Droplet plume emission during plasmonic bubble growth in ternary liquids

Plasmonic bubbles are of great relevance in numerous applications, including catalytic reactions, micro/nanomanipulation of molecules or particles dispersed in liquids, and cancer therapeutics. So far, studies have been focused on bubble nucleation in pure liquids. Here we investigate plasmonic bubble nucleation in ternary liquids consisting of ethanol, water, and trans-anethole oil, which can show the so-called ouzo effect. We find that oil (trans-anethole) droplet plumes are produced around the growing plasmonic bubbles. The nucleation of the microdroplets and their organization in droplet plumes is due to the symmetry breaking of the ethanol concentration field during the selective evaporation of ethanol from the surrounding ternary liquids into the growing plasmonic bubbles. Numerical simulations show the existence of a critical Marangoni number Ma (the ratio between solutal advection rate and the diffusion rate), above which the symmetry breaking of the ethanol concentration field occurs, leading to the emission of the droplet plumes. The numerical results agree with the experimental observation that more plumes are emitted with increasing ethanol-water relative weight ratios and hence Ma. Our findings on the droplet plume formation reveal the rich phenomena of plasmonic bubble nucleation in multicomponent liquids and help to pave the way to achieve enhanced mixing in multicomponent liquids in chemical, pharmaceutical, and cosmetic industries.

Figure 1

(a) Schematic of a gold nanoparticle sitting on a ${\mathrm{SiO}}_2$ island on a fused-silica substrate. (b) An energy-selective backscatter (left) and scanning electron microscope (right) image of the patterned gold nanoparticle sample surface. (c) Schematic diagram of the optical setup for plasmonic microbubble imaging.

Figure 2

Ternary diagram of ethanol-water-anethole mixtures. The black solid line is the measured phase separation line [30]. The filled triangles (points 1–7) along the phase separation curve represent the initial compositions of the ternary liquids that we have used in this study.

Figure 3

Bottom-view images of droplet plumes during plasmonic microbubble growth in ternary liquids. Parts (a), (b), (c), and (d) show the evolution of droplet plumes with ethanol-water relative weight ratio $r_{e/(e+w)}=65.0\%$, 75.0%, 80.0%, and 85.0%, respectively. With increasing $r_{e/(e+w)}$ from 65.0% to 80.0%, the initially emitted plume number increases from 1 to 3, as seen in (a) II, (b) II, and (c) II. For $r_{e/(e+w)}=75.0\%$, the two plumes finally merge into a single plume [(b) II–IV]. For $r_{e/(e+w)}=80.0\%$, the initial three plumes [(c) II] evolve into four plumes [(c) III] and finally merge [(c) IV]. Figure 3 shows the formation of an oil ring in the ternary liquid with $r_{e/(e+w)}=85.0\%$, corresponding to a high oil-weight fraction $y_a=24.52\%$.

Figure 4

(a) Bubble volume $V$ vs time and (b) maximum plume number $N_{\text{max}}$ vs ethanol-water relative weight ratio $r_{e/(e+w)}$ in the ternary liquids.

Figure 5

Plasmonic bubbles in ethanol and anethol oil mixtures with ethanol ratio $r_e=70\%$ (a) and $r_e=80\%$ (b). Plume formation is observed in both cases, and the maximum plume number increases with increasing ethanol ratio.

Figure 6

(a) Schematics of the numerical model. The domain is filled with the ethanol-water mixture, and a bubble of radius $R$ is placed near the bottom wall. (b) A zoom-in display for the area selected by the dashed box at the bubble surface in (a). Concentration differences along the bubble surface lead to a Marangoni force, which is balanced by the viscous force induced by the Marangoni flow.

Figure 7

Snapshots (top view) of the ethanol concentration field near a plasmonic bubble from the simulation in the ethanol-water mixtures during the vaporization process. The symmetry of the concentration field breaks for Marangoni numbers $\text{Ma}\ge 5\times {10}^4$. With further increasing Ma, the number of plumes increases from one to four. In (c)–(e), neighboring plumes can merge.

Figure 8

(a) Schematic illustration of the solutal Marangoni flow along the bubble interface caused by selective evaporation of ethanol. Ethanol vaporization occurs in a local area near the bubble bottom. $R$ and $H$ are the bubble radius and the height of the bubble where mass transfer into the bubble can occur, respectively. $u_0$ denotes the flow velocity along the substrate. The zoom-in image shows the thickness ${\delta }_T$ of the thermal boundary layer. The liquid in the region above the blue curve is at room temperature. The region of thickness ${\delta }_b$ (below the red curve) has liquid with a temperature above the boiling temperature $T_b$. It can be obtained by assuming a linear temperature decrease from the substrate to the blue curve. (b) Maximum plume number $N_{\text{max}}$ as a function of the Marangoni number Ma. Theory vs experiment.

Figure 9

The numerical setup for a bubble with a constant radius $R$ fixed at $h=1.2R$ above the bottom wall. There is a constant normal concentration gradient at the bubble surface due to the evaporation of ethanol in the surrounding ethanol-water mixture. Marangoni flow is induced by the ethanol concentration difference along the bubble surface. Note that the simulation is conducted in a three-dimensional domain, although the sketch is two-dimensional.

Figure 10

Schematic illustration of the immersed boundary method with staggered grids: Eulerian grids of velocity points and concentration points are denoted by different colors, and the Lagrangian points at the boundary are denoted by red circles. The governing equations are solved on the Eulerian grids, and the boundary conditions are satisfied at $L$ point based on the interpolated value at the probes $P_1$ and $P_2$ points.

Figure 11

Surface tension of ethanol-water binary mixtures as a function of $f_e$ at 20 ${}^{\circ }\mathrm{C}$, at 50 ${}^{\circ }\mathrm{C}$, and at the respective boiling temperature of the mixture (which depends on $f_e$).

Figure 12

Numerical calculated temperature field of the substrate immersed in vapor upon laser irradiation for 2 s.