Biaxially Morphing Droplet Shape by an Active Surface

Abstract

Drop morphology can be manipulated by designing localized solid/liquid interactions to create a favorable interfacial energy equilibrium. A topographical surface with hierarchical roughness can be harnessed to generate complex drop morphologies, enhance uniaxial and anisotropic spreading, in a designable fashion. Here, using an active surface is proposed with a responsive roughness (wrinkle patterns) under uniaxial compression/stretching, to morph droplet shape biaxially in a continuous and reversible manner. The keys to achieve biaxial drop shaping are the in‐plane confinement from lattice hole patterns and the programmable formation of roughness, to pin and guide contact line movement in both in plane directions. The complex interplay between wetting and the patterns is elucidated by both experiments and numerical analysis. The results enrich the current understanding of shaping droplets by managing the contact line pinning/movement on an engineered elastic substrate, and providing insights for emerging applications in the areas such as droplet emicrofluidics, liquid robotics, ink‐jet printing, 3D printing and healthcare.

Rose petal effect: A subtle combination of nano‐scale roughness and chemical variability

“…[28,41] Very high curvature of the surface morphology ("edges") and chemical heterogeneity lead -on the nanoscale-to contact line pinning [42,43] and -meso/macroscopically-to advancing ϑ adv and receding ϑ rec contact angles that differ from the equilibrium contact angle (with ϑ rec ≤ ϑ 0 ≤ ϑ adv ) [31,44] and thus to contact angle hysteresis, which implies on the one hand friction processes (of the contact line) and on the other hand, that the concept of "reaching equilibrium" is (highly) non-trivial. [19,45,46] In this context, a superhydrophobic surface-such as the upper side of sacred Lotus (Nelumbo nucifera) leaves-is defined as a surface with very high (pseudo-equilibrium) contact angle (ϑ 0 > 150 • , and thus larger than the maximum possible on a flat surface) and very low contact angle hysteresis (with Δϑ hys = ϑ adv − ϑ rec < 5-10 • ), [22,33,47,48] while the Rose petal effect implies a surface with very high (pseudoequilibrium) contact angle and very high contact angle hysteresis (ϑ 0 > 120 • , and Δϑ hys >50 • ). [20,21,23,49] Aware of the existing wettability literature and new findings on the nature of plant surfaces as described above, our hypothesis is that the difference between the Rose petal and the Lotus Effects cannot be simply explained via surface roughness, but requires consideration of further aspects associated with surface chemical composition.…”

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

“…Oriented wrinkled structures with controlled surface functionality and variable roughness can effectively control the wettability of the surface. [ 39 , 148 , 244 , 245 , 246 , 247 ] Lee et al. presented an elastomeric smart window with switchable wetting, as depicted in Figure 24 .…”

“…[238][239][240][241][242][243] Oriented wrinkled structures with controlled surface functionality and variable roughness can effectively control the wettability of the surface. [39,148,[244][245][246][247] Lee et al presented an elastomeric smart window with switchable wetting, as depicted in Figure 24. The nanopillar structure was first replicated onto a PDMS film surface by micromolding.…”

Drone Application for Spraying Disinfection Liquid Fighting against the COVID-19 Pandemic—Examining Drone-Related Parameters Influencing Effectiveness

“…The behavior of thin liquid layers on surfaces and the conditions of droplet dispersal have been studied by several authors, based on which it can be proved that the thickness of the film layer can be formed in nanometer dimensions [17,[31][32][33][34][35][36]. In the case of wiping, the thickness of the film layer is obviously thinner than when examining the spontaneous spreading of the droplets, so the results of the latter are also acceptable as a base of good practice for drone spraying.…”

“…12%. Although this initial coverage ratio can be considered low, it should be taken into account that with the help of a surface tension-reducing additive, the coverage ratio can be multiplied [32][33][34][35][36]. After the liquid comes to the surface, the droplets spread on the surface, reaching a multiple of their previous diameter size and losing their initial shape due to the surface tension-reducing additive.…”