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Download fileSimple-to-Apply Wetting Model to Predict Thermodynamically Stable and Metastable Contact Angles on Textured/Rough/Patterned Surfaces
journal contribution
posted on 21.02.2017, 00:00 authored by Yair Kaufman, Szu-Ying Chen, Himanshu Mishra, Alex M. Schrader, Dong Woog Lee, Saurabh Das, Stephen H. Donaldson, Jacob N. IsraelachviliRough/patterned/textured
surfaces with nano/microcavities that
broaden below the surfaceknown as “re-entrants”can
be omniphobic (macroscopic contact angle greater than 90° for
both water and oils). The existing theoretical models that explain
the effects of texture on wetting are complex and do not provide a simple procedure for predicting the thermodynamically
stable and metastable states and their corresponding contact angles
(for example, wetting states that involve partially filled cavities).
Here, we develop a simple-to-apply wetting model that allows for (1)
predicting a priori the wetting state (partially
or fully filled) of the cavities both under and outside the liquid
droplet and the corresponding macroscopic contact angles on any type
of textured surface; (2) determining the conditions under which metastable
states exist; and (3) engineering specific nano/microtextures that
yield any desired macroscopic contact angle, θt,
for a given intrinsic contact angle θ0. Subsequently,
we experimentally demonstrate how one can use the
model to predict the metastable and the thermodynamically stable contact
angles on nondeformable textured surfaces consisting of arrays of
axisymmetric cavities/protrusions. In this model, we do not consider
the effects of gravitational forces, Laplace pressure of the droplet,
line tension, droplet impact velocity, and quantitative aspects of
contact angle hysteresis. Nonetheless, the model is suitable for accurately
predicting the contact angles of macroscopic droplets (droplet volume ∼1
μL and base diameters <2 mm), which is of immense relevance
in engineering. In the experimental section we also discuss the suitability
of the model to be extended in order to include the effects of contact
angle hysteresis on the macroscopic apparent contact angle on textured
surfaces. Controlling these macroscopic contact angles, whether higher
or lower than the intrinsic angle, θ0, is desirable
for many applications including nonwetting, self-cleaning, and antifouling
surfaces and for completely wetting/spreading applications, such as
creams, cosmetics, and lubricant fluids.