posted on 2017-02-02, 00:00authored byAthanasios Milionis, Carlo Antonini, Stefan Jung, Anders Nelson, Thomas M. Schutzius, Dimos Poulikakos
Controlled handling
of liquids and colloidal suspensions as they interact with surfaces,
targeting a broad palette of related functionalities, is of great
importance in science, technology, and nature. When small liquid volumes
(drops on the order of microliters or nanoliters) need to be processed
in microfluidic devices, contamination on the solid/liquid interface
and loss of liquid due to adhesion on the transport channels are two
very common problems that can significantly alter the process outcome,
for example, the chemical reaction efficiency or the purity and the
final concentration of a suspension. It is, therefore, no surprise
that both levitation and minimal contact transport methodsincluding
nonwetting surfaceshave been developed to minimize the interactions
between liquids and surfaces. Here, we demonstrate contactless surface
levitation and transport of liquid drops, realized by harnessing and
sustaining the natural sublimation of a solid-carbon-dioxide-coated
substrate to generate a continuous supporting vapor layer. The capability
and limitations of this technique in handling liquids of extreme surface
tension and kinematic viscosity values are investigated both experimentally
and theoretically. The sublimating coating is capable of repelling
many viscous and low-surface-tension liquids, colloidal suspensions,
and non-Newtonian fluids as well, displaying outstanding omniphobic
properties. Finally, we demonstrate how sublimation can be used for
liquid transport, mixing, and drop coalescence, with a sublimating
layer coated on an underlying substrate with prefabricated channels,
conferring omniphobicity using a simple physical approach (i.e., phase
change) rather than a chemical one. The independence of the surface
levitation principle from material properties, such as electromagnetic,
thermal or optical, surface energy, adhesion, or mechanical properties,
renders this method attractive for a wide range of potential applications.