arXiv subjects

Ashish V. Orpe

Publications and source records attributed to Ashish V. Orpe.

16 recordsLinked to original sources

arXiv2026

Surface coating induced lubrication in flowing granular materials

We investigate the flow of spherical, bulk granular particles down an inclined plane mixed with small-sized spherical lubricant particles using discrete element method simulations. Predefined cohesive interaction is implemented between lubricant and bulk particles, enabling the coating of the former over the latter. The overall flow rate exhibits non-monotonic dependence on lubricant content. Initially, it increases with lubricant addition, reaches a maximum at an intermediate lubricant content, and decreases for even higher lubricant content. The increase in the flow rate is attributed to a lower inter-particle friction coefficient between lubricant-coated bulk particles. The decrease in the flow rate at higher lubricant content, on the other hand, is attributed to enhanced densification and increased damping between crowded particles. Both these occurrences are examined using various flow level characteristics. The simulation results are found to be in qualitative agreement with previous experimental results. Overall, the outcome integrates novel computational insights and prior experimental results to enhance the understanding of the powder lubrication phenomena.

cond-mat.soft↗

arXiv2024

Flow and clogging behavior of a mixture of particles in a silo

We investigated the clogging behavior observed during the flow of aspherical particles from a silo in the presence of spherical particles of different sizes and proportions using flow visualization experiments and discrete element method (DEM) simulations. The size of the avalanche, essentially the tendency of clogging, exhibits non-monotonic dependence on the spherical particle volume fraction. For small enough content of spherical particles, the clogging tendency intensifies, whereas it reduces rapidly for high enough spherical particle fractions, with a minimum in between. The non-monotonic behavior is observed to persist over for different spherical particle sizes. The overall behavior is shown to arise due to competing effects between the localized total particle fraction influencing avalanche strength and mean size of the particles exiting the silo, influencing the probability of arch formation.

cond-mat.soft↗

arXiv2022

Flow of lubricated granular material on an inclined plane

We have studied the gravity driven flow of spherical shaped, millimetric sized granular material coated with aspherical, micron-sized, near frictionless lubricant particles. Experiments were performed on an inclined plane using two different sized particles for varying concentrations of the lubricant. The particle volumetric flow rate exhibits a non-monotonic behavior with increasing lubricant concentration. It shows an increase at lower lubricant concentration followed by a decrease at higher lubricant concentration. The lubricant particles adhere to the granular particle surface thereby reducing the inter-particle friction. However, presence of lubricant particles at higher concentration damps out inter-particle collision thereby reducing the interparticle momentum transfer. Non-monotonicity in the observed behavior is then conjectured to arise due to competing effects of inter-particle friction and inter-particle collision. The present work and the overall observed behavior therein provides a simple experimental system to characterize the effects of added lubricant material in pharmaceutical and other relevant industrial applications.

cond-mat.soft↗

arXiv2021

Slip behavior during pressure driven flow of Laponite suspension

We investigate pressure driven pipe flow of Laponite suspension, as a model thixotropic fluid. The tendency of the suspension to age is controlled by addition of sodium chloride salt to vary the ionic strength. We use a syringe pump to prescribe the flow and observe that a steady state flow is obtained. Unusually, the steady state pressure drop required to maintain a constant flow rate decreases with an increase in the flow rate, in qualitative contrast to the expectation for Poiseuille flow. We demonstrate that experimental results obtained by varying the flow rate, salt concentration, and flow geometry (pipe diameter and length) can be collapsed onto a single universal curve that can be rationalized by invoking slip of the suspension at the tube walls. The Laponite suspension exhibits plug-like flow, yielding at the tube walls. Our results suggest that the slip length varies linearly with the flow rate and inversely with the tube diameter.

physics.flu-dyn↗

arXiv2020

Microstructure and yielding of capillary force induced gel

We have investigated the rheology and structure of a gel formed from a mixture of non-Brownian particles and two immiscible liquids. The suspension of particles in a liquid undergoes gelation upon the addition of a small content of second, wetting liquid which forms liquid bridges between particles leading to a sample spanning network. The rheology of this gel primarily exhibits a yield stress at low shear rates followed by a linear variation of shear stress at high shear rates. The apparent yield stress extracted from the flow curves increases rapidly with volume fraction of second liquid before saturation, while it exhibits a monotonic increase with increasing particle concentration. Rescaling of the yield stress curves using suitable shift factors results in an empirical expression for the yield stress showing squared dependence on liquid fraction and a rapid increase with particle fraction above a certain value, both combined in a highly nonlinear manner. The microstructural variations with changing secondary liquid content and particle fractions are captured using three dimensional X-ray tomography technique. The microstructure is observed to show increased local compactness with increased liquid content and increased spatial homogeneity with increased particle fractions. The images from X-ray tomography are analysed to obtain the distributions of particle-particle bonds (coordination number) in the system which serve to explain the observed yield stress behavior in a qualitative manner.

cond-mat.soft↗

arXiv2019

Friction-mediated flow and jamming in a two-dimensional silo with two exit orifices

We show that the interparticle friction coefficient significantly influences the flow and jamming behavior of granular materials exiting through the orifice of a two-dimensional silo in the presence of another orifice located in its vicinity. The fluctuations emanating from a continuous flow through a larger orifice results in an intermittent flow through the smaller orifice consisting of sequential jamming and flowing events. The mean time duration of jammed and flow events, respectively, increase and decrease monotonically with increasing interparticle friction coefficient. The frequency of unjamming instances ($n_{u}$), however, shows a nonmonotonic behavior comprising an increase followed by a decrease with increasing friction coefficient. The decrease on either side of the maximum, then, represents a system moving progressively towards a permanently jammed or a permanently flowing state. The overall behavior shows a systematic dependence on the interorifice distance which determines the strength of the fluctuations reaching the smaller orifice leading to unjamming instances. The probability distributions of jamming and flowing times are nearly similar for different combinations of friction coefficients and interorifice distances studied and, respectively, exhibit exponential and power-law tails.

cond-mat.soft↗

arXiv2016

Fingering instability of a suspension film spreading on a spinning disk

The spreading of a thin film of suspension on a spinning disk and the accompanying contact line instability is studied through flow visualization experiments. The critical radius for the onset of instability shows an increase with increase in the particle fraction ($ϕ_{p}$) before decreasing slightly at the highest value of $ϕ_{p}$ studied, while the instability wavelength ($λ$) exhibits a non-monotonic dependence. The value of $λ$ is close to that for a partially wetting liquid at lower $ϕ_{p}$, it decreases with increasing $ϕ_{p}$ to a minimum before increasing again at largest $ϕ_{p}$. The non-monotonic trends observed for $λ$ are discussed in light of the linear stability analysis of thin film equations derived for suspensions by Cook {\it et al.} [Linear stability of particle-laden thin films, Eur. Phys. J.: Spec. Top. {\bf 166}, 77 (2009)] and Balmforth {\it et al.} [Surface tension driven fingering of a viscoplastic film, J. Non Newtonian Fluid Mech. {\bf 142}, 143 (2007)].

cond-mat.soft↗

arXiv2014

Flow of granular matter in a silo with multiple exit orifices: Jamming to mixing

We investigate the mixing characteristics of dry granular material while draining down a silo with multiple exit orifices. The mixing in the silo, which otherwise consists of noninteracting stagnant and flow regions, is observed to improve significantly when the flow through specific orifices is stopped intermittently. This momentary stoppage of flow through the orifice is either controlled manually or is chosen by the system itself when the orifice width is small enough to cause spontaneous jamming and unjamming. We observe that the overall mixing behavior shows a systematic dependence on the frequency of closing and opening of specific orifices. In particular, the silo configuration employing random jamming and unjamming of any of the orifices shows early evidence of chaotic mixing. When operated in a multipass mode, the system exhibits a practical and efficient way of mixing particles.

cond-mat.soft↗

arXiv2014

Spontaneous jamming and unjamming in a hopper with multiple exit orifices

We show that the flow of granular material inside a 2-dimensional flat bottomed hopper is altered significantly by having more than one exit orifice. For the hoppers with small orifice widths, inter-mittent flow through one orifice enables the resumption of flow through the adjacent jammed orifice, thus displaying a sequence of jamming and unjamming events. Using discrete element simulations, we show that the total amount of granular material (i.e. avalanche size) emanating from all the orifices combined can be enhanced by about an order of magnitude difference by simply adjusting the inter-orifice distance. The unjamming is driven primarily by fluctuations alone when the inter-orifice distance is large, but when the orifices are brought close enough, the fluctuations along with the mean flow cause the flow unjamming.

cond-mat.soft↗

arXiv2008

Fast decay of the velocity autocorrelation function in dense shear flow of inelastic hard spheres

We find in complementary experiments and event driven simulations of sheared inelastic hard spheres that the velocity autocorrelation function $ψ(t)$ decays much faster than $t^{-3/2}$ obtained for a fluid of elastic spheres at equilibrium. Particle displacements are measured in experiments inside a gravity driven flow sheared by a rough wall. The average packing fraction obtained in the experiments is 0.59, and the packing fraction in the simulations is varied between 0.5 and 0.59. The motion is observed to be diffusive over long times except in experiments where there is layering of particles parallel to boundaries, and diffusion is inhibited between layers. Regardless, a rapid decay of $ψ(t)$ is observed, indicating that this is a feature of the sheared dissipative fluid, and is independent of the details of the relative particle arrangements. An important implication of our study is that the non-analytic contribution to the shear stress may not be present in a sheared inelastic fluid, leading to a wider range of applicability of kinetic theory approaches to dense granular matter.

cond-mat.soft↗

arXiv2007

Lubrication effects on the flow of wet granular materials

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.

cond-mat.soft↗

arXiv2006

Velocity correlations in dense granular flows observed with internal imaging

We show that the velocity correlations in uniform dense granular flows inside a silo are similar to the hydrodynamic response of an elastic hard-sphere liquid. The measurements are made using a fluorescent refractive index matched interstitial fluid in a regime where the flow is dominated by grains in enduring contact and fluctuations scale with the distance traveled, independent of flow rate. The velocity autocorrelation function of the grains in the bulk shows a negative correlation at short time and slow oscillatory decay to zero similar to simple liquids. Weak spatial velocity correlations are observed over several grain diameters. The mean square displacements show an inflection point indicative of caging dynamics. The observed correlations are qualitatively different at the boundaries.

cond-mat.soft↗

arXiv2005

Friction of a slider on a granular layer: Non-monotonic thickness dependence and effect of boundary conditions

We investigate the effective friction encountered by a mass sliding on a granular layer as a function of bed thickness and boundary roughness conditions. The observed friction has minima for a small number of layers before it increases and saturates to a value which depends on the roughness of the sliding surface. We use an index-matched interstitial liquid to probe the internal motion of the grains with fluorescence imaging in a regime where the liquid has no significant effect on the measured friction. The shear profiles obtained as a function of depth show decrease in slip near the sliding surface as the layer thickness is increased. We propose that the friction depends on the degree of grain confinement relative to the sliding surfaces.

cond-mat.soft↗

arXiv2005

Rheology of surface granular flows

The rheology of surface granular flows is investigated by means of measurements of velocity and number density profiles in a quasi-two-dimensional rotating cylinder, half-filled with mono-disperse steel balls. The measurements are made at the center of the cylinder, where the flow is fully-developed, using streakline photography and image analysis . The stress profile is computed from the number density profile using a force balance taking into account wall friction. The profiles for the mean velocity superimpose when distance is scaled by the particle diameter and the velocity by a characteristic shear rate and the particle diameter. The mean velocity is found to decay exponentially with depth in the bed with a decay length of $λ=1.1d$. The r.m.s. velocity is nearly constant near the free surface and below a transition point it decays linearly with depth. The shear rate, obtained by numerical differentiation of the velocity profile, shows a maximum which occurs at the same depth as the transition in the r.m.s. velocity profile. The velocity distribution is Maxwellian above the transition point and a Poisson velocity distribution is obtained deep in the layer. The variation of the apparent viscosity ($η$) with r.m.s. velocity ($u$) shows a relatively sharp transition at the shear rate maximum, and in the region below this point the apparent viscosity varies as $η\sim u^{-1.5}$. The experimental data is compared to predictions of three models for granular flow.

cond-mat.soft↗

arXiv2004

Surface Granular flows: Two Related Examples

Granular surface flows are common in industrial practice and natural systems, however, theoretical description of such flows is at present incomplete. Two prototype systems involving surface flow are compared: heap formation by pouring at a point and rotating cylinders. Continuum models for analysis of these flows are reviewed, and experimental results for quasi-2d systems are presented. Experimental results in both systems are well described by continuum models.

cond-mat.soft↗

arXiv2004

Solid-fluid transition in a granular shear flow

The rheology of a granular shear flow is studied in a quasi-2d rotating cylinder. Measurements are carried out near the midpoint along the length of the surface flowing layer where the flow is steady and non-accelerating. Streakline photography and image analysis are used to obtain particle velocities and positions. Different particle sizes and rotational speeds are considered. We find a sharp transition in the apparent viscosity ($η$) variation with rms velocity ($u$). In the fluid-like region above the depth corresponding to the transition point (higher rms velocities) there is a rapid increase in viscosity with decreasing rms velocity. Below the transition depth we find $η\propto u^{-1.5}$ for all the different cases studied and the material approaches an amorphous solid-like state deep in the layer. The velocity distribution is Maxwellian above the transition point and a Poisson velocity distribution is obtained deep in the layer. The observed transition appears to be analogous to a glass transition.

cond-mat.soft↗