Session Z11: Granular Porous Media and Multiphase Flows I

Hydrodynamically induced jamming in colloidal particles flow-driven across a potential landscape

We will show that when several colloidal particles are driven by the flow of the medium, hydrodynamic interactions can hinder their collective transport across potential barriers. This result contrasts with previous reports of force-driven systems, where hydrodynamic interactions facilitate particle motion. Experimentally, we observe this phenomenon by driving colloidal particles using rotating optical traps, which creates a vortex flow field in the corotating reference frame. This produces a jamming-like decrease of the current as density is increased, which can also be seen in simulations. A minimal theoretical model shows that hydrodynamic interactions between particles enhance the external potential, and this causes the average velocity reduction. The impact of hydrodynamic interactions is reversed compared to force-driven motion, suggesting that our findings are a generic feature of flow-driven transport.   

Interaction of shock waves in rapid granular flows

Granular flow around triangular obstacles results in the formation of parabolic shock waves, with fast-moving grains upstream of the shock front and almost stationary grains sitting on the nose. The flowfield becomes more interesting and notoriously complicated when more than one triangular obstacle is placed together in the granular stream. The shock waves from each obstacle interact with each other to give rise to special structures that are new to the granular community. For a detailed investigation on the shock-shock interaction phenomenon, experiments are performed for two and three wedges placed in close proximity. For large spacings between the wedges, thin streaks are formed, whereas, for small spacings, a big shock wave is formed that encapsulates the entire wedge array. When the wedge angle is increased to 60 degrees, the central diamond-shaped streak becomes unstable and starts oscillating from left to right about the mean position for the two wedge cases, and up and down in the streamwise direction for the three wedge cases. As a result of the instability, the shock front undergoes oscillatory motion with a frequency of 3 Hz. The underlying mechanism responsible for these instabilities is discussed in detail.   

Nonlinear transport and pattern formation in two-dimensional binary systems

The transport of charged colloidal particles in the environment of larger colloidal particles or polymers is ubiquitous in biological and chemical systems. By tuning the size, charge and dimensionality of the colloidal particles, the responses of the system to the external electric field can be varied and show exotic nonlinear effects. In this report, we focus on two-dimensional binary systems, where the colloidal particles have large size asymmetry. The interaction between the conducting small particles and the vibrational degrees of freedom of large particles is crucial for understanding the nonlinear transport behavior of the small particles and the formation of some non-trivial patterns of the density distribution. Proposals of experimental realization will be presented.   

The failure of two-dimensional particle raft under tension

We study particle rafts, which are submillimeter particles aggregated at an air-liquid interface, as they are pulled apart from their opposing sides. There is a distinct failure morphology which depends on the pulling velocity, V_pull. When V_pull is high, there are many microcracks that are distributed throughout the entire system. As V_pull is decreased, the cluster size, that is the number of particles between adjacent cracks, increases until it becomes comparable to the length of the raft. A one-dimensional linear instability analysis reveals that there is a healing velocity V_heal(k) for clusters to rearrange which decreases with the wavevector, k. That is, the time it takes for a cluster to heal (i.e., rearrange and find new neighbors) increases as the length of the cluster increases. This accounts for the rate dependence in the failure morphology: the pulling speed selects the length scale at which the clusters can relax effectively.  We further explore this raft failure under different parameters, such as changing the viscosity and surface tension of the  underlying liquid, the boundary conditions (using either repulsive or attractive interactions with the puller) and system size.   

Ultralow-Pressure-Drop Multiplexed Coalescence Inertial Filters

Multiphase flows containing small droplets are ubiquitous in engineering applications and often pose a significant challenge to the efficient and reliable design of thermofluidic systems. This work describes multiplexed coalescence inertial filters composed of parallel helical pathways, designed to capture fine droplets (< 40 µm) through inertial separation. These filters have a large number of flow passages, resulting in an ultralow pressure drop (< 400 Pa). Filtration efficiencies for streams of air laden with 7 µm and 30 µm droplets were characterized under varying flow conditions, with complete capture observed above a threshold flowrate. Models for filtration efficiency and pressure drop were developed and validated against experimental results to allow system design and optimization, enabled by the tunable additive manufacturing approach used to fabricate the filters. Furthermore, additive manufacturing methods were modified to create porous multiplexed coalescence inertial filters that may be continuously drained. This multiplexed coalescence inertial filtration approach could find use in HVAC systems, fog harvesting, chemical reactors, and microgravity droplet capture.   

Electric Field Induced Capillary Condensation in Micron Deep Channels

Condensation can occur in small scale systems due to the presence of Laplace pressure below the saturation pressure of the liquid. Lately capillary condensation has attracted lots of attentions due to its broad application in various fields from climate engineering to water purification. Thermodynamics arguments suggest that electric fields alongside capillaries can induce the condensation of fluids in larger channels or lower humidity. The validity of this hypothesis has not been looked at in previous works due to experimental difficulties such as reaching high electric fields necessary. Here we show an experiment to investigate how high electric field invoke condensation in micron deep channel. In order to study this effect water vapor was flowed through a channel with clear electrodes, permitting visualization of the condensed water. The formation of the liquid water was further inspected using a self assembled Raman Spectrometer to determine the effect of the electric field on the structure of the water. We will discuss the  validity of the thermodynamic model and the implications for applications such as vapor separation and gas drying.   

Withdrawn

We discuss the flow of turbulent suspensions of non-Brownian, non-colloidal, rigid spherical particles in a Newtonian fluid over a porous surface. In this study, we report the effect of the

momentum transfer coefficient τ on the turbulence statistics of suspension flows where the bulk particle volume fraction ranges from 0 to 0.2 for different wall porous permeability, while the thickness and porosity of the porous wall are fixed. We study three values of τ, i.e., −1, 0 and 1, because the momentum transfer coefficient τ of most of the porous media falls in this range that measures the transfer of stress at the porous/fluid interface and depends on the porous material and the texture of the solid interface. Direct numerical simulations (DNS) with an immersed boundary method (IBM) are employed to resolve the particles and flow phase where they coupled with the volume-averaged Navier-Stokes (VANS) to solve the flow within the porous layer. By changing τ from 0 to either 1 or -1 the velocity at the interface between the porous wall and the free fluid region increases significantly. The overall drag is found to grow by changing τ from 0 to 1 and -1 whereas the mean momentum balance analysis shows that the particle-induced stresses are the main factor for the overall drag increase for non-zero τ.   

The rheology of graphene oxide dispersions in confined geometries

Dispersions of high aspect ratio 2D nano-particles have seen a huge increase in research interest over the past decade, especially in the context of ink formulations for printed electronics. So far the rheology of these dispersions has only been measured in bulk. For inkjet applications, however, it is crucial to understand the flow properties in confined geometries with dimensions of ~10-100 particle diameters.  This work focuses on graphene-oxide particles (GO) with an aspect ratio (lateral size/thickness) of ~15000 dispersed in water. Optical coherence tomography is used to non-invasively measure the velocity profile of these suspensions in microchannels at different pressure drops. We study how confinement changes the flow properties and microstructure of these suspensions compared to the bulk rheology for a wide range of concentrations.   

Worm tunneling across entropic barriers in granular media

We will discuss the dynamics of limbless California blackworms in heterogenous media using model quasi-2D pores separated by a narrow channel. These aquatic worms are known to move using dual undulatory and peristaltic strokes [1]. Tracking the worms as they move across the entropic barrier presented by the channels due their large aspect ratios, we obtain the center of mass and high moments of the mass distribution within the system over long periods. The worms are observed to translocate with increasing probability with decreasing channel length and width. We will discuss the residence time \tau_r and transition time \tau_t as a function of the ratio of the channel to worm width, and the ratio of the worm length to channel length. Comparison will be presented of this active matter system versus thermally activated systems.  The effect of applied fields and entanglements will be further discussed.    

 

[1]: A. Kudrolli and B. Ramirez, PNAS 116, 25569 (2019).