Fluidization and wall slip of soft glassy materials by controlled surface roughness

Abstract

The motion of soft-glassy materials (SGM) in a confined geometry is strongly impacted by surface roughness. However, the effect of the spatial distribution of the roughness remains poorly understood from a more quantitative viewpoint. Here we present a comprehensive study of concentrated emulsions flowing in microfluidic channels, one wall of which is patterned with micron-size equally spaced grooves oriented perpendicularly to the flow direction. We show that roughness-induced fluidization can be quantitatively tailored by systematically changing both the width and separation of the grooves. We find that a simple scaling law describes such fluidization as a function of the density of grooves, suggesting common scenarios for droplet trapping and release. Numerical simulations confirm these views and are used to elucidate the relation between fluidization and the rate of plastic rearrangements. Controlling the slip and flow of soft-glassy materials (SGM) at the microscale is crucial for food and pharmaceutical processing, and for micro-manufacturing [1][2][3][4]. SGM include concentrated emulsions, gels, foams, pastes, and exhibit a complex, non-linear rheology [5][6][7]: they behave like elastic solids unless a stress large enough, known as the yield stress σ Y , is applied. Above σ Y SGM flow like non-Newtonian liquids. This solidto-liquid transition and the corresponding flowing properties have been widely studied [8], but still pose a series of challenging questions, relevant both for applications [9][10][11] and for a better understanding of the statistical mechanics of SGM [12][13][14][15][16][17][18][19][20]. Recent studies [21][22][23][24][25][26][27][28][29] showed that their flow bahavior is characterized by "nonlocality" [21,22], meaning that the relation between the local stress σ and the local shear rateγ cannot be explained with a unique master curve. This non-local behaviour depends on both confinement and surface roughness [22,25,26], and it is ascribed to the presence of plastic rearrangements [21,22], i.e. topological changes in the micro-structural configurations. These take place whenever the material cannot sustain the accumulated stress, then it undergoes an irreversible deformation and releases the excess stress in the form of elastic waves. The range of such perturbation introduces a new length, named "cooperativity length" ξ [21], which is typically on the order of a few diameters of micro-structural constituents (i.e. droplets for emulsions [21,22], bubbles for foams [27,28], blobs for gels [29], etc). Although the cooperativity length becomes relevant at the jamming point of SGM [30], it has been sharply argued that ξ is fundamentally different from the characteristic legnth that describes dynamical heterogeneities involved in spontaneous fluctuations [31][32][33]. Recently, many theoretical studies have been put forward in the recent years to account for these non-local effects [12,[15][16][17]20]. One of them, the kinetic elastoplastic (KEP) model [16], explores the effects of coopera...

Metastability at the Yield-Stress Transition in Soft Glasses

“…The values of the cooperativity scale ξ extracted from the model [41,47,48] are in agreement with experimental observations [7,49]. Recently, the model has been used in synergy with experiments on real emulsions in order to quantify the impact of the fluidization induced by the roughness of microchannels on the flow behavior of the emulsion [47,48]. As shown in the reminder of the paper, for this "model emulsion" the static and dynamic yield-stress values are found to differ.…”

A review on wall slip in high solid dispersions

“…Several experimental studies have reported results in the direction that rough surfaces affect surface fluidization in confined flows (Goyon et al 2008;Goyon et al 2010;Mansard et al 2014;Geraud et al 2013). A recent study analyzes the importance of surface texture with respect to surface fluidization by changing the amplitude and wavelength of the corrugations created by specific surface patterns (Derzsi et al 2016). Still more surprising significant alterations of the velocity profiles over distances to the wall much larger than a few particle sizes have been reported when surfaces are smooth and develop attractive interactions with the particles of the dispersions (Seth et al 2012;Paredes et al 2015).…”

Wall slip mechanisms in direct and inverse emulsions

“…The detailed impact of surface roughness or surface interaction between the fluid and the solid has been studied through detailed measurements of the flow characteristics at a local scale, by few authors [38][39][40], in the yielding regime.…”