Controllable Microfluidic Production of Microbubbles in Water-in-Oil Emulsions and the Formation of Porous Microparticles
The paper described the formation of micrometer gas-in-water-in-oil emulsions and the effects of liquid flow rates and gas pressure on the formation of water-encapsulated microbubbles. The effects of two different microfluidic geometries (flow-focusing and T-junction channels) and the addition of photopolymerizable monomers were investigated.
The method described in the paper is as follows: monodisperse microbubbles were generated in a continuous water phase using a flow-focusing geometry. Then, the gas-in-water system was detached into gas-in-water droplets by using a flow-focusing or T-junction geometries. They found that the double flow-focusing geometry’s (DFF) advantage is its ability of producing thin water shells while the T-junction geometry (FFT) allowed them control over the number of gas bubbles per water droplets. The addition of photopolymerizable monomers (acrylamide) into the aqueous phase allowed them to obtain porous polyacrylamide particle with lower elastic moduli compared to solid polyacrylamide particles.
Number of gas bubbles per droplets
By controlling the flow rate ratio of water to oil at constant gas pressure, they have found the different conditions that reproducibly give discrete numbers of gas bubbles per water droplets. By adjusting the gas pressure, they have also observed two different regimes, one where the number of gas bubbles per water droplets is independent of gas pressure (Qw/Q0=0.3-0.6) and another where the number of gas bubbles per water droplets is dependent of gas pressure (Qw/Q0=0.25-0.3, 0.6-0.9). When the flow rate ratio of water to oil is lower than 0.15 or higher than 1, the gas-in-water-in-oil emulsions didn’t form steadily and gives either a string of gas bubbles in a continuous water phase or water droplets with no bubbles at all.
The results of the variation of Qw/Q0 on the number of gas bubbles per droplets are shown below:
Thickness of the water layer
They noticed that even though the sizes of the bubbles were dependent on the relative flow rate (Qw/Q0), the sizes of the droplets did not depend significantly on Qw/Q0. This allowed them to control the thickness of the water layer. They also observed that the gas pressure played a minor role in the DFF while it significantly affected the droplet size in the FFT.
The dependence of thickness of the water layer on the relative flow rate for both DFF and FFT is shown below:
Addition of photopolymerizable monomers into the aqueous phase
By adding in photopolymerizable monomers (acrylamide) into the aqueous phase, they were able to fabricate porous particles by making droplets containing gas bubbles and then polymerizing the acrylamide. The effective elastic modulus of the dry photopolymerized polyacrylamide particles with and without entrapped microbubbles was then investigated using atomic force microscope. The result obtained is shown in the graph below:
Scott's comments: Were there any scaling relationships found for the size of the drops relative to the flow rates of the fluids? If so, how did they scale?