Controlled Cavitation in Microfluidic Systems

We report on cavitation in confined microscopic environments which are commonly called microfluidic or lab-on-a-chip systems. The cavitation bubble is created by focusing a pulsed laser into these structures filled with a light-absorbing liquid. At the center of a $20\mu \mathrm{m}$ thick and 1 mm wide channel, pancake-shaped bubbles expand and collapse radially. The bubble dynamics compares with a two-dimensional Rayleigh model and a planar flow field during the bubble collapse is measured. When the bubble is created close to a wall a liquid jet is focused towards the wall, resembling the jetting phenomenon in axisymmetry. The jet flow creates two counter-rotating vortices which stir the liquid at high velocities. For more complex geometries, e.g., triangle- and square-shaped structures, the number of liquid jets recorded correlates with the number of boundaries close to the bubble.

Figure 1

Sketch of the experimental setup to generate cavitation bubbles in microfluidic devices.

Figure 2

(a) Comparison of the stroboscopically measured bubble radii (○) and the model predictions for the 2D Rayleigh collapse (solid line) and 3D type (dashed line). The time $t=0$ is shifted to maximum bubble expansion $R_{\max }=43\mu \mathrm{m}$. (b) The velocity is plotted in a log-log graph as a function of the distance $r$ from the bubble center. The solid line shows the predicted $1/r$ scaling for a two-dimensional flow; the two lower dashed lines depict a $r^{-1.5}$ and a $r^{-2}$ dependency, respectively.

Figure 3

Comparison of the bubble dynamics for different sized bubbles with a two-dimensional model (solid line) and the 3D-Rayleigh collapse (dashed line). Markers are from measurements with the high-speed camera.

Figure 4

Planar bubble dynamics close to a channel wall. (a) Selected frames from a high-speed recording depicting the formation of a liquid jet towards the wall. The bar indicates a length of $150\mu \mathrm{m}$. (b) Velocity field of the flow after the bubble has split up due to the impacting jet flow. The jet flow and the radial inflow impact and lead to a splashing phenomenon (see text). The longest velocity vector corresponds to $7.4\mathrm{m}/\mathrm{s}$. (c) Vorticity plot after the bubble has collapsed averaged over the time interval $t\in [24\mu \mathrm{s};64\mu \mathrm{s}]$. Note that the bubble has already vanished during this time interval. Bright colors denote counterclockwise rotation and dark ones clockwise.

Figure 5

Snapshots for three different geometries from left to right: single bubble cavitation in a $150\mu \mathrm{m}$ wide and several millimeter long channel, in a $200\mu \mathrm{m}$ isosceles triangular structure with feeding channel attached to the corners, and in a square chamber with $200\mu \mathrm{m}$ long sides. From the time stamps it becomes evident that the jetting flow is much faster in the latter two cases.