Scaling of anisotropic droplet shapes on chemically stripe-patterned surfaces

Abstract

We present an experimental study of the tunable anisotropic wetting behavior of chemically patterned anisotropic surfaces. Asymmetric glycerol droplet shapes, arising from patterns of alternating hydrophilic (pristine SiO2) and hydrophobic (fluoroalkylsilane self-assembled monolayers) stripes with dimensions in the low-micrometer range, are investigated in relation to stripe widths. Owing to the well-defined small droplet volume, the equilibrium shape as well as the observed contact angles exhibit unique scaling behavior. Only the relative width of hydrophilic and hydrophobic stripes proves to be a relevant parameter. Our results on morphologically flat, chemically patterned surfaces show similarities with those of experiments on topographically corrugated substrates. They are discussed in terms of the energetics at the liquid-solid interface.

Directional wetting on patterned surfaces

“…6.2 (b)) shows the droplet in the Wenzel state. From the top-view, the elongation in the direction of the grooves can be observed, similarly to what was previously described for chemically patterned surfaces [44,85]. The side view photos show the difference in CAs (θ and θ ⊥ ) and diameter lengths of the wetted area in orthogonal directions.…”

“…4.4 (c) the droplet has detached from the needle and has moved completely off the unpatterned PFDTS rectangle onto patterns I and II. The droplet adopts an approximately cylindrical shape with two spherical caps, similar to those observed for static shapes on anisotropic patterned substrates [42,85]. The macroscopic surface energy of the striped patterns increases for decreasing α, therewith inducing motion of the liquid in the positive x direction.…”

“…The droplet translation is induced by the higher surface energies of the subsequent patterns, owing to an increasing fraction of hydrophilic area. To quantify the relative hydrophobicity of the patterns, we introduced a dimensionless parameter α [44,85]:…”

Low-power silicon-based thermal sensors and actuators for chemical applications

“…When the width of the hydrophilic part was increased compared to chemically modified hydrophobic areas, such as from 8 × 2 µm to 16 × 2 µm, elongation of the droplets was observed. Similar behavior was observed with water and glycerol with mechanism for this explained in [182], [183]. The difference in the affinity of the boehmite sol to hydrophobic and hydrophilic areas causes a deformation of the droplet and preferential orientation along the lines.…”

Evaporative gold nanorod assembly on chemically stripe-patterned gradient surfaces

“…We use a dimensionless parameter a ¼ w=s to quantify the relative hydrophobicity of the pattern [41], where w and s represent the hydrophobic PFDTS and hydrophilic SiO 2 stripe widths, respectively. Areas with smaller values for a correspond to larger overall surface energy, and as such are more hydrophilic.…”