The formation of dewetting structures after evaporation of n-dodecane on graphite studied by atomic force microscopy
Introduction
The drying and dewetting of liquid films is of fundamental importance to many aspects of science and technology. Numerous applications exist, for example the drying of ink and the lubrication of engines, which would benefit from a deeper understanding of the mechanisms involved in dewetting and drying. Obtaining this knowledge could lead to the prevention of dewetting and allow the formation of homogeneous films. Although this topic has been the subject of much interest over the last decade, a complete understanding is yet to be achieved.
During the last decade, several experimental studies regarding the instability and dewetting patterns of non-volatile thin liquid/polymer/metal films have been published (e.g. [1], [2], [3]). The experimental developments led to careful theoretical and simulation studies which drastically improved the understanding of the dewetting phenomena. Various theoretical frameworks are now available for the analysis of the experimental results (e.g. [4]). Dewetting has been characterised by three stages: the rupture of a film leading to the formation of holes, the growth of the holes resulting in a network structure and then the aggregation of material causing the formation of structures on the substrate [1]. Two mechanisms of triggering film rupture have been suggested: nucleation and spinodal dewetting. The first results from the nucleation of dry spots which are caused by defects/heterogeneities or thermal activation [2]. The second is due to attractive long- or short-range molecular forces [2], [3], [4], [5] destabilising the film and causing the exponential growth of surface fluctuations resulting in spontaneous rupture of the film and characteristic patterns of small holes or undulations. The origin of the destabilising forces depends on the specific system. For example, in the case of water on freshly cleaved mica they were attributed to “polar hydrophobic” interactions [6], [7] while in the majority of systems they are attributed to the long-range van der Waals attraction [2], [3], [4], [5]. Apart from the short- or long-range attractive forces, recent experimental [8] and theoretical [9] work has concentrated on the density variations (coupled with the local film thickness) caused by restructuring, confinement and layering in polymeric films. This effect seems to destabilise thin films which are thermodynamically stable (no short- or long-range attraction is present) and produces spinodal-like dewetting patterns.
Drying can be described as the removal of liquid material from a surface due to evaporation. The rate at which material is removed from the surface is dependent on factors such as surface area, temperature, vapour pressure and air velocity and can be described on the basis of kinetic theory. Experimental studies of the dewetting behaviour of evaporating films are rare and the theoretical framework is still vague. The dewetting/drying patterns can be influenced by a variety of antagonistic and possibly coupled effects: evaporation/condensation cycles, multilayer adsorption, hydrodynamic effects, evaporation rate, curvature of the droplets etc.
If the liquid film wets the solid surface (this corresponds to our experimental system) there are two options: (i) the thickness of the film remains uniform and decreases continuously until total evaporation has occurred (an ultra-thin adsorbed film can remain on the surface); or (ii) for a specific range of film thickness uniform films are unstable and rupture into two different phases (thicknesses). This second scenario is more complicated and it has been shown to produce pattern formation in the case of water on cleaved mica [10]. In the case of a non-wetting evaporating liquid (water on graphite) Thiele et al. [6] showed that the dewetting occurs by both mechanisms (spinodal and nucleation) at high evaporation rates while at low evaporation rates the spinodal dewetting is suppressed. Kagrupta et al. [11] used 2D non-linear simulations to investigate the dewetting patterns in evaporating thin liquid films. They found that the patterns (holes) do not form uniformly over the entire surface since a nucleated rupture can initiate a spinodal cascade of secondary holes. The average size of the resulting holes increases with a decreasing rate of evaporation.
In this work we study the patterning of an n-alkane (n-dodecane) on a (freshly cleaved) highly ordered pyrolytic graphite (HOPG) surface after drying and dewetting using atomic force microscopy (AFM). Previous work on these samples has shown the existence of stable nanoscale features on the “dry” surface. The dimensions of these are thought to be dependent on the size of the alkane used [12], [13]. The stability of these structures is believed to arise from the adsorption of the bottom n-alkane layer on to the graphite surface with the carbon chain lying parallel to the surface and with the hydrogen atoms fitting into the centres of the hexagonal structures formed by the carbon atoms of the graphite [14]. With alkanes, and other materials, for example liquid crystals, this adsorption can result in domains of aligned molecules [15]. At the meeting point of such domains, the formation of a characteristic chevron type structure is possible and has been observed for some materials by scanning tunnelling microscopy [16], [17].
In this paper, we show that a spinodal-like dewetting pattern co-existing with nucleated holes can occur in a system expected to present thermodynamic stability at all thicknesses. We argue that the only plausible explanation for this is the density variation due to layering and possible restructuring when the film is sufficiently thin. To the best of our knowledge, spinodal-like patterns have not previously been observed in a thermodynamically stable (complete wetting) volatile simple liquid.
Section snippets
Experimental
All experiments were carried out using a Molecular Imaging PicoSPM operating in intermittent contact mode. The cantilevers used have a spring constant of 1.2–5.5 N/m (typically 2.8 N/m), a resonant frequency of 60–100 kHz (typically 75 kHz) and nominal tip radius of ∼10 nm. The HOPG surface (AGAR) was initially examined directly after cleaving and found to have similar characteristics to those reported by Martin et al. [12]. The alkane used for these experiments is n-dodecane (C12H26) which has
Results
Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 show the drying and dewetting patterns which have been observed at various heating times and temperatures. For each permutation of heating time and temperature, sizeable quantities of dodecane remained on the HOPG surface to form patterns. These have been found to remain on the surface for many hours after the samples are prepared. Cross-sectional AFM images of the samples at different heating times and temperatures can be seen in Fig. 8 and show
Discussion
In our experiments, we have two competing phenomena occurring at the same time: evaporation and dewetting. As they occur simultaneously and since the majority of our AFM measurements are snapshots of an evolving system, it is difficult to judge their individual contributions to the observed structures.
A variety of patterns of various length scales have been observed previously in drying drops of colloid solutions [19] and have been associated with the antagonistic effects of flow during drying
Conclusion
We have shown that the evaporation of n-dodecane on graphite results in the formation of patterns which are attributed to two different dewetting scenarios: (i) heterogeneous and thermal nucleation of “dry” spots and (ii) spinodal-like dewetting at nanometer-size film thicknesses due to combination of apolar long-range repulsive forces and restructuring/layering-induced density variations.
Acknowledgements
The authors would like to thank F. Madani, S. Shirran, A. Wagner and D. Martin for helpful discussions. This work was funded by EPSRC, The Royal Society and The Nuffield Foundation.
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