Session Y06: Particle-Laden Flows: Deformable Particles (11:30am - 12:15pm CST)

Under Pressure: Mechanics of Swelling Hydrogels Under Confinement

Hydrogels hold promise in agriculture as reservoirs of water in dry soil, potentially alleviating the burden of irrigation. However, confinement in soil can drastically reduce the ability of hydrogels to absorb water and swell---in some cases, by as much as 90 vol{\%}---limiting their wide-spread adoption. Unfortunately, the underlying reason remains unknown. By directly visualizing the swelling of hydrogels confined in three-dimensional (3D) granular media, we demonstrate that the extent of hydrogel swelling is determined by the competition between the force exerted by the hydrogel due to osmotic swelling and the confining force transmitted by the surrounding grains. Further, we demonstrate that the medium can itself be restructured by hydrogel swelling, as set by the balance between the osmotic swelling force, the confining force, and inter-grain friction. Together, our results provide quantitative principles to predict how hydrogels behave in confinement, potentially improving their use in agriculture as well as informing other applications such as oil recovery, construction, mechanobiology, and filtration.   

Deformation and migration of finite-sized bubbles in turbulent channel flows

Through extensive efforts in measuring bubble dynamics in turbulence, we have developed two important models for bubbles, one for its center of mass motion and the other one for its deformation. In this talk, I will present a new effort to implement these two models together along with a background turbulent channel flows by using the JHU turbulence database. In particular, because the carrier phase is pre-calculated, the simulation can be repeated for many bubbles of different sizes and surface tension. The results will be compared with well-established data in bubble columns to understand how turbulence-bubble interaction at different flow rates modulates the bubble concentration at different radial locations.   

Experimental characterization of the breakup strength of marine diatom aggregates in oscillatory shear flow

While the aggregation of marine snow has been studied extensively, little is known about its breakup in response to turbulence, which limits our ability to quantify and predict particulate mass transport in marine ecosystems. To understand the hydrodynamic influence on the size of this particulate matter, we conducted breakup experiments on aggregates of laboratory-cultured diatoms, which are an abundant type of phytoplankton in the ocean. First, we produced marine aggregates in artificial sea water inside a cylindrical tank with solid body rotation. Next, we superimposed a harmonic oscillation at the tank wall and thus exposed the aggregates to a laminar oscillatory shear flow, which we quantified analytically. The local velocity gradients inside this facility were similar in magnitude to shear in the surface ocean. Finally, we implemented particle tracking techniques to identify and capture the breakup events with a high-speed camera and determined the local fluid shear stress causing aggregate breakup. With a large database of disruption events, we discuss the breakup behaviors of diatom aggregates in a size range from 100 $\mu $m to 10 mm and quantify their breakup strength.   

Bending of elastic fibers in shear flow

We investigate numerically the dynamics of an elastic fiber in a shear flow at low Reynolds number. The results are based on numerical simulations in the bead-spring model with hydrodynamic interactions evaluated in the Rotne-Prager-Yamakawa approximation and a precise multipole expansion corrected for lubrication forces. The fiber is initially straight and oriented in the flow direction. Typical time scales of the bending process and fiber shapes and their maximum curvatures are analyzed. We find universal scalings and identify three different dynamical modes in the phase space of the fiber bending stiffness A and aspect ratio n. A comparison with the elastica model is also performed, assuming as a new theoretical input the existence of a hydrodynamic force acting on the fiber tip.   

Dynamics of Semiflexible Colloidal Sheets in Shear Flow

As 2D materials such as graphene, transition metal dichalcogenides, and 2D polymers become more prevalent, solution processing and colloidal-state properties are being exploited to create advanced and functional materials. However, our understanding of the fundamental behavior of 2D colloidal sheets and membranes in flow is still lacking. In this work, we perform Brownian dynamics simulations of semiflexible colloidal sheets with hydrodynamic interactions in shear flow. For athermal sheets, buckling instabilities of different mode numbers are found to vary with bending stiffness and can be understood with simple elasticity arguments. Colloidal sheets exhibit different conformational behaviors that range from seemingly chaotic towel-wringing to periodic tumbling as parameters such as bending stiffness and shear flow strength are varied.   

Precision measurements of deformations of flexible particles in turbulent flows

We measure the deformation of flexible particles in the flow of a turbulent conical Taylor-Couette flow and a vertical water tunnel which generates homogeneous isotropic turbulence. The particles are tetrads and are 3D printed from 2 different polymers, they are made up of 4 slender arms separated by the tetrahedral angle (approximately 109.5 degrees) connected at the center by a weak joint. The joints are made from a soft flexible polymer and the arms are made from a rigid polymer allowing all deformation to be concentrated at the flexible joint. The particles are 2cm in diameter and have an effective ellipsoid which is a sphere, thus under small deformations, the particle will rotate with the same rate as the fluid rotation rate. As a result, the relative velocity between the arm of the particle and the fluid is due to the strain component of the velocity gradient tensor which causes the particle arms to bend. We are able to measure the small arm deformations experienced by the particle using 4 high-speed, high-resolution cameras. The arm deformations are larger in the Taylor-Couette apparatus compared to the vertical water tunnel since we are able to use other fluids that are more viscous than water.