Our study provides a promising microfluidic tool to study bubble formation and coalescence dynamics simultaneously.Įmulsifiers play a key role in the stabilization of foam bubbles. A minimum time of 0.4 ms is needed to immediately stabilize individual bubbles. Interaction between these bubbles causes finite coalescence to a diameter dcoal that increases for lower τ. When Pd∗ exceeds 1400 mbar, bubbles (fixed d0 16 μm) experience no adsorption lag and thus are formed at steeply increasing frequency, with τ < 1 ms. Individual bubbles (fixed d0 25 μm) are formed slowly with τ≫1 ms. When Pd∗<1400 mbar, protein adsorption is a requisite to lower the surface tension and initialize bubble formation. We observe two distinct bubble formation regimes, delimited by the pressure difference required to initiate bubble formation in pure water, Pd∗= 1400 mbar. Bubble behavior is studied as a function of the pressure difference imposed across the pores (Pd∗), and thus the bubble formation time (τ, ranging from μs to s), using highspeed recordings, quasi-static pressure arguments and a semi-empirical coalescence model. We use a microfluidic device to study Laplace pressure-driven formation of bubbles that are stabilized by whey proteins. We hypothesize that microfluidic tools provide direct insights to these effects, and that the final bubble size depends on the intersection of time scales for bubble formation versus adsorption of proteins. The bubble sizes (and the corresponding formation frequency) are indicated with two different symbols: í µí° The large error bars reflect the width of the size distribution, yet they are expected to be much smaller when studying individual pores. In 1,2, the arrows roughly indicate the position of active pores. It is noted that in 1,2, only pores located at a single tangent line of (part of) the membrane are in our focal plane, while in 3,4 bubbles formed from the whole membrane surface (circumference and length) are present. (A) Snapshots of bubble behavior obtained when using different transmembrane pressures (low-1,3 and high-2,4): 1,2-visualization of bubble formation at the membrane surface in the module 3,4-visualization of bubbles that are flowing through the flow cell (10 cm downstream). Our study provides insights into the membrane foaming process at relevant (micro-) length and time scales and paves the way for its further development and application.īubble formation as function of transmembrane pressure, í µí± í µí±¡í µí±í µí± (at constant í µí± í µí± = 1.5 × 10 −5 m 3 /s and í µí± = 1 wt %). The stabilizing effect is ascribed to the convective transport of proteins and the intersection of timescales (i.e., μs to ms) of bubble formation and protein adsorption. Particularly, in a 2.5 wt % whey protein system, coalescence could be suppressed with a coefficient of variation below 20%. Coalescence resulted in an increase in bubble size (Dcoal>D0), which can be strongly reduced by increasing either continuous phase viscosity or protein concentration-factors that only slightly influence D0. Bubble formation time and the snap-off bubble size (D0) were primarily controlled by the continuous phase flow rate (Qc) they decreased as Qc increased, from 1.64 to 0.13 ms and from 125 to 49 µm. Observations with high spatial and temporal resolution allowed us to study bubble formation and bubble coalescence processes simultaneously.

In this study, we provide a fundamental study of a cross-flow membrane foaming (CFMF) system to understand and control bubble formation for various process conditions and fluid properties.

Membrane foaming is a promising alternative to conventional foaming methods to produce uniform bubbles.

Thessa neef