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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Superhydrophilicity of alpha-alumina surfaces results from tight binding of interfacial waters to specific aluminols
Hypothesis
Understanding the microscopic driving force of water wetting is challenging and important for design of materials. The relations between structure, dynamics and hydrogen bonds of interfacial water can be investigated using molecular dynamics simulations.
Experiments and simulations
Contact angles at the alumina (0001) and (
) surfaces are studied using both classical molecular dynamics simulations and experiments. To test the superhydrophilicity, the free energy cost of removing waters near the interfaces are calculated using the density fluctuations method. The strength of hydrogen bonds is determined by their lifetime and geometry.
Findings
Both surfaces are superhydrophilic and the (0001) surface is more hydrophilic. Interactions between surfaces and interfacial waters promote a templating effect whereby the latter are aligned in a pattern that follows the underlying lattice of the surfaces. Translational and rotational dynamics of interfacial water molecules are slower than in bulk water. Hydrogen bonds between water and both surfaces are asymmetric, water-to-aluminol ones are stronger than aluminol-to-water ones. Molecular dynamics simulations eliminate the impacts of surface contamination when measuring contact angles and the results reveal the microscopic origin of the macroscopic superhydrophilicity of alumina surfaces: strong water-to-aluminol hydrogen bonds.
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- Award ID(s):
- 1828421
- NSF-PAR ID:
- 10477260
- Publisher / Repository:
- Elsevier B.V.
- Date Published:
- Journal Name:
- Journal of Colloid and Interface Science
- ISSN:
- 1095-7103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
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