techniques comprise an electric field, [21] acoustic waves, [22,23] thermal gradient, [24] optical actuation, [25] and magnetic field. [6,13,[26][27][28] However, despite the advancements, most DOI: 10.1002/admi.202300144 strategies mentioned above suffer from inherent design complexities. For example, manipulation via an electric field necessitates incorporating a complex electrode array and a digital device. [12,21,29] Similarly, as noted by Huang et al., [12] acoustic actuation requires compatible materials with reflecting properties and demands a complex fabrication of the device. While optical methods facilitate non-contact manipulation, they suffer from low throughput and can cause chemical changes due to photosensitivity. Thermally actuated droplet migration requires continuous heat flux and wettability gradient surfaces and thus has limited maneuverability. [24] Alternatively, magnetic manipulation is a relatively simple and easy-toimplement strategy, which has become quite popular due to the non-contact control of droplets. [26,[30][31][32][33][34][35] The presence of magnetically responsive agents inside or on the droplet's surface makes it easily maneuverable. [36] Unlike other techniques, magnetic systems lead to minimum heat generation in the case of electromagnets and no heat generation in the case of permanent magnets. Owing to the high saturation magnetization, better stability, and low toxicity of magnetic nanoparticles (MNPs), [10,11,37,38] the droplets are loaded with magnetic nanoparticles, which is commonly referred to as an MNP-laden droplet, [12] or by coating its surface with a magnetic powder, which is popularly known as magnetic liquid marble (MLM). [12,15,[39][40][41][42] Due to the promising prospects of magnetically actuated materials in multiple practical applications, including drug targeting, minimally invasive surgery, and others, there has been a recent upsurge in scientific efforts in this direction, especially toward the development of magnetically actuated micro/milli-robots. [42][43][44][45][46][47][48] Notably, Wang et al. [48] prepared magnetoactive phase transitional matter (MPTM) by dispersing neodymium-iron-boron magnetic microparticles in a liquid metal matrix. The prepared MPTM could switch between rigid and fluidic states due to magnetic induction heating and ambient cooling, respectively. Similarly, Chen et al. [49] have developed a magnetorheological fluid robot (MRF robot) that can switch between a Newtonian liquid to Bingham plastic in the absence or presence of a magnetic field, respectively. The MRF robot could efficiently perform complex locomotion, shape reconfiguration, and push-pull. Similarly, ferrofluids, which are colloidal suspensions of magnetic nanoparticles in a base fluid, have emerged as a popular choice in magnetic manipulation systems. [12,50] Ferrofluid manipulation finds application in liquid cargo transportation, [51][52][53] micro-lens fabrication, [19,20] droplet sorting, [13,34,54] controlled chemical synthesis, [13] oil recovery, [55] among others. Importantly, it has been shown that manipulation of encapsulated magnetic cargo can be used for targeted drug delivery for therapeutic applications. [10,11,[56][57][58] For example, Kato et al. [57] showed that magnetic microcapsules containing anti-cancer drugs could be successfully manipulated to deliver the medicine at the tumor site. Chen et al. [10] fabricated magnetic nanocapsules consisting of nano iron core encapsulated by a mesoporous silica shell. The space between the shell and the iron core was loaded with an anti-cancer drug. The drug was then controllably released by varying the pH of the outermost silica shell. The applications mentioned above demand the encapsulation of the target cargo (core) by a magnetic layer or the encapsulation of magnetic fluids inside a non-magnetic shell, so that the magnetic forces can manipulate it. This necessitates robust and efficient encapsulation techniques, which should facilitate strong protection of the core from the aggressive environment and provide stability to the encapsulated cargo. It should also facilitate a wider range of core and shell volume as required. Motivated by these gaps, the research community has investigated various encapsulation techniques for magnetic fluids, including phaseseparation, [55,59] microfluidics, [8,34,54,60] and emulsification. [61] Recently, Banerjee et al. [62] from our group demonstrated a facile magnet-assisted framework that facilitates the encapsulation of multiple ferrofluid droplets inside a thin polydimethylsiloxane (PDMS) shell. Their technique uses a magnet to pull the ferrofluid (FF) droplets through a thin PDMS layer floating on a water pool. It is observed that the PDMS acts as a durable shell that prevents coalescence of the ferrofluid droplets and protects the ferrofluid core (miscible with water), which remains stable in the water bath. The study has also demonstrated the magnet-assisted underwater manipulation of the encapsulated cargo, affirming the stability and integrity of the wrapping. While most of the literature is dedicated to the encapsulation of ferrofluids inside another liquid, studies on the encapsulation of non-magnetic liquids by ferrofluids are scarce. Note that non-magnetic droplets wrapped by a thin ferrofluid shell can be of immense practical relevance. They can impart manipulability to non-magnetic cargo with small volumetric doping of magneto-responsive liquid. A non-magnetic droplet encapsulated inside a ferrofluid shell is analogous to a magnetic liquid marble (MLM), a droplet coated with ferromagnetic particles. MLMs have been shown to facilitate controlled chemical reactions, [15] micro-mixing, [1] on-demand wrapping, and unwrapping of the magnetic coating. [39] However, despite the interfacial analogies between magnetic liquid marbles and ferrofluid encapsulated droplets, the latter remains much less explored. Importantly, MLMs lack mechanical robustness [63] due to interfacial breakage and particle jamming and thus have limited magnetic maneuverability. A survey of relevant papers on magnetic manipulation reveals that most of the effort is directed toward manipulating ferrofluid droplets and fabricating MLMs and MNP-Laden droplets. Few of the studies, [42,64] report the magnetic actuation of liquid metal droplets.
Herein, we demonstrate the underwater magnet-assisted manipulation of ferrofluid-wrapped droplets. The encapsulation of the core liquid (laser oil) inside the oil-based ferrofluid is achieved by using the robust impact-driven liquid-liquid encapsulation technique developed by our group. [65,66] In those works, Misra et al. [65,66] exploited the thermodynamically favorable tendency of a liquid-triplet to demonstrate impact-driven stable and ultrafast encapsulation by impinging a target core analyte on the interfacial layer of a shell-forming liquid floating on the pool of another host liquid. This technique is further utilized by Yin et al. [67] for the fabrication of triple-layered encapsulated droplets, indicating the robustness of the technique. Here, based on the above-mentioned technique, the droplets of laser oil are allowed to impact the interfacial layer (oil-based ferrofluid) floating on a quiescent water pool resulting in the formation of the ferrofluid-wrapped droplets inside the water bath. Thereafter, we employ a cubic neodymium permanent magnet fixed on a computer-controlled linear translator to demonstrate the post-encapsulation manipulation of the encapsulated cargo. Under the manipulations, we demonstrate the underwater magnetassisted transportation of the encapsulated cargo and its characteristics. We show that the ferrofluid layer protecting the laser oil (core) plays a significant role in manipulating the encapsulated cargo. We further demonstrate the stability of the ferrofluid layer (shell) by wrapping and unwrapping the ferrofluid shell using the magnetic field.
Further, we exploit the magnetic manipulability of the encapsulated cargo to design and demonstrate a novel platform for controlled actuation, coalescence of a water-soluble analyte, and the release of the cargo inside a water bath. This system bears a high degree of practical relevance in controlled reactions, drug targeting, and release. We use ethylene glycol (EG) as the model watersoluble analyte. We first enclose the EG droplet inside a carrier droplet of laser oil using a Y-junction flow arrangement and then encapsulate the laser oil -EG compound droplet with a ferrofluid shell layer using the same impact-driven liquid-liquid encapsulation technique. [65,66] Subsequently, we use the controlled motion of a permanent magnet to manipulate the compound cargo and demonstrate on-demand coalescence, allowing controlled mixing of the inner EG core without perishing in the surrounding aqueous medium. Finally, we show the release of the EG core inside the water bath using the magnetic field. The necessity of employing carrier droplets in this platform and the rationale behind its selection are elaborated in Section 3.1. To the best of our knowledge, this is a first-time demonstration of the controlled coalescence of the encapsulated compound droplets via magnetic manipulation.

Materials
The materials used in this work and their relevant properties are mentioned as follows. The core droplet of laser oil (Cargille Laboratories Inc., Cedar Grove, NJ, USA) has a density of lo = 1900 kg m −3 , dynamic viscosity of μ lo = 1024 mPa s, and liquid-air surface tension of lo = 50 mN m −1 . The host liquid is deionized (DI) water (Milli-Q, Millipore Sigma, Ontario, Canada) with density w = 1000 kg m −3 , dynamic viscosity μ w = 1 mPa s, and liquidair surface tension w = 72 mN m −1 . A commercially available oil-based ferrofluid (EMG900, Ferrotec, USA) with density = 1740 kg m −3 was mixed with a solvent (density 790 kg m −3 ) in the volumetric ratio of 1:10 (ferrofluid: solvent). The density of the prepared 10-fold diluted ferrofluid suspension used as the interfacial layer is ff = 885 kg m −3 . The surface tension of the interfacial layer in air is ff = 25.7 mN m −1 and in water ff − w = 23.6 mN m −1 . The laser oil-water interfacial tension is lo − w = 39.4 mN m −1 . The laser oil-ferrofluid interfacial tension lo − ff = 4 mN m −1 was calculated using the interfacial tension formula for non-polar liquids as lo−ff = lo + ff − 2 √ lo ff . [68] Ethylene glycol was purchased from Sigma-Aldrich, USA, which has a density of gly = 1110 kg m −3 , dynamic viscosity of μ gly = 16.1mPa s, and liquid-air surface tension of gly = 48.6 mN m −1 . Distortion-free glass cuvettes were purchased from Cuvet.Co (Hong Kong) with inner dimensions 50 × 50 × 50 mm. Figure 1 shows the schematic of the experimental setup for the underwater generation of ferrofluid-wrapped droplets. Before each experiment, the glass cuvette was thoroughly cleaned by dipping it in a glass beaker containing hexane, followed by ultrasonication (Branson 5800, Emerson Electric Co., USA) for 30 min. Thereafter, the cuvette was thoroughly rinsed with DI water and acetone, followed by drying with compressed nitrogen. Next, the cleaned cuvette was treated in air plasma (PE-25, PLASMA ETCH, USA) for 10 min. Then cuvette was placed over a vertically movable stage (Kruss GmbH, Hamburg, Germany). At first, the cuvette was partially filled with host liquid (DI water, 90 ml), resulting in a water column height of ≈ 36 mm. Then, 200 μl of the 10-fold diluted oil-based ferrofluid suspension was dispensed on the host bath using a pipette (DiaPETTE, Canada) which was allowed to spread uniformly on the water surface, lead-ing to the formation of the floating interfacial layer (i.e., which acts as the shell layer). Figure 1a shows the impact of a laser oil droplet on the interfacial layer. In this case, the laser oil droplets were dispensed using a disposable flat-tipped stainless-steel needle (gauge 14, part no. 7018035, Nordson EFD, USA) having an inner diameter of 1.53 mm attached to a 1 ml NORM-JECT sterile luer-slip plastic syringe (Henke-Sass, Wolf GmbH, Germany). The syringe containing laser oil was securely affixed vertically using a retort stand and centrally positioned over the cuvette (see Figure 1a). Laser oil droplets of volume 15 ± 0.8 μl were dispensed using a programmable syringe pump (Chemyx Fusion 4000) at a controlled rate of 36 μL min −1 from a height of 5 cm. Figure 1b shows the impact of a compound droplet on the interfacial layer. Throughout the study, the compound droplet indicated an ethylene glycol (EG) droplet wrapped in laser oil. A Y-junction flow arrangement was employed to generate a compound droplet, as shown in Figure 1b  3 μL min −1 . Then the compound droplet was dispensed by pumping laser oil using the programmable syringe pump (Chemyx Fusion 4000) at a controlled rate of 36 μL min −1 .

Compound Droplet Generation
For both single and compound droplet generation, the impact height, which was the difference between the needle tip and the ferrofluid interfacial layer, was kept constant at 5 cm, which corresponded to an impact Weber number We i = 57 for single droplet impact and We i = 68 for compound droplet impact. The impact Weber number We i = lo v 2 R d lo ≈ 2 lo gHR d lo was estimated by considering the properties of laser oil droplets. Here, v = √ 2gH is the velocity of impact, g is the acceleration due to gravity, H is the impact height, and R d is the radius of the droplet (assuming a spherical shape) before impact. For magnetic manipulation, a cubic neodymium (NdFeB) permanent magnet of size 1.27 cm (N52, remnant flux density B r = 1.48T, K&J Magnetics Inc. USA) was used, placed over a movable linear translator (Zaber Technologies, Canada), and positioned beneath the cuvette. The magnetic field was estimated using a 3D numerical simulation carried out in the computational framework of COMSOL Multiphysics. The variation of the magnetic field and the details of the COMSOL simulation are provided in Section S1, Supporting Information.
The complete dynamics of the encapsulation process were captured using a high-speed camera (Photron, FASTCAM mini) coupled with a lens interfaced with a personal computer. The manipulation of the encapsulated cargo and the color images in the study were captured using a macro lens (Tokina, 100 mm F2.8 MACRO) coupled with a DSLR camera (Nikon D5200). The postprocessing of the raw video files was done using an in-house MATLAB script to acquire relevant quantitative data reported here.

Results and Discussion
In this section, we discuss the impact-driven encapsulation of a single laser oil droplet and the compound droplet (EG drop engulfed within the laser oil drop) using the ferrofluid shell layer as the wrapping layer. Following that, we demonstrate the underwater magnetic manipulation of the ferrofluid-wrapped laser oil droplet by studying its transportation characteristics and the role played by the ferrofluid shell layer in the magnetic manipulation. Finally, we discuss the magnetic manipulation of the compound droplets, where we studied the role of the magnetic field in the underwater coalescence of two compound droplets. In addition, we also experimentally demonstrate a sample case of release of the innermost EG core in the compound droplet. In doing so, we demonstrate a novel platform that addresses the physiologically relevant problem of controlled underwater mixing of watersoluble analytes. Figure 2 shows the interfacial evolution during the encapsulation process when a laser oil drop and a compound drop pierce through the interfacial layer (ferrofluid), giving rise to encapsulated droplets. The impact is captured at 5000 frames per second.

Encapsulation of a Single Core Droplet and Compound Droplet using Ferrofluid Wrapping Layer
In both cases, the laser oil droplet and the compound droplet are released from a height of 5 cm onto the interfacial layer. The generation process of the compound droplet remains the same, as discussed in Section 2. Owing to the sufficient kinetic energy, the droplet pierces through the interfacial layer, where it overcomes the interfacial barrier offered by the ferrofluid-water interface and results in the formation of the encapsulated droplet as reported by Misra et al. [65,66] In both situations, the interface undergoes a similar evolution process, as evident from the high-speed timestamped images of Figure 2a,b, respectively. The color images of the final encapsulated droplets are shown in Figure 2a (rightmost) and Figure 2b (rightmost). It can be observed from the color images that, there is a significant difference in the concentration of the magnetic nanoparticles (MNPs) between the apex and the bottom of the droplet as evident from the appearance of the dark spherical cap at the apex of the droplet. This excess layer forms due to the extra ferrofluid volume the droplet pulls inward during impact. Ferrofluid ( ff = 885 kg m −3 ) being lighter than water ( w = 1000 kg m −3 ), the excess ferrofluid accumulates at the top due to buoyancy. As discussed in the subsequent sections, this excess layer plays a significant role in the magnetic manipulation of the encapsulated cargo.
Misra et al. [65] elucidated the thermodynamic criterion for the formation of a stable encapsulated droplet. They showed that for liquid-liquid encapsulation, the interfacial tension of the participating fluids should satisfy the following criteria.
The above equations are based on interfacial energy minimization before and after impact and depend only on the surface and interfacial tensions. Suppose we substitute the interfacial tensions mentioned in the previous section in Equations (1) and (2).
In that case, we get that for our combination involving laser oil (core), ferrofluid (shell), and water (host liquid).
which satisfies both the above criteria. For the compound droplet, the core liquid that plays a role in surface energy minimization during the impact-driven encapsulation process is laser oil, as in the case of the single-core droplet. Thus, based on the experimental images and the fulfillment of the above-mentioned criteria, we can confirm that the laser oil droplet is stably encapsulated inside the ferrofluid layer. In the case of a compound droplet, the droplet of ethylene glycol (marked with the red arrow in Figure 2b) remains stable during the interfacial evolution, as evident from the high-speed images of Figure 2b. In addition to that, the EG droplet remains stable after encapsulation. Note that EG being lighter than laser oil ( gly = 1110 kg m −3 , lo = 1900 kg m −3 ) migrates to the apex of the compound droplet because of buoyancy (see the color image, Figure 2b). By employing the ferrofluid interfacial layer, we fulfill two important aspects -i) controlling the dissolution of water-soluble ethylene glycol droplet in water which affirms the robustness of the encapsulation tech- nique and ii) providing the magnetic responsiveness to the compound droplet which is exploited for the magnet-assisted manipulation of the compound droplet as shown in Section 3.4. Figure 3 shows the schematic for the manipulation of the ferrofluid-wrapped droplets. In the preceding discussions, we emphasized the role of the non-contact manipulation of droplets and how it can be useful in related applications. Thus, the ability to controllably manipulate encapsulated droplets using a moving magnet is indispensable. Here we demonstrate the controlled actuation of the ferrofluid-wrapped droplet via two different motions of an external permanent magnet, as shown in Figure 3. First, magnet motion along the x-axis, where we fix the gap D m between the cuvette bottom and the magnet top and translate the magnet horizontally along the x-axis. Second, magnet motion along the z-axis, where the magnet is positioned at a fixed xlocation and moved up and down along the z-axis. In both cases, the interaction between the magnetic field and the MNPs gives rise to the actuation of the ferrofluid layer, resulting in controlled droplet manipulation, as discussed. The variation in the magnetic field is accounted for by using the magnetic Bond number, defined as [7,62] which is the ratio of magnetic force and interfacial tension force. Here μ 0 = 4 × 10 −7 H m −1 is the magnetic permeability of free space, H is the magnitude of the applied H-field (A m −1 ), R d is droplet radius, and ff − w is the ferrofluid-water interfacial tension. Thus, if the value of H-field (magnetic field) is known at a particular magnet height D m , then Bo m can be calculated. The variation of the H-field of the magnet and the associated simulation details used in this work are discussed in Section S1, Supporting Information. Figure 4 illustrates the complete mechanism of actuation of the ferrofluid-wrapped droplets via the movement of the permanent magnet in the respective x-and z-directions. Note that during the motion of the magnet, there always exists a finite time delay between the movement of the magnet and the effect of that movement being realized by the magneto-responsive droplet, irrespective of the direction of the motion. During translation in the x-direction, this delay in response results in a horizontal offset (x-offset) between the axis of magnetization and the axis of the droplet (please see Video S1, Supporting Information, which depicts this x-offset. For caption, refer to Section S8, Supporting Information). Consequently, the direction of the overall magnetic force, F m experienced by the ferrofluid-wrapped droplet is not vertical, as shown schematically in Figure 4a Figure 4a-2 experimentally delineates the translation motion of the ferrofluid-wrapped laser oil droplet owing to the motion of the magnet along the x-axis. In this case, the magnet-cuvette gap is kept fixed, that is, D m = 1.5 cm, which corresponds to Bo m = 88.8, and the magnet is moved with a speed v m = 3.8 cm s −1 . As explained previously, in the absence of an external magnetic field (i.e., when the permanent magnet is far away from the cuvette), the excess ferrofluid layer predominantly remains at the top (t = 0 s) as it has a lower density than the surrounding medium (water). When the magnet moves closer to the droplet from the left, owing to the interaction between the MNPs and the magnetic field, the ferrofluid layer experiences an attractive magnetic force toward the oncoming magnet. The MNPs always try to attain the position of maximum magnetic field and try to migrate towards the magnet. Owing to the attractive force experienced by the MNPs due to the incoming magnet from the left-hand side (see Movie S1, Supporting Information), the excess ferrofluid layer starts to rotate in an anti-clockwise direction (t = 0.2 − 0.8s). During this motion, when the magnet is directly beneath the droplet at t = 0.9s, the ferrofluid layer attains a position closest to the magnet. At this position, the axis of magnetization coincides with the axis of the droplet, and the MNPs experience the maximum magnetic force. Beyond this point (t ≥ 1.0s), the droplet begins to translate along the direction of the magnet (along the x-axis). It can be noted that the excess layer remains firmly attached to the droplet and causes it to translate, which affirms the stability of the magnetic manipulation. Once the droplet starts to move along the magnet motion, it experiences a viscous drag (due to the surrounding water bath) opposing its motion.

Mechanism of Droplet Manipulation
Depending on the competition between the magnetic pull and viscous retardation, the droplet moves a certain distance before coming to rest (termed as "disengagement," see the timestamp t = 3.5s in Figure 4a-2). The disengagement is defined as the condition when the encapsulated droplet can no longer follow the motion of the magnet and comes to rest instead. It is interesting to note that, just after the disengagement, the excess ferrofluid layer moves back to the top of the droplet due to buoyancy. The translational behavior of the ferrofluid-wrapped droplets is discussed in detail in Section 3.3, where the mechanism of disengagement is explained. Thus, the movement of the encapsulating ferrofluid layer propels the droplet. Unlike bulk ferrofluid droplets or magnetic liquid marbles, we achieve translation with a thin encapsulating layer of low-concentration ferrofluid. We estimate that our protocol allows robust, controlled actuation of the wrapped cargo with less than 0.5% volumetric concentration of the magnetic fluid (EMG-900) in the final encapsulated droplet, which indicates the sustainability of our framework. Importantly, we can also generate and subsequently manipulate droplets of different volumes, as discussed in Section S2, Supporting Information. Figure 4b-1 shows the manipulation of the ferrofluid layer across the droplet when the magnet is moved along the z-axis. We schematically show the involved forces and the resulting morphology of the compound droplet during the magnet's motion along the z-axis. In this case, the magnet's axis of magnetization coincides with the encapsulated droplet's axis which ensures that there is no x-component of the magnetic force experienced by the droplet and, therefore, no translational motion along the x-axis. The net magnetic force, F m acting vertically downward on the magneto-responsive encapsulated droplet increases as the cuvette-magnet separation, D m reduces due to the magnet approaching the base of the cuvette from the bottom. As a result, the excess ferrofluid layer experiences an increased attractive pull toward the bottom of the droplet. This is manifested in a visible decrease in the width of the excess ferrofluid layer, w ff . Herein, we experimentally capture the response of the encapsulated droplet towards the motion of the magnet along the z-axis for a single encapsulated droplet (laser oil) of radius 1.86 mm, the results of which are shown in Figure 4b The experiment is repeated for a total of five complete cycles similarly during which the ferrofluid layer undergoes the aforementioned morphological evolution cyclically. The cyclic variation of the layer morphology during the experiment can be confirmed by the quantitative estimation of the ferrofluid layer width w ff and the ferrofluid layer coverage area A ff with time as shown in Figure  4b-4,5. Here, w ff is the width of the excess ferrofluid layer across the droplet along the vertical direction, as shown in Figure 4b-4, which is calculated after processing the images in MATLAB. Similarly, A ff is the area of the excess layer on the droplet, as shown in Figure 4b-5, which is calculated assuming spherical cap geometry. The details of image processing and calculation scheme of w ff and A ff can be found in Section S3, Supporting Information. It can be observed from Figure 4b-4,5 that, the variation w ff and A ff follows similar cyclic patterns. Both w ff and A ff first increase owing to a decrease in D m , reach a maximum value and then begin to decrease again, which affirms the stability of the ferrofluid layer. The above result is presented in Movie S3, Supporting Information. One of the implications of the layer manipulation discussed above is that it allows one to expose the core droplet to the water environment by varying the magnetic field (Bo m ). This can be useful in the controlled coalescence of the core droplet by unwrapping the ferrofluid shell layer in a controlled fashion. Please note that we are not unwrapping the laser oil drop (core) completely, as demonstrated by Wang et al. [13] In this case, the magnetic force can only actuate the excess ferrofluid layer and cannot unwrap the thin ferrofluid layer that has an intrinsic thermodynamic tendency to adhere to the laser oil core, as shown theoretically in Section 3.1. The following sections will discuss the translational characteristic of the ferrofluid-wrapped laser oil droplets and the manipulation of compound droplets based on the above mechanisms.

Translational Characteristics of the Ferrofluid-Wrapped Droplets
Based on the above discussion, it can be understood that using a moving magnet, a ferrofluid-wrapped droplet can be transported from one location to another inside the water bath. Figure 5 shows the translational characteristics of the ferrofluid-wrapped laser oil droplets based on the magnet speed v m and the magnetic Bond number Bo m . The experimental procedure adopted to obtain the results shown in Figure 5 are described in Section S4, Supporting Information. The side view image sequence (denoted by numeric 1 − 5) shown in Figure 5a is obtained from imaging at a frame rate of 50 frames per second and represents the disengagement length of the droplets for five different magnet speeds v m at a particular Bo m = 42. Here, we define the disengagement length as the distance traversed by the droplet before it can no longer follow the motion of the magnet and comes to rest instead. Thus, is estimated as the absolute value of the difference between the initial and final location of the droplet (along the x-axis) for a given magnet speed v m at which the droplet ceases to follow the motion of the magnet. It can be observed from Figure 5a that as the magnet speed is decreased from v m = 4 cm s −1 to v m = 2 cm s −1 , the difference v m − v d reduces, which indicates that the tendency of the droplet to follow the magnet increases. Also, the corresponding disengagement length of the droplet increases ( 1 to 5 ), which can be observed in Figure 5a. This fact can be understood in the following manner as discussed here.
It is mentioned that the interaction between the magnetic field and the MNPs gives rise to the actuation of the ferrofluid layer which in turn results in the manipulation of the encapsulated cargo. The droplet motion is governed by the interplay between the magnetic force on the MNPs and the viscous drag between the encapsulated droplet and the host water bath. Once the droplet starts to move along the magnet motion, it experiences a viscous drag opposing its motion. At a lower magnet speed v m = 2 cm s −1 , the viscous drag experienced by the encapsulated cargo is less as compared to the same at a higher magnet speed v m = 4 cm s −1 as also noted by Mandal et al. [52] Thus, at a higher magnet speed, the magnetic force on the MNPs is unable to overcome the viscous drag experienced by the encapsulated cargo resulting in a shorter disengagement length. Figure 5b,c shows the temporal variation of the droplet displacement l and the droplet velocity v d for various magnet speeds v m and at a fixed Bo m . Here the droplet displacement l is defined as the horizontal length traveled by the droplet as v m is varied. Here the left wall of the cuvette (marked with the blue arrow in Figure 5a) denotes l = 0. The results of Figure 5a-c are presented in Movie S4, Supporting Information. A detailed discussion of image processing methodology to obtain droplet displacement and velocity is presented in Section S5, Supporting Information. The corresponding disengagement lengths and the droplet velocities for the cases shown in Figure 5a are marked in Figure 5b,c, respectively. Theoretically, the droplet translation in the x-direction can be described as a balance of magnetic force F m,x, and viscous drag F visc as shown in Equation (5).
As discussed in Section 3.2, F m,x is the x-component of the net Here, Δ is the difference in the magnetic susceptibility of the ferrofluid-encapsulated droplet and the surrounding water medium. V ff is the volume of the ferrofluid shell encapsulating the droplet, B is the magnetic flux density generated by the permanent magnet, and m d is the mass of the droplet. The viscous drag F visc can be modeled as Stokes' drag for a spherical body moving in a fluid, [9] such that F visc = −6 μ w R d v d . Here, μ w is the dynamic viscosity of the surrounding water medium. As noted by Bijarchi et al. [28] the above force equation cannot be solved analytically and hence requires numerical computations using appropriate boundary conditions. Nevertheless, it can be used to explain the results in Figure 5a-c qualitatively. Once the droplet starts moving, it experiences viscous drag due to the surrounding water medium. The ensuing retardation causes the offset between the magnet and droplet to increase, thus resulting in a decrease in magnetic force. At higher magnet speeds, this offset increases proportionally, so at higher magnet speeds, the droplet either does not engage with the magnet or disengages early. Secondly, the viscous drag is also proportional to the droplet speed v d , hence even if the droplet gets engaged at higher magnet speeds, it experiences a larger drag force that causes early disengagement.
The above discussion corresponds to a fixed Bo m and it is important to obtain the translational characteristics of the encapsu-  Figure 5d. As v m increases, decreases, such that at a sufficiently high v m , the droplet practically remains stationary. This is reflected in the near-flat nature of the curve between 7 and 8 cm s −1 . Conversely, at a sufficiently low v m , approaches the value of maximum traversal length, which is found to be 4.5 cm (see Section S4, Supporting Information). As we increase the magnetic field by lowering the gap D m (i.e., increase Bo m ), the curve shifts rightwards, which means that the magnet speed at which droplet disengagement occurs increases. Moreover, it can be assumed that for the cuvette size used in the current work, if ≈ 4.5 cm, one would not observe disengagement below that corresponding v m . This fact can be understood from Figure 5d. For every value of Bo m , we see that there is a minimum threshold value of v m , corresponding to ≈ 4.5 cm, the maximum available length for the droplet to travel. Now, if v m were to decrease even further, the droplet would remain engaged entirely with the magnet. We tested this hypothesis by doing a separate experiment, wherein the starting magnet speed is kept sufficiently low at 0.5 cm s −1 , and is then increased in discrete steps for D m = 1.8 cm. The results are presented in Figure 5e, wherein the blue dotted line represents the experiment with increasing magnet speed. For increasing v m , the droplet remains completely engaged with the magnet until v m = 1.8 cm s −1 . Beyond this value of v m , the droplet starts to disengage and with further increase in v m it travels shorter distances. Conversely, for the experiments on decreasing v m , the droplet approaches the same threshold point found in the case when v m is increased, which is found to be 2 cm s −1 . Hence, in our experiments, we observe the same threshold value of v m for disengagement, irrespective of the starting magnet speed indicating a robust magnetic manipulation. In addition, we also demarcate the result of Figure 5e into complete, partial, and no engagement zones. Note that the partial engagement zone is an essential attribute of efficient, targeted delivery systems where retrieving the magnet without disturbing the target analyte once the analyte has been delivered to the target is often necessary. Therefore, this boundary of the various zones is critical for the efficient design of magnetic manipulation systems, where one can appropriately estimate the dimensions for the actuation length or the required magnet speeds for the target application.
In the next section, we discuss the underwater magnetic manipulation of the encapsulated compound droplet, controlled coalescence, and the release of the inner core.

Underwater Manipulation of an Encapsulated Compound Droplet
As discussed in the introduction section, one of the primary motivations for developing magnet-assisted manipulation techniques is its suitability for non-contact transport and release of targeted materials packaged inside tiny droplets. Secondly, as noted by many authors, [1,15,27] the technique can also be applied for applications such as micro-mixing and micro-reactions, wherein two droplets can be controllably coalesced for miniaturized applications. Naturally, these applications involve droplet manipulation such as translation, rupture, or wrapping/unwrapping. With this motivation, we demonstrate a robust platform for controlled actuation and mixing of water-soluble cargo inside a water medium. As the introduction section discusses, a water-soluble EG core is used as the model analyte. EG is first enclosed inside a carrier medium of laser oil before executing the impact-driven wrapping process of the compound droplet by the magneto-responsive ferrofluid layer. The laser oil carrier phase serves two distinct purposes here. First, as identified in our previous work, [65] a critical condition for the success of the impact-driven liquid-liquid encapsulation is the mutual compatibility between the shell layer and the core droplet as they remain in direct contact in the final encapsulated cargo. However, our experiment found that if EG droplets are used for the core, they are incompatible with the oilbased ferrofluid suspension during encapsulation. Upon contact with the ferrofluid interfacial layer, the EG core droplet immediately disrupts the colloidal stability of dispersed superparamagnetic particles of the interfacial layer leading to a depletion zone (see Figure S6, Supporting Information). It is likely due to the attractive interaction between the EG and the oleic acid surfactant in the ferrofluid suspension. Therefore, unlike laser oil, EG can not be wrapped directly using the ferrofluid interfacial layer. However, to impart underwater magnetic manipulability to EG, it must be packaged alongside the ferrofluid. Introducing an intermediate carrier phase of non-polar laser oil compatible with EG and the ferrofluid suspension solves this challenge. Second, the ferrofluid-wrapped compound droplet essentially has a triplelayered morphology where the inner EG core is surrounded by two shell layers, namely, a laser oil shell in direct contact with EG and the outer ferrofluid layer. The intermediate viscous laser oil provides an additional diffusion barrier that prevents unwanted mixing of the EG droplet with the surrounding water bath. It improves the controllability of the manipulation and subsequent coalescence of the encapsulated cargo and aids in the robustness of the developed platform. The experimental scheme shown in Figure 1b can generate encapsulated compound droplets with good repeatability of laser oil-EG composition, which remains stable in the water bath (see Movie S5, Supporting Information; for captions, see Section S8, Supporting Information). Figure 6 shows the experimental scheme adopted in studying the magnet-assisted coalescence of two compound droplets inside the water bath.
The compound droplet consists of an ethylene glycol (EG) droplet encapsulated inside a laser oil droplet. The compound droplet is then encapsulated inside a ferrofluid shell wrapping layer using the impact-driven technique discussed in Section 3.1. The controlled coalescence of the two compound droplets is achieved in four steps -i) two compound droplets are generated inside the water bath with an excess ferrofluid layer at the apex of the droplet (Figure 6a), ii) then the magnet is brought beneath the cuvette and fixed at a gap D m which causes the two compound droplets to come in contact and also causes the excess layer to migrate towards the bottom (Figure 6b), iii) the gap D m is then reduced (increasing the magnetic field), which causes the excess ferrofluid layer to settle at the bottom of the droplet (Figure 6c), and iv) keeping the magnet at the same location D m for a while, results in the migration of the MNPs toward the zone of the highest magnetic field which results in the continuous drainage of the excess ferrofluid layer, which in turn results in the coalescence of the compound droplets ( Figure 6d).
We discuss the results of compound droplet coalescence through the time-stamped color images shown in Figure 7, which are obtained from a single experiment by varying the gap D m . The coalescence of compound droplets indicates the merging of the two outer laser oil droplets (containing the EG drops) with each other when the ferrofluid layer is brought to the droplet's bottom. The coalescence of the EG droplets encapsulated inside the laser oil is discussed separately. Initially, the magnet is positioned at a distance of D m = 2.3 cm (Bo m = 15.6) beneath the cuvette when the first compound droplet of radius 1.8 mm impacts the ferrofluid interfacial layer (Figure 7a), resulting in the formation of the "first encapsulated compound droplet." Owing to the magnetic pull experienced by the MNPs due to the magnet, the encapsulated droplet attains the position of the highest magnetic field. It stays there, as shown in Figure 7a (t = 0 s). Then, the second compound droplet impacts the ferrofluid interfacial layer at a distance from the first impact point resulting in the formation of the "second encapsulated compound droplet" at t = 1.2 s. The droplet, upon experiencing the magnetic pull, migrates (t = 1.2 to 2.1 s) toward the highest magnetic field and eventually comes in contact with the first droplet at t = 2.5 s. It can be observed from Figure 7a that the magnetic interaction between the MNPs and the magnetic field at Bo m = 15.6 is not sufficient to bring the ferrofluid layer from the apex of the encapsulated droplet to the bottom of the droplet. Owing to this fact, the drainage of the ferrofluid does not occur, thus, the coalescence of the droplets is not observed even after 63 s. The EG droplets remain intact inside the laser oil droplet, which affirms that the laser oil and the ferrofluid layer successfully prevent the mixing of the EG droplets and the water.
On the contrary, when the magnetic field is enhanced by bringing the magnet closer to a location D m = 1.8 cm (Bo m = 50.3), the excess ferrofluid layer migrates from the apex of the droplet toward the bottom of the droplet. This fact can be visually confirmed from the experimental images shown in Figure 7b (t = 162.6 to 219.3 s). It is interesting to observe that, even though the two droplets remain in contact with each other during the process, the ferrofluid layer cannot migrate below the contact point even at t = 219.3 s, which prevents the coalescence of the two laser oil droplets (containing EG drops). Thereafter, we further increase the magnetic field by reducing the gap to D m = 1.5 cm (Bo m = 105.1) at around 220 s (see Figure 7c). The increase in magnetic force pulls the excess ferrofluid layer below the point of contact between the two laser oil drops (t = 225.2 s). Owing to the drainage of the excess ferrofluid layer at the point of contact, the interface of the laser oil drops begins to fuse ("onset of coalescence" at t = 229.8 s), as shown in Figure 7c. Finally, the two laser oil drops merge at t = 240 s with two EG droplets intact inside the laser oil. This experiment provides an insightful understanding that the magnetic field can control the coalescence time of the two drops. Also, the experimental demonstration further affirms the stability of our impact-driven encapsulation technique and the role of the laser oil and the ferrofluid layer in preventing the mixing between the EG drops and the water. Thus, the ferrofluid layer facilitates translation and provides an ingenious way to control coalescence (see Movie S6, Supporting Information).
The above results indicate that the position of the excess ferrofluid layer has a significant role in controlling the coalescence time of the two laser oil drops containing the EG drops. Based on the experimental observations, we theorize that in the absence of the magnetic field, the drainage of the ferrofluid film at the point of contact would have occurred due to the combined effect of Laplace and disjoining pressure, [69] and hence would have taken much longer time for coalescence. However, in the presence of the magnetic field, the magnetic force pulls a significant portion of the ferrofluid layer from the droplet's apex to the bottom such that only a thin nanometric ferrofluid film is present at the point of contact. Thereafter, the attractive disjoining pressure causes this thin ferrofluid film to rupture, resulting in the coalescence of the laser oil droplets (containing EG drops). Based on this premise, it should be possible to facilitate immediate coalescence of the laser oil drops if the magnet is positioned closer to the cuvette bottom, that is, the magnetic force is sufficiently strong to bring the ferrofluid excess layer below the point of contact. To test this hypothesis, we performed another experiment wherein, instead of varying D m from 23 to 1.5 cm (as in the case of Figure 7), we positioned the magnet at D m = 1.5 cm before impacting the first droplet. The experimental results are shown in  . Please note that this is the same value of D m at which the ferrofluid layer moves below the point of contact between the two droplets, as shown in Figure 7c. We record the observation until the inner EG drops inside the laser oil merge.
Let us first discuss the coalescence of the outer laser oil droplets, as shown in Figure 8a. At t = 0 s, the first encapsulated compound droplet is shown. Since the magnet is already placed at a position (D m = 1.5 cm), where the droplet experiences a stronger magnetic force. Thus, we see that the excess ferrofluid layer occupies a position at the bottom of the encapsulated cargo (t = 0 s). Thereafter, at t = 0.4 s, the second encapsulated compound droplet forms and eventually comes in contact (t = 7.0 s) with the first droplet due to the strong magnetic pull.
Shortly after this, the outer laser oil droplets coalesce at t = 18.0 s, which follows the same mechanism discussed before. Thus, unlike the results shown in Figure 7, here we achieve the coalescence of outer laser oil drops in a much shorter time after the impact of the initial droplet. Please note that the coalescence of the outer laser oil droplets leads to a single encapsulated compound droplet with two ethylene glycol droplets inside the laser oil droplet (at t = 18.0 s in Figure 8a, see Movie S7, Supporting Information). For the applications such as micro-mixing and microreactions, the fusion of the cargo packaged inside encapsulated droplets is an important aspect that should be addressed. We next show the merging of the two ethylene glycol droplets (inside the laser oil) for the same experiment, as shown in Figure 8b. It is found that owing to the buoyancy (see Movie S7, Supporting Information), the two inner EG droplets slowly approach each other at the apex of the droplet (t = 198.5 s) and finally merge at t = 246.4 s. It is to be noted that due to the high viscosity of the laser oil (μ lo = 1024 mPa s), it takes much longer for the inner ethylene glycol droplets to coalesce. It is to be noted that in practical applications, the coalescing droplets will not necessarily have equal volumes. Hence, the coalescence of droplets of unequal volume is of paramount importance from a practical standpoint. We further explored the coalescence of encapsulated droplets with dissimilar volumes using a high-speed camera which is discussed in Section S7, Supporting Information.
We mentioned in Section 1 that a significant driving motivation for the development of magnetically active materials is their ability for targeted cargo delivery and release. In this context, we present a sample experimental result of releasing the inner EG core in a compound droplet via magnetic actuation. For visualization, we dyed EG with a commercially available orange dye (Bright Dyes FLT Orange Liquid, Product number: 106006, Kingscote CHEMICALS, USA) and used a white LED light for better color contrast to show post-release mixing. The results are presented in Figure 8c. In this experiment, we generate an encapsulated compound droplet, as described in Section 3.1. The initial value of D m = 4 cm, which corresponds to Bo m = 0.56. Then the magnet is quickly moved upwards to D m = 2.0 cm (Bo m = 27.3), which causes the migration of the excess layer from the droplet's apex toward the bottom in a short time. As a result, the EG core becomes visible at t = 13.2 s (marked with a red arrow). We postulate the following event taking place at this time instant. A high shear rate is applied on the laser oil drop by the excess ferrofluid layer due to the quick migration of the excess ferrofluid layer from the apex to the bottom of the droplet. This results in the immediate rupture of the laser oil drop (which otherwise is under stable encapsulation in water during a normal impact-driven liquid-liquid encapsulation process) and releases the inner EG core (which has moved near the apex of the compound droplet due to density stratification) into the surrounding water medium as shown at t = 13.3 s. Thereafter, EG being heavier than water, it migrates toward the bottom and slowly dissolves, as evidenced by the misty orange appearance in the vicinity of the droplet bottom at t = 31.3 s (see Movie S9, Supporting Information). It can be clearly noted that the ruptured EG core displaces the ferrofluid layer after the release (t = 31.3 s), thus indicating their mutual incompatibility, as we discussed previously.
While we have not performed dedicated experiments for the controlled release of the inner cargo, we attribute the thinning of the laser oil film contained between the inner EG drop and the outer ferrofluid film to the abrupt release of the EG drop. The thinning can be accelerated via magnetic actuation, which pulls the excess layer and, in turn, distorts the droplet shape at the apex (see Figure 8c, t = 13.2s). Any deviation from the spherical cap shape leads to non-uniform curvature along the interface, which may accelerate thinning of the laser oil film. However, other factors such as droplet pinning, offset with the axis of magnetization, and post-impact internal movement of the inner droplet may also contribute significantly to the release phenomenon. A detailed analysis of these factors is beyond the scope of the current work. Nonetheless, we show that magnetic manipulation of encapsulated droplets is well-suited for targeted cargo delivery and can be enhanced further in future works.
Thus, in this section, we demonstrate that using magnetic actuation, we can control the total coalescence time of the outer layer (laser oil) of the encapsulated compound droplets. In addition, we also show that the inner EG droplets inside the laser oil drop spontaneously coalesce to form a larger encapsulated EG cargo. Note that the buoyancy-driven coalescence of the internal EG core droplets inside the laser oil carrier medium is a hydrodynamic phenomenon that can be tailored further by tuning the viscosity of the carrier phase and the density ratio between the carrier phase and the target analyte. Optimizing the fluidic properties of the carrier phase remains an exciting avenue for future research toward a fundamental understanding of the dynamics of compound droplet coalescence and controlled on-demand micromixing and micro-reaction. Importantly, we also demonstrate the release of the inner EG cargo, which can be taken up as a stepping stone to develop magnet-assisted controlled release technologies.

Conclusion
In summary, we report the underwater magnet-assisted manipulation of ferrofluid-wrapped droplets. We use an impact-driven liquid-based encapsulation technique to encapsulate laser oil drops and compound drops inside an oil-based ferrofluid shell inside a water bath. The compound droplets contain ethylene glycol encapsulated inside a laser oil drop. We found that the magnetic interaction between the magnetic nanoparticles with the magnetic field results in the migration of the ferrofluid shell layer from the apex to the bottom of the droplet. The migration of the shell layer occurs in two situations: the magnet is translated along the horizontal direction (x-axis) at a fixed gap from the cuvette bottom or by moving the magnet along the vertical direction (z-axis) at a fixed location. In this case, we find that the ferrofluid shell layer remains intact even after multiple cycles of magnet motion along the vertical direction (z-axis). We quantify the shell layer characteristics in terms of the variation of ferrofluid layer width w ff and ferrofluid layer coverage area on the droplet A ff . We exploit the first mechanism of shell layer migration to study the transportation characteristics of the encapsulated droplet, where we quantify the translational characteristics in terms of disengagement length ( ), magnet speed (v m ), and magnetic Bond number (Bo m ). We found that at a fixed Bo m , increase in v m results in a decrease in and at a fixed v m , increase in Bo m results in an increment in . We also show that for every Bo m , there is a threshold value of v m below which the droplet travels all along the cuvette length, and disengagement is not observed. Then, we use the second mechanism of shell layer migration to demonstrate the underwater coalescence of the encapsulated compound droplets. For the first time, we show the magnetassisted coalescence of compound droplets, wherein we conclusively demonstrate that the magnetic Bond number significantly influences the coalescence time of the compound droplets. We found that the increase in Bo m , results in the faster coalescence of the laser oil droplets by partially unwrapping the ferrofluid shell layer, forming a larger encapsulated compound droplet containing two drops of ethylene glycol. We then observe that owing to the buoyancy, the two inner ethylene glycol droplets slowly approach each other toward the apex of the droplet and finally merge. Finally, we show that magnetic actuation can also be used to release the inner cargo in compound droplets, thus paving the way for targeted cargo delivery. We believe that the present study provides an insightful understanding of the magnet-assisted manipulation of the encapsulated droplets and coalescence of the encapsulated compound droplets and is expected to be useful in experiments or applications that require targeted drug delivery, micro-mixing or micro-reaction and controlled release of the encapsulated core.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.