Breath figures of two immiscible substances on a repellent surface

The understanding of the competition between different substances while condensing on a cold surface is of high interest in situations in which it is desirable to control their condensation rates and the formed morphologies. We do the experiments for mixtures of water and hexamethyldisiloxane vapors at several concentrations. The dropwise condensation of the vapors forms breath figures on a substrate that is repellant to both substances. We report the average radius of the drops for each specie as a function of time. Also, we pay attention to the evolution of the corresponding morphologies and the appearance of hybrid clusters.

Figure 1

Classical water BF on our substrate. (a) Average radius (squares) and power laws $t^{1/3}$ (dashed line) and $t$ (dashed-dotted lines) for comparison. (b) Number of drops (circles) with a fit to a power law (solid line), and (c) three snapshots of the breath figure. Scale bar is 100 $\mu$m.

Figure 2

HMDSO BF on our substrate. (a) Average radius (squares) and a linear fit (dashed line) to the rightmost data and a translation of the fit to the left for comparison. (b) Number of drops (circles) with a power-law fit to the rightmost data (solid line) and a scale of the fit to the left for comparison, and (c) three snapshots of the breath figure. Scale bar is 100 $\mu$m.

Figure 3

First stage after HMDSO saturated vapor is flowed over a water breath figure (darker droplets). The lighter smaller droplets correspond to HMDSO. Concentric regions to the drops, where condensation is inhibited, can be observed.

Figure 4

Occupation factor for H-Wc ${\varepsilon }_{\text{H-Wc}}$ as the initial occupation factor for the water BF changes. The dotted line is a guide for the eye and it classifies the morphologies obtained. I stands for HMDSO ponds with water droplets immersed in them, and II stands for H-Wc regions. The solid line is a linear fit to the data excluding region I.

Figure 5

BF evolution snapshots for HMDSO and water at different fluxes of water-HMDSO (mL/min) for increasing times. (a) 250–230, (b) 250–240, (c) 250–250, (d) 240–250, and (e) 230–250. Scale bar is 152 $\mu$m.

Figure 6

The average radius $\langle r\rangle$ for the water droplets in the BF presents two stages: the small values represent the intermediate stage close to a power law with exponent $1/3$ (solid line). The rest of the values represent the mean size of the H-Wc (see text). The dashed line is a guide for the eye representing a power law with exponent 1. The coding (a–e) for the fluxes follows that of Fig. 5.

Figure 7

The average radius $\langle r\rangle$ for the HMDSO droplets in the BF and a power-law fit to the leftmost data (dashed line) and scales of the fit to the right for comparison. The coding (a–e) for the fluxes follows that of Fig. 5.