Self-Propelled Detachment upon Coalescence of Surface Bubbles

The removal of microbubbles from substrates is crucial for the efficiency of many catalytic and electrochemical gas evolution reactions in liquids. The current work investigates the coalescence and detachment of bubbles generated from catalytic decomposition of hydrogen peroxide. Self-propelled detachment, induced by the coalescence of two bubbles, is observed at sizes much smaller than those determined by buoyancy. Upon coalescence, the released surface energy is partly dissipated by the bubble oscillations, working against viscous drag. The remaining energy is converted to the kinetic energy of the out-of-plane jumping motion of the merged bubble. The critical ratio of the parent bubble sizes for the jumping to occur is theoretically derived from an energy balance argument and found to be in agreement with the experimental results. The present results provide both physical insight for the bubble interactions and practical strategies for applications in chemical engineering and renewable energy technologies like electrolysis.

Figure 1

(a) Schematics of the experimental setup. A PDMS container sticking to a glass coverslip is filled with 30% (w/w) ${\mathrm{H}}_2{\mathrm{O}}_2$ solution with a volume of $10\times 10\times 7.5{\mathrm{mm}}^3$. The coverslip with a thickness of $170\mu \mathrm{m}$ is coated with a 18-nm thick gold-titanium binary layer (15 nm for gold layer and 3 nm for titanium layer) patterned with microholes. The decomposition of ${\mathrm{H}}_2{\mathrm{O}}_2$ is taking place at the gold-solution interface, followed by the formation of bubbles which pin at the edge of the microholes (see inset). The bubble dynamics is measured by a high-speed camera and confocal microscopy. (b) Scanning electron microscopic image showing the hole-patterned binary layer. Each hole has a radius of $R_p=1\mu \mathrm{m}$ and the center distance is $5\mu \mathrm{m}$. The scale bar is $10\mu \mathrm{m}$. (c) Three-dimensional schematics of the hole-patterned catalyst substrate.

Figure 2

Snapshots of the coalescence event between two parent bubbles resulting in a (a) sticking, (b) moving, or (c) jumping merged bubble. Coalescence occurs at 0.13 s. The sticking merged bubble in (a) sticks to the location of the large parent bubble; the moving and jumping bubbles in (b),(c) relocate to the center of mass of the parent bubbles. Note that the merged bubble in (c) is out of focus after coalescence, indicating jumping followed by rising due to buoyancy. The scale bars in (a),(b), and (c) are all $20\mu \mathrm{m}$. See Supplemental Material, video [45]. (d) Schematic of the coalescence geometry and notation. The parent bubbles are shaded in dark blue; the resulting merged bubble in light blue.

Figure 3

Coalescence preference showing the relative position of the merged bubble as a function of the parent size ratio. Experimental data (symbols) were collected immediately after coalescence. The sticking bubbles pin at the location of the larger parent bubble, resulting in the data with $d_l/d_s\approx 0$. The solid line shows the theoretical prediction Eq. (2) following from momentum conservation. The inset shows a double logarithmic plot of the released surface energy for the jumping bubbles as a function of the parent size ratio $x=R_l/R_s$, see Eq. (8). The released surface energy increases as the size ratio decreases, which favors the jumping motion of the merged bubble.

Figure 4

Phase diagram of the motion regimes for the merged bubbles. The theoretical threshold curve [solid line, Eq. (7)] divides the phase diagram into two regimes, namely, nonjumping (to the left), including sticking and moving, and jumping (to the right) motions upon coalescence. Experimental data (symbols) were collected immediately before coalescence. $f$ is the mass fraction of glycerol in the 30% (w/w) ${\mathrm{H}}_2{\mathrm{O}}_2$ solution.