Computational investigation of actuation mechanisms of droplets on porous air-permeable substrates.

We study the actuation of droplets on porous substrates by air that permeates through pores. Air pockets are created between the droplets and the substrate which, eventually, incite the droplets to a quasi-moving state. We observe this mechanism computationally and verify it experimentally, using various case studies involving water droplets of different volume that are initially pinned on a porous substrate which has been set to different inclination levels and start to slide down when actuated by permeating air. The computational model employs the continuity equation and the equations of momentum transfer that are coupled with the Volume of Fluid (VOF) method, to track the shape of the droplet. We identify two dominant actuation mechanisms - seen in computations and experiments - that are given the names 'donut' and 'tunnel'. Both of them are characterized by the formation of small air pockets between the droplet and the substrate that coalesce into larger ones that finally escape the droplet, by collapsing its free surface. The two mechanisms differ in the way that the free surface of the droplet collapses. The donut mechanism has the free surface collapsing at its center, thus forming a hole in the middle of the droplet (hence the name, donut), whereas the tunnel mechanism has the free surface collapsing at its rear side, forming a horizontal hole that resembles a tunnel (hence the name). We compare each mechanism in terms of the event (mechanism) occurrence frequency and droplet displacement, and also provide the dependence of the droplet speed with respect to the flow rate of permeating air, substrate inclination and droplet volume.

Droplet Mobility Manipulation on Porous Media Using Backpressure.

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.

Plasma directed assembly and organization: bottom-up nanopatterning using top-down technology.

Fabrication of periodic nanodot or nanocolumn arrays on surfaces is performed by top-down lithographic procedures or bottom-up self-assembly methods, which both make use of plasma etching to transfer the periodic pattern. Could plasma etching alone act as an assembly--organization method to create the pattern and then transfer it to the substrate? We present data that support this idea and propose a mechanism of periodicity formation where etching and simultaneous deposition take place.

Mechanisms of oxygen plasma nanotexturing of organic polymer surfaces: from stable super hydrophilic to super hydrophobic surfaces.

Plasma processing is used to fabricate super hydrophilic or super hydrophobic polymeric surfaces by means of O2 plasma etching of two organic polymers, namely, poly(methyl methacrylate) (PMMA) and poly(ether ether ketone) (PEEK); a C4F8 plasma deposition follows O2 plasma etching, if surface hydrophobization is desired. We demonstrate high aspect ratio pillars with height ranging from 16 nm to several micrometers depending on the processing time, and contact angle (CA) close to 0 degrees after O2-plasma treatment or CA of 153 degrees (with CA hysteresis lower than 5 degrees) after fluorocarbon deposition. Super hydrophobic surfaces are robust and stable in time; in addition, aging of super hydrophilic surfaces is significantly retarded because of the beneficial effect of the nanotextured topography. The mechanisms responsible for the plasma-induced PMMA and PEEK surface nanotexturing are unveiled through intelligent experiments involving intentional modification of the reactor wall material and X-ray photoelectron spectroscopy, which is also used to study the surface chemical modification in the plasma. We prove that control of plasma nanotexture can be achieved by carefully choosing the reactor wall material.