The rheology of soft-flowing systems, such as concentrated emulsions, foams, gels, slurries, colloidal glasses and related complex fluids, has a larger and larger impact in modern science and engineering. Much of the fascination of these systems stems from the fact that they do not fall within any of three basic states of matter, gas-liquid-solid, but live rather on a moving border between them. To understand the flow mechanism, it is necessary to have a look at the micro-scale dynamics of its constituents (i.e, droplets for emulsions, bubbles for foams, blobs for gels, etc.). In fact, in these fluids, the flow occurs via successive elastic deformations and plastic rearrangements, which create fragile regions enhancing the “fluidization” of the material. Despite the fluidization of Soft Glassy Materials (SGMs) is strongly affected by the surface roughness, the role played by the density, the orientation and the periodicity of rough elements has not been quantitatively addressed so far. In fact, predict and control the flow of SGMs is particularly important for an ample variety of technological applications from food to pharmaceutical industries.
In this work, we study the flow of concentrated emulsions in microfluidic channels, one wall of which is patterned with micron-size grooves with different patterns. Using equally spaced grooves, we find a scaling law describing the roughness-induced fluidization as a function of the density of the grooves, thus fluidization can be predicted and quantitatively regulated. Furthermore, we quantitatively report the existence of two physically different scenarios. When the gap is large, compared to the droplets in the emulsion, the droplets hit the solid obstacles and easily escape scrambling with their neighbors. Conversely, as the gap spacing is reduced, droplets get trapped inside, creating a “soft roughness” layer, i.e., a complementary series of deformable posts. Introducing an asymmetrical micro-roughness (herringbone pattern), the flow presents, in turn an asymmetric behavior. The emulsion flows faster in the same direction of the herringbone groove respect when it flows in the opposite direction.
Our experimental observations are suitably complemented and confirmed by lattice Boltzmann simulations. These numerical simulations are key to highlight the change in the spatial distribution of the plastic rearrangements caused by surface roughness and to elucidate the micro-mechanics of the roughness induced fluidization.