(37b) A Simple-to-Apply Predictive Wetting Model for Textured (Rough/Patterned) Surfaces and the Role of Re-Entrant Cavities

Authors: 
Chen, S. Y., University of California Santa Barbara
Kaufman, Y., Ben Gurion University
Mishra, H., King Abdullah University of Science and Technology
Schrader, A., University of California Santa Barbara
Lee, D. W., University of California at Santa Barbara
Das, S., University of California, Santa Barbara
Donaldson, Jr., S. H., UC Santa Barbara
Israelachvili, J. N., University of California, Santa Barbara
Rough/patterned/textured surfaces with nano/micro- cavities that broaden below the surface â?? known as re-entrant cavities â?? can be omniphobic (macroscopic contact angle greater than 90° for both water and oils). The underlying physical principles/models that explain texture-driven omniphobicity have been studied extensively. However, existing models do not provide a simple procedure for predicting the thermodynamically stable and, in particular, the kinetically-trapped metastable states and contact angles (for example, wetting states that involve partially-filled cavities). Here, we develop a model that allows for deriving general conclusions and demonstrate the applicability of the model to analyze/determine/predict the macroscopic contact angle(s) on any textured surface.

In general, when liquid is placed on any textured surface, the liquid can either (1) partially- or (2) fully-fill the cavities. Both states (partially- or fully-filled) can occur inside and/or outside (by condensation) of the droplet (bulk liquid). Importantly, the macroscopic contact angles manifested for each state can be significantly different, and either of these states can be a transient (unstable) state, a kinetically-trapped (short or long-lived) metastable state, or the thermodynamic equilibrium state. Utilizing an energy minimization approach, we derive a â??general wetting modelâ?? that (1) predicts apriori the state (partially- or fully-filled) of the cavities both under (in contact with) and outside of the liquid droplet, and the corresponding macroscopic contact angles on any type of textured surface; (2) allows for determination of the conditions under which metastable states exist; and (3) allows for engineering of specific nano/micro- textures that yield any desired macroscopic contact angle, θt, for a given intrinsic contact angle θ0 (contact angle on smooth and flat surface).

During this talk, our â??general wetting modelâ?? will be discussed. In addition, we will demonstrate how this model can be used to design specific nano- and micro- textures to yield any desired macroscopic contact angle. Controlling the macroscopic contact angle, whether above or below the intrinsic contact angle, is desirable for many applications including both non-wetting, self-cleaning and anti-fouling surfaces, and completely-wetting/spreading applications, such as cosmetics and lubricant fluids.