Droplet Mobility Manipulation on Porous Media Using Backpressure

Abstract

Wetting phenomena on hydrophobic surfaces are strongly related to the volume and pressure of gas pockets residing at the solid-liquid interface. In this study, we explore the underlying mechanisms of droplet actuation and mobility manipulation when backpressure is applied through a porous medium under a sessile pinned droplet. Reversible transitions between the initially sticky state and the slippery states are thus incited by modulating the backpressure. The sliding angles of deionized (DI) water and ethanol in DI water droplets of various volumes are presented to quantify the effect of the backpressure on the droplet mobility. For a 50 μL water droplet, the sliding angle decreases from 45 to 0° when the backpressure increases to ca. 0.60 bar. Significantly smaller backpressure levels are required for lower surface energy liquids. We shed light on the droplet actuation and movement mechanisms by means of simulations encompassing the momentum conservation and the continuity equations along with the Cahn-Hilliard phase-field equations in a 2D computational domain. The droplet actuation mechanism entails depinning of the receding contact line and movement by means of forward wave propagation reaching the front of the droplet. Eventually, the droplet skips forward. The contact line depinning is also corroborated by analytical calculations based on the governing vertical force balance, properly modified to incorporate the effect of the backpressure.

Controlled deposition of fullerene derivatives within a graphene template by means of a modified Langmuir-Schaefer method

“…After this first static mea- surement and at the same point, dynamic water CA measurements were performed. Advancing and receding CA were measured on a water droplet of decreasing/increasing volume [59].…”

Pancake Jumping of Sessile Droplets

“…Coalescence‐induced droplet jumping occurs on superhydrophobic surfaces caused by the release of surface energy during the condensation process, [ 22 , 23 , 24 , 25 , 26 ] but it is limited by the low energy transfer efficiency and thus suffers from a small jumping velocity. [ 27 , 28 ] The directional movement of sessile droplets can be achieved on responsive surfaces through constructing energy gradient, or resorting to stimuli such as temperature, [ 29 , 31 ] pressure, [ 32 , 33 , 34 , 35 ] optical, [ 36 , 37 ] electrical [ 38 , 39 ] or magnetic fields, [ 40 , 41 , 42 , 43 , 44 , 45 , 46 ] etc. In particular, owing to its advantages of instantaneous response, [ 42 ] low energy consumption, flexible/convenient/safe controllability, and good biocompatibility, magnetic actuation has emerged as a promising approach to manipulate droplet motion.…”

Multiple air-bubble enhanced oil rupture on nanostructured cellulose fabric for easy-oil cleaning fouled in a dry state

“…Later, when put into water, this trapped air further pushes off the oil drop on top of the nanostructure as the water wicks through the fabric with the pushing air. This air beneath the oil droplet is known to act as a backpressure with improved liquid mobility by inducing a de-pinning of the oil 34–36 , as schematically shown in Fig. 4d.…”