Liquid nanodroplets spreading on chemically patterned surfaces

G. S. Grest, D. R. Heine, and E. B. Webb III, Langmuir, 22, 4745-4749 (2006).

Controlling the spatial distribution of liquid droplets on surfaces via surface energy patterning can be used to deliver material to specified regions via selective liquid/solid wetting. Although studies of the equilibrium shape of liquid droplets on heterogeneous substrates exist, much less is known about the corresponding wetting kinetics. Here we present large-scale atomistic simulations of liquid nanodroplets spreading on chemically patterned surfaces. Results are presented for lines of polymer liquid (droplets) on substrates consisting of alternating strips of wetting (equilibrium contact angle theta0 = 0degree) and nonwetting (theta0 asymptotically equal to 90degree) material. Droplet spreading is compared for different wavelength lambd of the pattern and strength of surface interaction on the wetting strips. For small lambda, droplets partially spread on both the wetting and nonwetting regions of the substrate to attain a finite contact angle less than 90degree. In this case, the extent of spreading depends on the interaction strength in the wetting regions. A transition is observed such that, for large A, the droplet spreads only on the wetting region of the substrate by pulling material from nonwetting regions. In most cases, a precursor film spreads on the wetting portion of the substrate at a rate strongly dependent on the width of the wetting region.

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