Deviation of sliding drops at a chemical step

Abstract

The motion of partially wetting liquid drops in contact with a solid surface is strongly affected by contact angle hysteresis and interfacial pinning. However, the majority of models proposed for drops sliding over chemical surface patterns consistently neglect the difference between advancing and receding contact angles. In this article, we present a joint experimental and numerical study of the interaction of gravity-driven drops with a chemical step formed at the junction between a hydrophilic and a hydrophobic region. It demonstrates the strong impact of a contact angle hysteresis contrast on the motion of drops at a linear chemical step. Surprisingly, the smallest driving force required to drag the drop across the step onto the lower hydrophobic surface is not observed at a right angle of incidence. Our model reveals that the non-monotonous response of this passive drop 'filter' is solely due to the higher advancing contact angle on the lower surface, and creates an instance where drop motion is affected by dissipation at the contact line rather than by surface energy.

Pinning of a drop by a junction on an incline

“…where V is the volume of the liquid drop and R is the radius of the initial circular contact line, [21]. The range of the Bond numbers showed in this table are similar to the ones measured by other authors in experiments on the shape and motion of millimetre-size drops of silicon oil sliding down over an homogeneous plane [18] and of glycerol-water mixtures over a substrate decorated with linear chemical steps [26].…”

Stretching of viscoelastic drops in steady sliding

“…Controlling and manipulating drops on open surfaces is a crucial step for applications in many fields, including chemistry, biomedicine, ink-jet printing, food and pharmaceutical industry [1][2][3][4][5][6]. The vast majority of such applications involves non-Newtonian fluids, e.g.…”

Drop mobility on chemically heterogeneous and lubricant-impregnated surfaces

“…When a gravity-driven water drop impinges on a chemical step formed at the junction between a hydrophilic and a hydrophobic region at a finite tilt angle, the presence of the step breaks the reflection symmetry of the drop shape while the induced deformation creates a coupling of the drop mobility into parallel and perpendicular directions [30]. Such a 'cross-talk' would be hindered if the surfaces had a contrast in surface energy but vanishing contact angle hysteresis.…”

“…The possibility of using chemically heterogeneous surfaces (CHS) to guide wetting drops along certain directions has recently attracted much attention both theoretically [24][25][26][27][28][29][30][31][32] and experimentally [33][34][35][36][37][38]. In these studies, the driving force is provided by the down-plane component of the drop weight F g = ρVgsinα, acting on a drop of volume V and density ρ sliding down an inclined plane tilted by an angle α (see Figure 3).…”