Bubble dispenser in microfluidic devices

This Brief Report presents experimental and computational results on bubble formation in microfluidic devices. Bubbles are generated at the right-angle intersection of four identical square microchannels. When the pressure gradient generated by the liquid flow dominates the pressure gradient generated by gas flow, the length of the produced confined bubbles follows a law based on the channel size and fluid volume fraction. This bubble production technique was used to produce monodisperse aqueous foam in two-dimensional and three-dimensional microchannels.

Figure 1

Bubble formation for different water and air superficial velocities. (1) $J_L=28.6\mathrm{cm}∕\mathrm{s}$, $J_G=11.3\mathrm{cm}∕\mathrm{s}$, ${\alpha }_L=0.72$; (2) $J_L=26\mathrm{cm}∕\mathrm{s}$, $J_G=19.3\mathrm{cm}∕\mathrm{s}$, ${\alpha }_L=0.57$; (3) $J_L=22\mathrm{cm}∕\mathrm{s}$, $J_G=72.4\mathrm{cm}∕\mathrm{s}$, ${\alpha }_L=0.23$; (4) $J_L=18.1\mathrm{cm}∕\mathrm{s}$, $J_G=109.9\mathrm{cm}∕\mathrm{s}$, ${\alpha }_L=0.14$. Liquid inlet pressure was kept constant while gas inlet pressure was increased.

Figure 2

Experimental and computational results on bubble length $d$ as a function of homogeneous liquid fraction ${\alpha }_L$.

Figure 3

Results from numerical simulations using the level set method showing the formation of the gas bubble due to pinching by the liquid cross flow ($J_L=88\mathrm{cm}∕\mathrm{s}$, $J_G=44\mathrm{cm}∕\mathrm{s}$, ${\alpha }_L\approx 0.67$, $d∕h\approx 1.5$).

Figure 4

Evolution of the distance $L$ between bubbles as a function of bubble length $d$. Region of bubble formation: (a) pseudoconstant volume and (b) pseudoconstant frequency.

Figure 5

Aqueous foam flow generated in different geometries with an on-chip liquid-gas mixer. (a) ${\alpha }_L\sim 0.91$; (b) ${\alpha }_L\sim 0.57$; (c) ${\alpha }_L\sim 0.09$; (d) ${\alpha }_L\sim 0.47$; (e) ${\alpha }_L\sim 0.30$; (f) ${\alpha }_L\sim 0.20$; (g) ${\alpha }_L=0.11$; (h) ${\alpha }_L\sim 0.01$; (i) ${\alpha }_L\sim 0.06$. Polydispersity observed in (c) is the result of bubble break during the flow such as indicated with Eq. (1). Three-dimensional flow exhibits ordered and disordered structures.