Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B15: Flow of Complex Fluids: Rheology, Structure, and Instabilities II |
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Sponsoring Units: DFD DSOFT Chair: Ehssan Nazockdast, Univ of NC - Chapel Hill Room: 210/212 |
Monday, March 2, 2020 11:15AM - 11:51AM |
B15.00001: Cell nucleus as a mcirorheological probe to study the rheology of the cytoskeleton Ehssan Nazockdast, Moslem Moradi Mechanical properties of the cell are important biomarkers for probing its pathological changes, and are increasingly used for cancer diagnosis and detecting rare cells. Yet, determining the time-dependent contributions of different cellular components --including the cell membrane, the cell cytoskeleton and the nucleus-- to the mechanical response of the whole cell has remained challenging. We propose a novel method to decouple the mechanics of membrane and cytoskeleton, by analyzing the correlation between the membrane deformations that are induced by external microfluidic flows and nucleus displacements induced by those membrane deformations i.e. we use the nucleus as a microrheological probe to study the rheology of the interior cytoskeleton, independent of membrane rheology. To demonstrate the applicability of this method, we consider a proof of concept model consisting of a rigid spherical nucleus centered in a spherical membrane. We obtain analytical expressions for time-dependent nucleus velocity as a function of membrane deformations, when the interior cytoskeleton is modelled as a viscoelastic and a poroelastic material, and demonstrate how the nucleus velocity can be used to characterize the rheology of the cytoskeleton over a wide range of forces and time-scales. |
Monday, March 2, 2020 11:51AM - 12:03PM |
B15.00002: The divergence-conforming immersed boundary method: Application to vesicles, capsules, and red blood cells under flow Hugo Casquero, Yongjie Jessica Zhang The divergence-conforming immersed boundary (DCIB) method is presented to tackle a long-standing issue of immersed boundary (IB) numerical methods for fluid-structure interaction (FSI), namely, the challenge of accurately imposing the incompressibility constraint at the discrete level. In the DCIB method, the Eulerian velocity-pressure pair is discretized using divergence-conforming B-splines, which leads to inf-sup stable, H1-conforming, and pointwise divergence-free Eulerian solutions. In order to discretize the higher-oder derivatives that appear in vesicle and capsule formulations, we use C2-continuous cubic B-splines with periodic knot vectors and C1-continuous bi-cubic analysis-suitable T-splines in 2D and 3D settings, respectively. Non-negligible spurious changes of the fluid volume inside of closed co-dimension one solids is a well-known issue of IB methods. The DCIB method results in volume changes various orders of magnitude lower than conventional IB methods. Benchmark and application problems of vesicle, capsules and red blood cells are solved, including mesh-independence studies and comparisons with other numerical methods. |
Monday, March 2, 2020 12:03PM - 12:15PM |
B15.00003: Flow-induced microstructural rearrangement and rheological consequences for model red blood cell suspensions in microvessels Yeng-Long Chen, Chih-Tang Liao Flow-induced structural evolution of soft particle clusters in microflow is a key driver of non-Newtonian fluid response. We investigated how inter-particle attraction affect cluster and particle migration in microvessels, where the near wall particle depletion layers due to hydrodynamics significantly alters fluid viscosity. We found that strong particle aggregation correspond to structurally intact large clusters at low shear rates, resulting in large near wall depletion layer and low suspension viscosity. Shear-induced cluster break-up at moderate shear rates reduces the wall depletion layer, corresponding to viscosity increase akin to shear-thickening. At high shear rates, particle deformation and inter-particle ordering leads to shear-thinning. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B15.00004: Cross-stream migration of non-spherical particles in a second-order fluid Shiyan WANG, Cheng-Wei Tai, Vivek Narsimhan Particle migration in viscoelastic suspensions is vital in many applications in the biomedical community and the energy industries. Previous studies have provided insight on the motion of spherical particles in simple viscoelastic flows, yet the combined effect of more complex flow profiles and particle shapes is underexplored. Here, we develop approximate, analytical expressions for the polymeric force and torque on an arbitrary-shaped particle in a second-order fluid, subject to a general quadratic flow field. This model is exact for the case when the first and second normal stress coefficients satisfy ψ1 = -2ψ2. In shear driven flows, we observe that spheroidal particles adjust their orientation to align their longer axis along the vorticity direction, although significant deviations from slender body theories occur for finite aspect ratios. In pressure driven flows, we identify scaling theories to quantify how the particle lift depends on shape for a wide variety of shapes. We find that prolate particles slowly transition to a log-rolling state as they approach the flow center, with the lift initially being larger than that of an equal-volume sphere, but then becoming smaller as log-rolling emerges. Lastly, we extend our analysis towards more complicated systems (ψ1 ≠ -2ψ2). |
Monday, March 2, 2020 12:27PM - 12:39PM |
B15.00005: Vortices in 2D Colloids Myeonggon Park, Bo Li, Steve Granick Particles and molecules composing real materials generally contain anisotropic properties such as a spin or an electric dipole. To understand their roles in material phase states, we observed colloidal crystals comprised of Janus colloids whose anisotropic properties can be controlled. Here, we found a BKT transition by tuning an interparticle interaction and could see dynamics of topological defects in real time. We think that this model system can eventually give an insight to apprehend roles of molecular and atomic orientational dynamics in 2D crystal phases. |
Monday, March 2, 2020 12:39PM - 12:51PM |
B15.00006: On the mechanisms of superspreading Panagiotis Theodorakis, Edward Smith, Erich A Müller, Richard V Craster, Omar K Matar Superspreading is the rapid and complete spreading of surfactant-laden droplets on hydrophobic substrates, which is caused by certain surfactant molecules known as superspreaders. Despite significant experimental efforts, the precise mechanisms of this phenomenon and the characteristic properties of superspreading surfactants had remained elusive. Here, we report on extensive molecular dynamics simulations of a coarse-grained model based on the SAFT force-field. Our studies highlight the mechanisms of superspreading, the features of superspreading surfactants and a range of parameters that affect the spreading efficiency of surfactant-laden droplets. We anticipate that our investigations will pave the way for the design of molecular architectures tailored specifically for applications that rely on the control of wetting. |
Monday, March 2, 2020 12:51PM - 1:03PM |
B15.00007: Microfluidic flow-processing of soft matter systems Liva Donina, Joao Cabral Arguably one of the most striking properties of soft matter systems is the propensity to have a major response to minor perturbations. One of such perturbations is flow, which allows to induce phase changes and structural rearrangements. Here, two pseudo-quaternary systems containing sodium dodecyl sulfate (SDS) and medium to long chain alcohols as co-surfactants in a lamellar phase (Lα) are studied. The structural transformation from planar lamellar sheets to multilamellar vesicles (MLVs) is observed upon the application of shear stress to the solution. This transformation is characterised with nuclear magnetic resonance (NMR), rheology and small angle neutron scattering (SANS). Furthermore, the kinetics of the transformation is resolved with realtime measurements within a microfluidic device to obtain well-defined shear fields. Time-resolved kinetics are investigated by tracking the changes in birefringence pattern employing cross-polarised microscopy. The results point to MLV formation and indicate that it is a time-dependent and shearrate dependent process. The combination of microfluidics with high resolution spatio-temporal measurements allows characterisation of out-of-equilibrium transformations in complex surfactant mixtures. |
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