Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session ZC02: Vesicles and Micelles |
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Chair: Vivek Narsimhan, Purdue University Room: Ballroom B |
Tuesday, November 26, 2024 12:50PM - 1:03PM |
ZC02.00001: Hydrodynamics of a vesicle enclosing an active particle suspension Yuan-Nan Young, Bryan Quaife, Herve Nganguia, On Shun Pak, Jie Feng, Vinit Kumar A viscous drop enclosing active particles (such as bacteria, active colloids, or synthetic microswimmers) can propel through its hydrodynamic interactions with the encaged swimmers. A vesicle, often modeled as a self-enclosing inextensible elastic membrane, can encage microswimmers and gain propulsion only in a much more specific way, a stark contrast to a viscous drop. In this work, we examine the hydrodynamics of a vesicle enclosing an active particle suspension using both theoretical analysis and numerical simulations based on an integral formulation, where we show the conditions for the vesicle propulsion in terms of the type of singularity of the microswimmers. Using boundary integral simulations, we show that the formation of a tube from a microswimmer pushing the vesicle membrane is insufficient for vesicle propulsion. |
Tuesday, November 26, 2024 1:03PM - 1:16PM |
ZC02.00002: Analysis of stability of cylindrical multicomponent vesicles Anirudh Venkatesh, Aman Bhargava, Vivek Narsimhan Multicomponent vesicles are sacs of fluid containing multiple phospholipids and cholesterol molecules on the lipid bilayer. The lipid bilayer can phase separate into ordered/disordered domains that give rise to inhomogeneous bending resistances and line tension between domains. These systems give rise to a richer set of hydrodynamic instabilities than simple fluids and single component vesicles. In this study, we inspect the linear and non-linear stability of a multicomponent, cylindrical vesicle by solving the Stokes equations along with the Cahn-Hilliard equations. We delineate the effects of phase separation on pearling and how it aids the process depending on the underlying critical dimensionless variables. We determine the conditions under which axisymmetric and non-axisymmetric modes are unstable and supplement our results with an energy analysis that shows the sources for these instabilities. We provide a comparison between linear and weakly-nonlinear dynamics in this study to shed light on the coarsening dynamics and the dependence on physical parameters. Moreover, we share a qualitative comparison with previous experiments. This study could be instrumental in understanding a multitude of physical phenomena surrounding cells, organelles, and even active fibers. |
Tuesday, November 26, 2024 1:16PM - 1:29PM |
ZC02.00003: Stresses in lipid membrane models James Hanna, Sanjay Dharmavaram The stresses in a membrane model are significant for two reasons. One is that the model-dependent tractions on the surface directly generate the external fluid velocity field in the boundary integral representation of Stokes flows. Another is that the nature of the stresses may or may not be compatible with constitutive restrictions on the membrane itself, such as fluidity. A fluid constitutive model restricts dependence on a referential configuration to information about change in area, and is often presumed to preclude static shear stresses. Geometric bending energies, such as those based on mean curvature, are not as simple as one might assume, as they inherently couple bending and tangential stresses. For this reason, Helfrich-like energies generate shear stresses, despite being fluid, as the deviatoric part of the bending tensor leads to a corresponding deviatoric part of the stress tensor. By contrast, the correct continuum limit of a Seung-Nelson-like bending energy produces a stress with purely isotropic tangential part, despite not being fluid. |
Tuesday, November 26, 2024 1:29PM - 1:42PM |
ZC02.00004: Arbitrary Lagrangian–Eulerian finite element method for lipid membranes Amaresh Sahu Biological membranes are unique two-dimensional materials in which lipids flow in-plane as a Newtonian fluid, while the entire membrane bends out-of-plane as an elastic sheet. Though the dynamical equations governing lipid membranes are known, they are analytically intractable: membrane dynamics are highly nonlinear and involve spatial derivatives on a surface which is itself arbitrarily curved and deforming over time. The challenges in analytically solving the membrane equations extend to their numerical solution as well: standard computational techniques from fluid and solid mechanics cannot model a two-dimensional material with arbitrarily large shape deformations, in-plane flows, and out-of-plane elasticity. We address this issue by developing an arbitrary Lagrangian–Eulerian (ALE) finite element method for lipid membranes. The membrane surface is endowed with a mesh whose in-plane motion can be specified independently of the material velocity; the out-of-plane mesh motion is required to be Lagrangian such that the mesh and material always overlap. A new in-plane mesh motion is implemented, in which the mesh evolves according to the dynamical equations governing a two-dimensional area-compressible viscoelastic fluid film. Our scheme is used to simulate the long-time dynamics of unstable lipid membranes in biologically-relevant scenarios, including the non-axisymmetric buckling and wrinkling of an membrane tube. The surface tension propagation in situations where tethers are pulled from membrane sheets and cylinders is also quantified. Finally, we show how to extend our numerical implementation to describe surfaces with more complex rheology. |
Tuesday, November 26, 2024 1:42PM - 1:55PM |
ZC02.00005: Interaction of per- and polyfluoroalkyl substance with phase-separated phospholipid vesicles Seungsu Han, Sangwoo Shin Per- and polyfluoroalkyl substances (PFAS), a newly emerging pollutant, are surface-active, amphiphilic molecules that are known to strongly interact with biological membranes. Recent studies have indicated that important cellular activities governed by membrane phase separation, such as cell signaling, may be disrupted by PFAS through lipid-PFAS interactions. Here, we experimentally investigate how the phase separation of ternary phospholipid vesicles (DOPC/DPPC/cholesterol) is affected by perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), which are common PFAS compounds widely found in the environment. To uncover how PFAS interacts with each phase domain, we deconstruct the ternary system into solitary (DOPC, DPPC) and binary (DOPC/DPPC, DOPC/chol, DPPC/chol) vesicles, with which PFAS impact is evaluated. Using a microfluidic platform and fluorescence microscopy, we show that phase-separated lipid vesicles undergo various morphological changes upon exposure to PFAS. We rationalize our findings by quantifying the interaction strengths of multicomponent lipid/chol/PFAS mixtures. Our results may provide helpful insights into the potential health impacts of PFAS. |
Tuesday, November 26, 2024 1:55PM - 2:08PM |
ZC02.00006: Modeling of transport of triacylglycerols during lipid droplet formation and growth Vladimir S Ajaev, Kaiyan Zhao, Tobias Walther, Robert Farese Jr. Lipid droplets are cellular membrane-less organelles, consisting of a hydrophobic lipid core enclosed by a monolayer of phospholipids, which act as surfactants with their hydrophobic tails oriented toward the core and their hydrophilic head groups toward the aqueous cytoplasm. Formation and growth of lipid droplets allows cells to store excess fatty acids, thus providing a buffer for fluctuations in demands for energy or membrane lipid synthesis. Despite tremendous progress in understanding the molecular pathways of lipid droplet formation, the mechanisms of transport of triacylglycerols, the main component of the hydrophobic core, into a growing lipid droplet are not fully understood. We developed a model that accounts for the effects of surface tension, viscous flow, and diffusion of triacylglycerols in the endoplasmic reticulum, where their synthesis is taking place. The model predicts the droplet radius as a function of time and shows two phases of growth: a relatively rapid initial increase of the radius followed by slower evolution, in qualitative agreement with experimental observations. The effects of lipid degradation on the droplet surface are discussed. |
Tuesday, November 26, 2024 2:08PM - 2:21PM |
ZC02.00007: Dynamics of multicomponent vesicles in simple shear flow Vivek Narsimhan, Anirudh Venkatesh In biology, cell membranes are often multi-component in nature, made up of multiple phospholipids and cholesterol mixtures that give rise to interesting phase separation and coarsening dynamics. In this talk, we consider the motion of a nearly spherical, giant unilamellar vesicle (GUV) in shear flow, where the lipid membrane is made up of a ternary mixture of a saturated phospholipid, an unsaturated phospholipid, and cholesterol. The bending energy of the vesicle is governed by the Helfrich model and the mixing energy is governed by a Landau-Ginzburg model with an order parameter that represents the phospholipid composition. We use spherical harmonics basis sets to come up with reduced order equations that solve the fluid flow (Stokes equations) in the limit of small deformations (small excess area), as well as the nonlinear Cahn-Hilliard equations for phospholipid distribution on the membrane surface. We observe a wide range of dynamical regimes for shape and concentration, including tank treading, phase treading, swinging, and tumbling depending on the characteristic dimensionless numbers governing the line tension, average bending stiffness, and shear rate. This talk discusses what gives rise to the observed shape and phase separation behaviors. |
Tuesday, November 26, 2024 2:21PM - 2:34PM |
ZC02.00008: Investigating von Willebrand Factor and Extracellular Vesicle Interactions Under Shear Flow Sruthi Chengalrayan, Mustafa Usta The interaction between von Willebrand factor (vWF) and extracellular vesicles (EVs) under hydrodynamic flow conditions remains largely unexplored, despite its potential implications in events such as vascular leakage and systemic coagulation. This study introduces a computational approach using Langevin dynamics to investigate these interactions, focusing on how EV properties impact vWF conformational behavior under varying shear stress conditions. Langevin dynamics accurately captures the stochastic movement and binding behavior of vWF and EVs. This model aims to elucidate how shear stress, particularly above and below the critical threshold for vWF unfurling, along with the physical properties of EVs, such as size and surface charge, influence vWF activity and potentially its role in promoting platelet aggregation and endothelial adhesion. It is observed that charged interactions between vWF and EV alleviate the tendency of vWF unfurling even at elevated shear rates. This implies a change in vWF thrombotic activity. Simulations set the stage for understanding the mechanistic pathways of bonded and nonbonded vWF-EV interactions, which could lead to significant advancements in therapeutic strategies for managing thrombosis and inflammatory diseases. By pioneering this computational approach, we provide a foundation for future research into the biomechanical and biochemical factors that govern vWF and EV interactions in dynamic blood flow environments. |
Tuesday, November 26, 2024 2:34PM - 2:47PM |
ZC02.00009: Amphiphilic Block Copolymer Vesicles Under Shear Flow: A Novel Equlibrated Shear-Induced Structure Radhakrishna Sureshkumar, Senyuan Liu The phase behavior of block copolymer (bcp) solutions (Liu & Sureshkumar, Colloids and Interfaces, 7, 40 (2023); 8, 12 (2024); Liu et al., Colloids and Interfaces, 8, 29 (2024)) and morphology evolution of bcp vesicles under uniform shear flow are studied by molecular dynamics simulations. Flow strength is characterized by the Weissenberg number Wi defined as the ratio of the vesicle shape relaxation time to the inverse shear rate. Above a critical Wi ~ 10, triblock polymer vesicles undergo flow-alignment, stretching and breakup into lamellar fragments. Equilibration of the solution after flow cessation leads to the reorganization of the fragments into a dumbbell-shaped morphology in which two vesicular structures are connected by a dynamic molecular bridge. By quantifying the polymer-solvent interfacial area and individual molecular configurations, the evolution of certain energetic and entropic markers of the system is analyzed suggesting that the dumbbell-like, novel equilibrated shear-induced structure (NoESIS) lowers the system’s free energy. Implications of this inference to flow-mediated irreversible morphology transitions observed in amphiphilic solutions will be discussed (Vasudevan et al., Nature Materials, 9, 436 (2010)). |
Tuesday, November 26, 2024 2:47PM - 3:00PM |
ZC02.00010: ABSTRACT WITHDRAWN
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