Abstract
The wetting kinetics of water droplets on chemically heterogeneous surfaces is important in several industrial technologies, such as biomedicine and microfluidics. Surfaces with different wettabilities can be designed to control the spread of droplets. In this study, nanosized water droplet spreading on chemically heterogeneous surfaces was investigated using molecular dynamics simulations. Chemically heterogeneous surfaces with different wetting patterns were investigated, and the equivalent spreading radius and dynamic contact angle during the spreading process were analyzed. Results showed that droplet spreading is mainly dependent on the area fractions of hydrophobic and hydrophilic regions and the shape of the wetting pattern has a minor influence on the spreading process. The dynamic contact angle can be well predicted by molecular kinetics theory. The static contact angle data remarkably deviate from Cassie’s equation, while they agree better with the modified Cassie’s equation as a function of the hydrophobic length fraction, indicating that the wetting pattern has a substantial influence in the vicinity of the contact line.
Highlights
When a droplet is deposited on a solid surface, it is influenced by surface tension and viscous force, and spontaneously spreads toward equilibrium
The geometric parameters for 15 surfaces with different patterns are listed in Table I, where the Type column shows the pattern types as listed in Fig. 1. w, l, and d are the length parameters defined in Fig. 1, and a0 is the lattice constant. f is the area fraction of the hydrophobic region on the surface and is obtained by dividing the hydrophobic atom number by the total atom number on the surface
The result shows that the dimensionless equivalent spreading radius and dynamic contact angle change when the number of molecules increases from 5000 to 9000, whereas they are almost identical for droplets with 9000 and 15 000 molecules
Summary
When a droplet is deposited on a solid surface, it is influenced by surface tension and viscous force, and spontaneously spreads toward equilibrium. As a droplet reaches the equilibrium state on chemically heterogeneous surfaces, Cassie’s equation is widely used to describe the static contact angle of the droplet. The validity of the power law for droplet spreading on chemically heterogeneous surfaces needs to be examined, and the influence of surface properties, especially near the triple contact line, on the spreading of droplet needs to be understood. The spreading of water nanodroplets on chemically heterogeneous surfaces with wetting patterns of alternating hydrophilic and hydrophobic regions was investigated through MD simulations. The spreading radius and dynamic contact angle were compared to previous experimental and theoretical studies and were analyzed to understand the influence of surface properties on nanodroplet spreading, and the influence of the length scale of chemical heterogeneity, wetting pattern, and fraction of hydrophobic surface is discussed
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