Droplets Capped with an Elastic Film Can Be Round, Elliptical, or Nearly Square

Abstract

We present experiments which show that the partial wetting of droplets capped by taut elastic films is highly tunable. Adjusting the tension allows the contact angle and droplet morphology to be controlled. By exploiting these elastic boundaries, droplets can be made elliptical, with an adjustable aspect ratio, and can even be transformed into a nearly square shape. This system can be used to create tunable liquid lenses, and moreover, presents a unique approach to liquid patterning.Wetting has been the subject of intense research for well over a century, motivated largely by industrial applications, ranging from designing tires to treating textiles and choosing coatings for surfaces, but also by the academic interest in a field which is undeniably rich [1]. A common theme in wetting phenomena is that boundary conditions and substrate play a pivotal role. For instance, when the substrate is replaced by a soft solid, elastocapillary interactions lead to deviations from the classical Young-Dupré's law due to the formation of a wetting ridge at the contact line [2][3][4][5][6][7][8][9]. The substrate can also be replaced by a thin free-standing elastic film, serving as a compliant boundary for the droplet [10][11][12][13][14][15][16][17]. In this geometry, the contact angles are set by a Neumann construction with mechanical and interfacial tensions balanced at the contact line [11,12, 14,16,17].Compliant elastic surfaces also show novel wetting behaviours and morphologies. For instance, droplets have been observed to migrate towards regions of increased compliance [18][19][20], and interact with other droplets due to deformations induced in the elastic films [15,21]. Dewetting liquid films capped by a thin elastic layer can have their dynamics and dewetting morphologies controlled by adjusting the tension [23]. Anisotropic tension in a supporting free-standing film causes sessile droplets to elongate along the high tension direction, and thus, droplets map out the stress field in the elastomer [17]. Furthermore, droplets pressed between a rigid surface and a soft solid acquire the shape of a flattened ellipsoid [22].Although partial wetting on soft or compliant solids has received significant attention, here, we examine partial wetting in a novel geometry wherein droplets are capped by a thin elastic film under tension. This system could serve as a model for blisters or droplets trapped beneath drying paint [24]. We show that the contact angle of these droplets decreases with increased tension which is well described through an analogy made with Young-Dupré's law which incorporates mechanical tension. The model contains a free parameter from which the elastomer-liquid interfacial tension may be determined. We extract reasonable values for this quantity for four different liquid-solid combinations. Finally, we show that biaxial stresses in the capping elastic boundary can produce elliptical droplets with tuneable aspect ratio, and even droplets with nearly-square morphology using a suitable sample geometry.Thin elast...

Elastocapillary cleaning of twisted bilayer graphene interfaces

“…Apparently, the application of pretension flattens the nanopocket profile. Similar behavior has been recently observed in droplets capped with thin polymeric films, in which pretension overwhelms the tension associated with the out-of-plane deformation 14 . What is new in our experiments is that the latter ( ) is more likely to be greater than or at least comparable to the pretension ( ).…”

“…This implies that the associated elastic energies have been underestimated in our model. The deviation may be ameliorated by a more detailed analysis considering the initial pretension, the non-uniformity of wrinkling patterns, the effect of the drumhead curvature 14 , 43 as well as the “mechanics” arising when pretension is relatively weak 44 .…”

“…Similar scenarios of thin solids confining liquids have been extensively investigated at larger scales, including liquid blisters and lenses 13 , 14 . What makes the vdW material system particularly intriguing is the mobility of the confined liquids.…”

Time-Dependent Pinning of Nanoblisters Confined by Two-Dimensional Sheets. Part 1: Scaling Law and Hydrostatic Pressure

“…By analyzing the morphology changes of wrinkles on both graphite flakes that extend to the overlaid area, we determine that flake 2 is on top of flake 1. A closeup image such as the one in Figure (c) shows that the confined blisters are mostly in nearly round shapes, except for a few elongated ones which indicate local anisotropic tensions in the capping sheet . For example, blisters near wrinkles are seen to be elongated along the wrinkles’ directions.…”

“…On the other hand, blisters enclosing liquids represent a novel partial wetting phenomenon that stimulates research interest. In such a case, referred to as elastic-wetting elsewhere, the wetting behaviors of the confined droplets are governed together by the elasticity of the capping film and the surface tension. , …”