Evaporation-Triggered Segregation of Sessile Binary Droplets

Droplet evaporation of multicomponent droplets is essential for various physiochemical applications, e.g., in inkjet printing, spray cooling, and microfabrication. In this work, we observe and study the phase segregation of an evaporating sessile binary droplet, consisting of a miscible mixture of water and a surfactantlike liquid (1,2-hexanediol). The phase segregation (i.e., demixing) leads to a reduced water evaporation rate of the droplet, and eventually the evaporation process ceases due to shielding of the water by the nonvolatile 1,2-hexanediol. Visualizations of the flow field by particle image velocimetry and numerical simulations reveal that the timescale of water evaporation at the droplet rim is faster than that of the Marangoni flow, which originates from the surface tension difference between water and 1,2-hexanediol, eventually leading to segregation.

Figure 1

Confocal images of the segregation process during droplet evaporation in a semi-side view (a)–(d) and bottom view (a′)–(d′) taken at the same times. (a)–(d) The confocal microscope scans the rectangular box with the volume $590\mu \mathrm{m}\times 590\mu \mathrm{m}\times 90\mu \mathrm{m}$. Water (blue) and 1,2-hexanediol (yellow) are labeled with different dyes for the observation. (a),(a′) In the beginning, the droplet is homogeneously mixed. (b),(b′) At about 34 s after recording started, 1,2-hexanediol nucleates at the contact line of the droplet, which is revealed as yellow round shapes. (c),(c′) The nucleated microdroplets of 1,2-hexanediol gradually grow and coalesce. (d),(d′) The evaporation ends when 1,2-hexanediol fully covers the surface of the droplet.

Figure 2

(a)–(d) Bottom-view snapshots of the droplet seeded with fluorescent particles in different life phases. (a) The flow is directed radially outwards near the substrate, as shown by particles transported to the contact line (blue arrows). (b) All the particles are released from the contact line and flow to the upper center (orange circle) of the droplet (red arrows). (c) The particle-filled upper (water-rich) region is squeezed by growing nucleated microdroplets. A star-shaped pattern (purple dashed line) emerges. At the fingers of the star (green arrows), the particles flow down to the bottom of the droplet. (d) When the droplet stops evaporating, the particles are deposited homogeneously on the substrate, without leaving a coffee stain. (e),(f) Schematics of the flow inside the binary droplet at different phases. (e) Before segregation, the surface tension gradient drives a Marangoni flow from the edge to the apex of the droplet. (f) After segregation, the nucleated microdroplets of 1,2-hexanediol grow and coalesce. At the same time, water-rich liquid from the upper layer of the droplet flows down through the streams between neighboring nucleated microdroplets.

Figure 3

Particle image velocimetry results showing the flow field near the substrate in terms of velocity vectors and vorticity, allowing us to identify evaporation stages. The five vertical lines show the moments of the five snapshots. (Blue line, 10 s; red line, 80 s; yellow line, 112 s; purple line, 122 s; green line, 167 s.).

Figure 4

Experimental (data points) and numerical results (solid line) for the temporal evolution of the geometrical parameters (a) footprint diameter $L$, contact angle $\theta$, and (b) volume $V$ from the experiment and numerical simulation. The error bars are deduced from the experimental accuracy.

Figure 5

These snapshots from the simulation of an evaporating droplet with initially 10% of 1,2-hexanediol with the axisymmetric finite element model at different times $t$. In the gas phase, the water vapor concentration $c_w$ is shown, and the corresponding evaporation rate $J_w$ is indicated by the arrows at the interface. Inside the droplet, the mass fraction of 1,2-hexanediol (left) and the velocity (right) is depicted. Note the very different phenomena at $t=10\mathrm{s}$ (upper) and at the two later times $t=145\mathrm{s}$ and $t=180\mathrm{s}$ (lower).