[highlights] The Soft matter, Fluidics and Interfaces group (SFI) is addressing interfacial phenomena that are relevant for microfluidic processes. Such phenomena include multiphase flow, phase contacting, interface geometry, wetting, and separations, mostly related to mass/heat transport control. Careful interfacial design and fabrication will allow manipulating (multiphase) flow on a (sub)micrometer level. Fabrication of well-defined structures is foreseen as a crucial aspect, in order to study
Prof. dr. ir. Rob G.H. Lammertink
the fundamentals of interfacial phenomena. The connection between microfluidics and interfaces
“Understanding and
understanding at the microscopic length scale is required to advance microfluidics further in the
optimizing processes at
mentioned areas, as well as possible new application fields. Micro flow structures can be further
the interface.�
explored for operation units and conditions that are unprecedented on a macro scale.
is evident as interfacial phenomena start to dominate at small length scales. A fundamental
Soft Matter, Fluidics and Interfaces Evaporation triggered wetting transition Superhydrophobic surfaces repel water by means of their chemical composition and structure. Such structured surfaces can typically accommodate a water droplet in two distinct configurations. One where the droplet is in full contact with the surface (Wenzel state), Figure 1: Snapshots of evaporating droplets with 15 s time
and one where the droplet sits on top of the protrusions of the surface (Cassie Baxter state). The Cassie Baxter state provides a hybrid
intervals. Initially the droplets evaporate with constant contact
interface as the liquid is partly in contact with the solid, and partly with the gas present between the protrusions.
angle. After the transition, the contact line pins and evaporation
Fundamental studies related to the wetting states on superhydrophobic surfaces are relevant for many scenarios. In fluid mechanics
results in a strong contact angle decrease.
for instance, the flow along superhydrophobic walls is influenced by the wetting state via the occurrence of slip velocity near the wall. A careful design of the wetting state is required in many chemical processes, including heterogeneous catalyzed processes and wet chemical etching processes. In this paper, we address the observation of the wetting state transition induced during evaporation of the droplet. Evaporating droplets are frequently encountered in production of (bio)arrays, inkjet printing, and coating processes. We have addressed the wetting transition during evaporation by means of a global energy argument. This argument estimates the interfacial energy of the droplet as a function of drop size. It can thereby predict for each drop size which wetting state is the most favorable. The predicted drop size at which the transition should occur is thereby easily obtained. The predicted drop size for the transition was found to match very well with the experimentally observed one.
Figure 2: Global interfacial energy of the droplet as a function of droplet base radius. The inset displays the energy difference between the two wetting states from which the predicted transition point can be derived (ΔE=0).
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HIGHLIGHTED PUBLICATION: Tsai et al. Evaporation-triggered wetting transition for water droplets upon hydrophobic microstructures, Phys. Rev. Lett. (2010) 104 (11).