Capture of colloidal particles by a moving microfluidic bubble

Abstract

Foams can be stabilized for long periods by the adsorption of solid particles on the liquid–gas interfaces. Although such long-term observations are common, mechanistic descriptions of the particle adsorption process are scarce, especially in confined flows, in part due to the difficulty of observing the particles in the complex gas–liquid dispersion of a foam. Here, we characterise the adsorption of micron-scale particles onto the interface of a bubble flowing in a colloidal aqueous suspension within a microfluidic channel. Three parameters are systematically varied: the particle size, their concentration, and the mean velocity of the colloidal suspension. The bubble coverage is found to increase linearly with position in the channel for all conditions but with a slope that depends on all three parameters. The optimal coverage is found for 1 μm particles at low flow rates and high concentrations. In this regime the particles pass the bubbles through the gutters between the interface and the channel corners, where the complex 3D flow leads them onto the interface. The largest particles cannot enter into the gutters and therefore provide very poor coverage. In contrast, particle aggregates can sediment onto the microchannel floor ahead of the bubble and get swept up by the advancing interface, thus improving the coverage for both large and medium particle sizes. These observations provide new insight on the influence of boundaries for particle adsorption at an air–liquid interface.