Droplets Capped with an Elastic Film Can Be Round, Elliptical, or Nearly Square

We present experiments that show that the partial wetting of droplets capped by taut elastic films is highly tunable. Adjusting the tension allows the contact angle and droplet morphology to be controlled. By exploiting these elastic boundaries, droplets can be made elliptical, with an adjustable aspect ratio, and can even be transformed into a nearly square shape. This system can be used to create tunable liquid lenses and, moreover, presents a unique approach to liquid patterning.

Figure 1

(a) Schematic of a droplet capped by a taut elastic film and supported by a film of the same elastomer on a silicon substrate. The heuristic tension balance to determine the contact angle is shown. (b) Optical image under a red filter of the top view of a glycerol droplet capped by an Elastollan film with $\epsilon =0.15$ and $h=570\mathrm{nm}$. (c) Height profile of the capped droplet in (b) as a function of horizontal position. The solid curve is a circular cap fit from which we determine ${\theta }_{\mathrm{c}}=7.8\pm 0.2°$.

Figure 2

Contact angle as a function of $\epsilon h$, which is proportional to tension, for two elastomers: (a) Elastollan and (b) SIS, and with two test liquids: glycerol (glyc) and PEG. The Elastollan with glycerol data include sets where the strain is held constant, $⟨\epsilon ⟩=0.16\pm 0.03$ (solid circle) and $⟨\epsilon ⟩=0.08\pm 0.02$ (solid rectangle), and only film thickness is changed. Plotted are also data attained from biaxial strain experiments along the low- (solid left triangle) and high- (solid right triange) tension axes. Solid (glyc) and dashed (PEG) curves correspond to fits of Eq. (1), with ${\gamma }_{\mathrm{el},l}/E$ as the only free parameter. Vertical error bars represent the standard deviation in contact angles measured for a sample, and horizontal error bars stem from uncertainties in thickness and strain.

Figure 3

(a) Optical image under red filter of an elliptical droplet aligned with the high-tension direction in the capping film (${\epsilon }_{\mathrm{high}}=0.2$, ${\epsilon }_{\mathrm{low}}=-0.04$, $h=586\mathrm{nm}$). (b) Measured aspect ratio as a function of strain along the low-tension axis. ${\epsilon }_{\mathrm{high}}$ is held constant at $\sim 0.2$. (c) Measured aspect ratio plotted against the theoretically expected aspect ratio [Eq. (2)] given ${\epsilon }_{\mathrm{high}}$ and ${\epsilon }_{\mathrm{low}}$. The solid line represents the relationship $a_{\exp }=a_{\mathrm{th}}$. Horizontal error bars are due to uncertainties in the two strains. Vertical error bars are standard deviations in measured $a_{\exp }$.

Figure 4

(a) Top view schematic of the sample with droplets capped by biaxially stretched films on either side, all freestanding over the hole in the washer. (b)-(c) Optical images of droplets with square morphology. (d) Focal spot (with diffraction pattern) of a laser shone through a square droplet.