Experimental investigation of interface deformation in free surface flow of concentrated suspensions

Abstract

It is well known that during the free surface flow of concentrated suspension of non-colloidal particles, the suspension-air interface becomes highly corrugated. This surface corrugation changes the interfacial area which could have important implications in various applications involving heat and mass transfer across the interface. Surface corrugation in free surface flow has been studied in the past, but its mechanism is not fully understood. We report detailed experiments on quantitative measurement of the surface deformation of concentrated suspension of non-colloidal particles in open channel flow. The motion and location of the interface and the velocity field of the bulk flow beneath the free surface were measured using the particle image velocimetry technique. Experiments were performed to study the effect of particle size, particle concentration, and viscosity of suspending fluid on the corrugation. The interface fluctuation was found to increase linearly with the flow rate. The deformation of the interface increased with increase in particle concentration until an optimum concentration is reached and thereafter it decreases. Our observation supports the previous studies on surface corrugation interpreted from the power spectra of the reflected light from the interface. Suspension of larger particles and less viscous fluid gives larger deformations of the suspension-air interface. These results can be used to determine the optimum parameters to control the interfacial area in free surface flow of concentrated suspensions.

Stability of gravity-driven particle-laden flows – roles of shear-induced migration and normal stresses

“…Surface corrugations arising due to shear-induced migration of particles in free-surface flows have been experimentally observed in the context of shear flows (Tirumkudulu, Tripathi & Acrivos 1999;Loimer, Nir & Semiat 2002), gravity-driven film flows (Timberlake & Morris 2005) and open channel flows (Singh, Nir & Semiat 2006;Kumar et al 2016). Our predictions of an enhanced instability of the surface mode remained consistent with the experimental observations by Timberlake & Morris (2005).…”

“…Although they study particle suspensions with Péclet numbers that are significantly larger, the experiments showed surface deformations happening in a highly viscous slow flow down an incline with Re 1. The occurrence of larger surface corrugations with suspensions of larger particles reported by Kumar et al (2016) can also be related to our predictions of higher growth rates for increasing Péclet number. However, our analyses were limited to infinitesimal disturbances and dealt with the waves' inception, whereas the experimental observations showed highly nonlinear wave formations.…”

“…For a pressure-driven channel flow with a layer of fluid suspended with negatively buoyant particles flowing below a layer of fluid devoid of any particles, Abedi, Jalali & Maleki (2014) identified a Kelvin-Helmholtz-like interfacial instability. In the specific context of particle-laden flows with free surfaces, experimental and theoretical investigations have been performed by several authors to study the surface topography of gravity-driven, neutrally buoyant non-Brownian particle-laden free-surface flows (Timberlake & Morris 2005;Ancey, Andreini & Epely-Chauvin 2013;Kumar, Medhi & Singh 2016). Most studies find that an increase in particle concentration leads to increased surface deformation and decreased entrance lengths.…”

Characterizing the surface texture of a dense suspension undergoing dynamic jamming

“…A corrugated free surface has been reported for a wide range of particle sizes and packing fractions and in several experimental setups (Loimer et al 2002;Timberlake and Morris 2005;Singh et al 2006;Kumar et al 2016). In the inclined plane experiment by Timberlake and Morris (2005), two dimensional (2D) power spectra of free surface images indicate that the features exhibit anisotropy, specifically, the corrugations are shorter in the flow direction.…”

Directionally controlled open channel microfluidics

“…[5][6][7] Recent studies on free-surface flows in miniaturized devices have promised potential applications in fluid mixing, chip cooling and cleaning. 8,9 Such progresses have been possible as a consequence of a phenomenal recent advancements in microfluidic and nanofluidic device fabrication processes, 10,11 which brought to focus several reduced-scale electrokinetic transport mechanisms [12][13][14][15][16][17] and their derived technology. 18 Recently, electric field-driven actuation has emerged tremendously in microscale transport system due to its several advantages, such as chip scale integrability, no moving part, high reliability, portability, etc.…”