An electric field applied across the interface has been shown to enable transitions from the Cassie to the Wenzel state on superhydrophobic surfaces with miniature corrugations. Molecular dynamics (MD) simulations manifest the possibility of reversible cycling between the two states when narrow surface wells support spontaneous expulsion of water in the absence of the field. With approximately 1 nm sized wells between the surface asperities, the response times to changes in the electric field are of O(0.1) ns, allowing up to GHz frequency of the cycle. Because of the orientation preferences of interfacial water in contact with the solid, the phenomenon depends on the polarity of the field normal to the interface. The threshold field strength for the Cassie-to-Wenzel transition is significantly lower for the field pointing from the aqueous phase to the surface; however, once in the Wenzel state, the opposite field direction secures tighter filling of the wells. Considerable hysteresis revealed by the delayed water retraction at decreasing field strength indicates the presence of moderate kinetic barriers to expulsion. Known to scale approximately with the square of the length scale of the corrugations, these barriers preclude the use of increased corrugation sizes while the reduction of the well diameter necessitates stronger electric fields. Field-controlled Cassie-to-Wenzel transitions are therefore optimized by using superhydrophobic surfaces with nanosized corrugations. Abrupt changes indicate a high degree of cooperativity reflecting the correlations between the wetting states of interconnected wells on the textured surface.
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Controlling states of water droplets on nanostructured surfaces by design
Surfaces that exhibit both superhydrophobic and superoleophobic properties have recently been demonstrated. Specifically, remarkable designs based on overhanging/inverse-trapezoidal microstructures enable water droplets to contact these surfaces only at the tips of the micro-pillars, in a state known as the Cassie state. However, the Cassie state may transition into the undesirable Wenzel state under certain conditions. Herein, we show from large-scale molecular dynamics simulations that the transition between the Cassie and Wenzel states can be controlled via precisely designed trapezoidal nanostructures on a surface. Both the base angle of the trapezoids and the intrinsic contact angle of the surface can be exploited to control the transition. For a given base angle, three regimes can be achieved: the Wenzel regime, in which water droplets can exist only in the Wenzel state when the intrinsic contact angle is less than a certain critical value; the Cassie regime, in which water droplets can exist only in the Cassie state when the intrinsic contact angle is greater than another critical value; and the bistable Wenzel–Cassie regime, in which both the Wenzel and Cassie states can exist when the intrinsic contact angle is between the two critical values. A strong base-angle dependence of the first critical value is revealed, whereas the second critical value shows much less dependence on the base angle. The stability of the Cassie state for various base angles (and intrinsic contact angles) is quantitatively evaluated by computing the free-energy barrier for the Cassie-to-Wenzel state transition.
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- Award ID(s):
- 1665324
- NSF-PAR ID:
- 10076246
- Date Published:
- Journal Name:
- Nanoscale
- Volume:
- 9
- Issue:
- 46
- ISSN:
- 2040-3364
- Page Range / eLocation ID:
- 18240 to 18245
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c7nr06896d
