Lubrication effects on the flow of wet granular materials

Abstract

We investigate the dynamics of a partially saturated grain-liquid mixture with a rotating drum apparatus. The drum is partially filled with the mixture and then rotated about its horizontal axis. We focus on the continous avalanching regime and measure the impact of volume fraction and viscosity of the liquid on the dynamic surface angle. The inclination angle of the surface is observed to increase sharply to a peak and then decrease as a function of liquid volume fraction. The height of the peak is observed to increase with rotation rate. For higher liquid volume fractions, the inclination angle of the surface can decrease with viscosity before increasing. The viscosity where the minima occurs decreases with the rotation rate of the drum. Limited measurements of the flow depth were made, and these were observed to show only fractional changes with volume fraction and rotation speeds. We show that the qualitative features of our observations can be understood by analyzing the effect of lubrication forces on the timescale over which particles come in contact.

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

“…Like any thin fluid film between moving particles, the bridge generates a viscous force, which is repulsive when the particles approach or attractive when they separate, and whose magnitude increases with both liquid viscosity and interparticle velocity [124]. Moreover, the capillary force exerted by the bridge may increase the interparticle friction or decrease it, if the liquid viscosity or the deformation rate is high enough to prevent solid-solid contact [153,156,158]. Hence, there exists a complex interrelation between forces, which can be treated by means of the capillary number Ca = D/cos ( is the liquid viscosity and  is the rate) as the ratio of viscous to capillary forces [33].…”

“…Above a critical stress (yield stress), particles flow is initiated in the system, exhibiting a liquid-like behavior: a yield stress fluid. On the other hand, the interstitial liquid introduces a viscous force acting between the particles through the capillary bridge [33,124,156,157] that alters both the frictional behavior and the energy dissipation in the material, and introduces additional time-dependences in the system performance. While a dry granular material generally behaves as a viscoplastic medium [37] (flow properties depend on the deformation rate at sufficiently high rates), liquid may greatly enhance such viscous character.…”

“…Additionally, the energy dissipation due to rupture and formation of bridges between moving particles increases with both liquid viscosity and separation velocity [157], also leading to higher yield stress at faster deformation rates. Only a few studies have investigated these mechanisms causing rate dependence in unsaturated granular media containing rather large amounts of highly viscous liquids [33,156,157,187].…”

“…In any case, this estimate shows that the 'shear resistance' of each sphere contact against deformation of the CC in the wet range is largely due by friction and viscous forces, probably dominating over capillary interaction. In this sense, F would approach the dynamic liquid bridge force, which is the sum of capillary and viscous forces [156,157]. To evaluate how these mechanisms correlate with the characteristics of the bridge, whose viscosity is unknown, thickness-dependent and probably much higher than that of bulk water, is a challenging issue to be tackled in the future.…”

Transformation around intruders in granular media

“…Volume changes due to dilatation coefficients [1] or phase transitions may obviously affect the particles [2]. Consequently, the particle's shapes of particles evolves [3] as well as their contact properties through lubrication [4] or formation of capillary bridges [5]. In addition, chemical reactions also modify the surface and the nature of each individual grain [6].…”