Passage of surfactant-laden and particle-laden drops through a contraction

Abstract

We report on the experimental behavior of oil drops suspended in water and passing in a constricted capillary tube under an imposed pressure gradient. The surfaces of droplets are covered either by colloidal solid particles or soluble surfactants. We investigate the coupling between the flow behavior and the concentration gradient in adsorbed species that are induced by surface expansion, when a water lubrication film persists between the drop and the capillary walls. For both particle-laden and surfactantladen drops, we evidence the formation of strong concentrations gradients resulting in surface tension gradients. We show how local values of surface tension can be monitored using flowrate measurements. In the case of particle-laden drops, we demonstrate the surface tension gradient is balanced by viscous friction in the lubrication film. In the case of surfactant-laden drops, we suggest a Marangoni flow opposes the decrease of surfactant concentration at the front of the drop, up to a threshold value of the surface expansion rate. Finally, we discuss how these effects increase the passage time of surfactant-laden drops in the constriction.

Stability of particle laden interfaces of drops flowing through a pore

“…besides, from the typical images in Fig. 2a) we find that, as the drop flows through the pore, the front interface is free of silica particles so that γ F = γ o/w , the oil/water interface tension, at all times [26]. At the back of the drop where surface coverage in adsorbed silica particles varies, the interfacial tension γ B will account for the subsequent change in surface energy.…”

“…The model system we used was developed in a previous study [26]. As model pore, we used a cylindrical tube made out of borosilicate glass with a central converging-diverging part, as depicted in Figure 1(a,b).…”

“…The pressure difference is denoted ∆P and varies between 1.2 and 5 kPa. The lower limit for ∆P is actually the pressure for which clogging is observed when drops do not cross the pore [26]. The dependence between pressure difference ∆P and water flowrate Q w was carefully calibrated to measure the pore hydraulic resistance Ψ defined as ∆P = ΨQ w for water : Ψ = (1.4 ± 0.1) × 10 12 Pa.s.m −3 .…”

“…The particles are first dispersed in 10 ml NaCl solution using an ultrasonic probe (20 000 Hz, 40% of maximum intensity). Oil is further added and the emulsion is obtained by mixing at 18 000 rpm for 30 s. The volume of oil is adjusted so that all particles and oil are emulsified [26]. We obtained drops with radius R = 125 ± 25 µm and a surface coverage in particles measured at C = 0.86±0.04.…”

“…In a recent paper [26], we described the behavior of oil-in-water drops laden with micrometer-sized silica particles flowing through a single convergent-divergent axi-symmetrical pore. The drop was initially 5 times larger than the narrowest part of the pore, so that it strongly deformed in the pore.…”