Abstract:
Porous surfaces are widely used for wetting phenomena control exploiting either the gas
entrapped on the porous network, or their ability to be infused, or impregnated by lubricants.
Still gas entraps (pockets) tend to collapse after prolonged exposure to liquid environments, or
at high liquid pressures, while lubricants tend to cloak the droplets and gradually dry out. In our
study we combine the two aforementioned approaches and we use porous media and have
them perfused with gas to dynamically actuate, and manipulate droplets on their surfaces. By
adjusting the backpressure, i.e. the pressure from the rear side of the porous medium, the
droplet may be actuated, and its downward velocity may be controlled without completely
levitate it. This entails low values of backpressure, in the order of few mbar, depending on the
porous network characteristics.
In this work we are going to present the basic principles of this approach, and
demonstrate it in various applications including droplet impingement, valving in digital
microfluidics, and droplet logic operations.
The mechanisms of actuation have been studied by means of simulations encompassing
the momentum conservation and the continuity equations along with the Cahn−Hilliard phasefield
equations in a 2D computational domain. The droplet actuation mechanism involves
depinning of the receding contact line and movement by means of forward wave propagation
reaching the front of the droplet, yielding to a forward skipping of the droplet.
New experimental results with highly viscous liquids and on non-symmetric surfaces will
be shown.
Keywords:
droplets, porous surfaces, wetting, hydrophobic, actuation, ceramics
