Droplet Memory on Liquid-Infused Surfaces

Abstract

The knowledge of droplet friction on liquid-infused surfaces (LIS) is of paramount importance for applications involving liquid manipulation. While the possible dissipation mechanisms are well-understood, the effect of surface texture has thus far been mainly investigated on LIS with highly regular solid topographies. In this work, we aim to address this experimental gap by studying the friction experienced by water droplets on LIS based on both random and regular polysilsesquioxane nanostructures. We show that the available models apply to the tested surfaces, but we observe a previously unreported droplet memory effect: as consecutive droplets travel along the same path, their velocity increases up to a plateau value before returning to the original state after a sufficiently long time. We study the features of this phenomenon by evaluating the motion of droplets when they cross the path of a previous sequence of droplets, discovering that moving droplets create a low-friction trace in their wake, whose size matches their base diameter. Finally, we attribute this to the temporary smoothing out of an initially conformal lubricant layer by means of a Landau-Levich-Derjaguin liquid film deposition behind the moving droplet. The proposed mechanism might apply to any LIS with a conformal lubricant layer.

Figure 1

SEM images of the surfaces used for this study: (a and b) SNFs, (a) tilted view and (b) side view; (c and d) rods, (c) tilted view and (d) side view.

Figure 2

(a) Scheme of the experiment used to determine the scaling of the droplet friction as a function of Ca, (b) detail of a single experiment, showing the plateauing effect observed for successive droplets, (c) driving force as a function of the expected friction scaling, and (d) ratio of the plateau velocity over the first droplet velocity as a function of the time interval between successive droplets.

Figure 3

(a–c) Scheme of the droplet crossing experiments, showing its three main steps: (a) reference, (b) trace, and (c) probe droplet motion and (d–g) detail of a single experiment (rods, V_trace = 15 μL, V_probe = 10 μL): (d) velocity and (e) acceleration of the reference and first probe droplet and (f) velocity and (g) acceleration of consecutive probe droplets.

Figure 4

Normalized acceleration of the first probe droplet (V_probe = 10 μL) crossing a trace (V_trace = 10 μL) as a function of the (a) advancing and (b) receding front position. The shaded areas represent the nominal extent and position of the trace.

Figure 5

Position of the global maximum and minimum of acceleration in terms of the (a and b) receding front and (c and d) their spacing as a function of the base diameter of (a and c) trace d_trace and (b and d) probe d_probe droplets. The shaded areas represent the nominal extent and position of the trace.

Figure 6

Scheme of the proposed mechanism for the droplet memory effect: (a) lubricant is initially conformal to the surface texture; (b) droplet deposits a thin lubricant at its back; (c) LIS surface is temporarily smoothed; and (d) after some time, the LIS returns to its original configuration.