Wall slip mechanisms in direct and inverse emulsions

Abstract

We carry out a series of experiments with the aim of completing our knowledge of wall slip characteristics, through a deductive approach based on macroscopic behavior observations. More precisely, we use model materials (direct and inverse emulsions) and determine the variations of wall slip properties depending on the material parameters (droplet size, concentration) and boundary conditions of the flow (free surface or flow between two solid surfaces, normal force, flow beyond yielding, and coated or rough surface). The wall slip characteristics are determined from long creep tests at different levels and from internal measurements of the velocity profile in the capillary or the Couette flow as determined by magnetic resonance imaging. First, we show that the slip yield stress is due either to edge effects in relation with evaporation then pinning around the line of contact or to a kind of adhesion of the suspended elements to the wall. This adhesion effect varies with the characteristics of the solid surface (interaction with elements, roughness), and wall slip (below the yield stress) disappears when the adhesion or adherence leads to a wall slip yield stress expected to be larger than the material yield stress. Then, we show that, below the yield stress, the slip velocity vs shear stress (from which the slip yield stress has been removed) relationship is linear. The corresponding value for the apparent slip layer made of interstitial liquid appears to be independent of the concentration and to vary only slightly with the droplet size. Moreover, it is independent of the normal force (below the critical value inducing elongation) and other experimental conditions, e.g., it is the same for free surface flows. Although the origin of this phenomenon remains to be found, the following scheme appears to be consistent with all observations: the droplets are attracted at a very short distance from the wall, forming regions of small area in which the liquid layer thickness is very small, the shear stress being dominated by the shear in these regions. Finally, this apparent layer thickness increases at the approach of the yield stress and beyond, or if a slightly rough surface is used, leading to a faster (quadratic?) variation of the slip velocity as a function of the stress.

Wall slip regimes in jammed suspensions of soft microgels

“…Note that all measurements are performed for wall stresses above the yield stresses σ Y , while previous measurements by Seth et al [17] suggested a transition in slip regimes at σ w ≈ σ Y . In this stress range, we also detect no measurable wall yield stress, contrary to measurements from the recent literature [10,17,21].…”

“…To conclude, we have demonstrated that the slip friction of jammed microgel suspensions exhibits a robust transition from a non-linear regime to a linear one at large wall stresses and slip velocities. We thereby reconcile seemingly contradictory measurements from the literature regarding slip laws of yield stress fluids above the yield stress [5,8,17,21].…”

“…In emulsions and microgel suspensions, either a linear [8,12,17] or a square dependency of V upon σ w [5,9,10,[18][19][20] is evidenced, depending on the stress and the chemical nature of the solid substrate, with respect to the one of the soft spheres. In addition, it has been suggested that the friction exponent switches from p = 2 to 1 as the wall stress σ w becomes higher than the bulk yield stress σ Y [17], but other measurements have evidenced a non-linear scaling above the yield stress [5,10,21]. In this context, the aim of this article is first to quantify the influence of both the wall stress and the velocity on the value of the exponent p by exploring many decades, and second to rationalize the friction regimes in terms of local dissipation mechanisms (figure 1).…”

Sliding flows of yield-stress fluids

“…Meeker et al (2004b) and Piau (2007) tried to theoretically describe the slip origin in soft particle pastes/microgels from an elastohydrodynamic perspective. Rheometric tests with slippery plates in the cone-plate/plate-plate geometries have been conducted to quantify the slip behaviours of yield-stress fluids ranging from Carbopol gels to emulsions (Meeker, Bonnecaze & Cloitre 2004a;Meeker et al 2004b;Poumaere et al 2014;Zhang et al 2018). Moreover, very recently, in a series of experiments using optical coherence tomography (known as OCT) and particle tracking velocimetry (known as PTV), Daneshi et al (2019) characterized the slip behaviour of Carbopol gel (a model 'simple' yield-stress fluid) inside a capillary tube.…”

“…2014; Zhang et al. 2018). Moreover, very recently, in a series of experiments using optical coherence tomography (known as OCT) and particle tracking velocimetry (known as PTV), Daneshi et al.…”