A sequence shows the progression of bidisperse foam generation in a microfluidic device created at Rice University. When bubbles enter, they pinch the preceding bubble into two before becoming a wall against which the next bubble will be pinched. Credit: Biswal Lab/Rice UniversityScientists from Rice University in the US have shows the underlying mechanics involved in producing foam that contains hundreds of thousands of bubbles of distinct sizes. The team developed a microfluidic device capable of pumping out over 15,000 microscopic bubbles every second, and which can be tuned to produce them in up to three specific sizes, in research that could lead to chemical and biological applications.
In the study, which was reported in the journal Soft Matter [Vecchiolla et al. Soft Matter (2018) DOI: 10.1039/C8SM01285G], chemical and biomolecular engineer Sibani Lisa Biswal and lead author and graduate student Daniel Vecchiolla demonstrated an approach for enabling customizable and "wet" foams in tiny amounts, providing a new microfluidic foam generation system using the production of segregated, mono- or bidisperse bubbles.
The mechanism produces elongated bubbles that emerge through a tube into an input channel. Vidya Giri helped develop the microfluidic channels, which are about one-twentieth of an inch wide with a feeder channel of about 70 microns. In the system, every bubble is arrow-like and has sufficient force to divide the bubble ahead of it with the arrow staying intact. The bubble therefore becomes installed between the new so-called “daughter” bubbles, acting as a kind of “wall” that then holds the next bubble in place for splitting. This means that only every other bubble entering the expansion divides from the inter-bubble forces.
"We're able to generate well-ordered foam systems and control the size distribution”Sibani Lisa Biswal
When the input is centered and the other parameters, such as what kind of liquid is used and its viscosity, as well as the flow rate and the diameter of the channel, are at optimum levels, the device becomes full of large bubbles in the middle with two ranks of identical but smaller bubbles along the edges. When the input is offset, bubbles produced by the stream come in three sizes. As Sibani Lisa Biswal points out, “There's interest in using monodisperse bubbles for material applications and miniaturized reactors, so there's been a lot of studies about the generation of uniformly sized gas bubbles”. She added, “But there have been very few that looked at using neighboring bubbles to create these daughter bubbles. We're able to generate well-ordered foam systems and control the size distribution”.
The whole technique is based on geometrically mediated splitting and bubble–bubble break-up, which has been called “metronomic”, with the tick being a bubble splitting and the tock being a bubble that remains whole. The team hope that the simplicity of the system will assist further research on both dynamic bubble interactions and ordered, wet foam applications.