Armoring confined bubbles in the flow of colloidal suspensions

Abstract

We study the process of coating the interface of a long gas bubble, which is translating in a horizontal circular capillary tube filled with a colloidal suspension. A typical elongated confined bubble is comprised of three distinct regions: a spherical front cap, a central body that is separated from the tube wall by a thin liquid film, and a spherical cap at the back. These three regions are connected by transitional sections. Particles gradually coat the bubble from the back to the front. We investigate the mechanisms that govern the initial accumulation of the particles and the growth of the particle-coated area on the interface of the bubble. We show that the initial accumulation of particles starts at the stable stagnation ring on the rear cap of the bubble, and the particles will completely coat the spherical cap at the back of the bubble before accumulating on the central body. Armoring the central interface of the bubble with particles thickens the liquid film around the bubble relative to that around the particle-free interface. This effect creates a rather sharp step on the interface of the bubble in the central region, which separates the armored region from the particle-free region. After the bubble is completely coated, the liquid film around the body of the bubble will adjust again to an intermediate thickness. We show that the three distinct thicknesses that the liquid film acquires during the armoring process can be well described analytically.

Time-dependent motion of a confined bubble in a tube: transition between two steady states

“…Experiments are performed in a refractive index matching set-up (Yu et al 2017(Yu et al , 2018, where a 5× objective (Mitutoyo) is used in the imaging apparatus. Pure glycerol is used as the continuous phase, with viscosity µ = 1.00 Pa s (Anton Paar, Physica MCG 301) and surface tension γ = 65.4 mN/m (pendant drop method), respectively.…”

“…Several accounts have been carried out considering the effects of impurities in the continuous fluid phase, including surfactants and suspensions. While both impurities thicken the film, surfactants increase the film thickness by introducing additional fluid flux due to a Marangoni stress (Ratulowski & Chang 1990;Park 1992;Stebe & Barthes-Biesel 1995;Olgac & Muradoglu 2013), and suspensions, which lead to particles being adsorbed on the interface, thicken the film by modifying the boundary condition on the bubble surface (Yu et al 2017). Moreover, as the tube radius R increases, buoyancy effects can become significant, which breaks the film thickness uniformity or leads to film rupture (Leung et al 2012;Atasi et al 2017;Lamstaes & Eggers 2017).…”

Capillary filtering of particles during dip coating

“…Over the past few years, studies have explored the influence of particles on interfacial dynamics, when the length scales of a liquid thread or film and the particle diameter are comparable. Because the particles deform the air-liquid interface, they strongly modify the geometry and instabilities of the liquid film [17][18][19][20][21][22][23][24][25]. In the dip-coating configuration, only a few studies have investigated the fate of individual non-Brownian particles in the meniscus and their entrainment in the film [26][27][28].…”

Separation of particles by size from a suspension using the motion of a confined bubble

“…The translation of a long bubble confined in a circular capillary of radius R filled with pure liquid of viscosity l and surface tension c is well documented in the literature, 22-29 in contrast to that in a capillary filled with a complex liquid, such as a colloidal suspension. [30][31][32][33] In a reference frame translating at the bubble velocity U b , most of the liquid phase in the front and at the back of the bubble remains on separate streamlines and do not mix with one another. However, a fraction of the liquid ahead of the bubble leaks towards the back of the bubble through an annular liquid-filled gap of uniform thickness b around the bubble [ Fig.…”

“…The larger particles are, however, found forming a monolayer coating at the back of the bubble. 33 In order to test the sensitivity of the system to the thickness of the liquid gap, we increase the Ca downstream in the same experiment to reach the condition d p =b < 1 again [ Fig. 2(c)].…”