Vesicle dynamics in large amplitude oscillatory extensional flow

Abstract

Although the behaviour of fluid-filled vesicles in steady flows has been extensively studied, far less is understood regarding the shape dynamics of vesicles in time-dependent oscillatory flows. Here, we investigate the nonlinear dynamics of vesicles in large amplitude oscillatory extensional (LAOE) flows using both experiments and boundary integral (BI) simulations. Our results characterize the transient membrane deformations, dynamical regimes and stress response of vesicles in LAOE in terms of reduced volume (vesicle asphericity), capillary number ( ${Ca}$ , dimensionless flow strength) and Deborah number ( ${De}$ , dimensionless flow frequency). Results from single vesicle experiments are found to be in good agreement with BI simulations across a wide range of parameters. Our results reveal three distinct dynamical regimes based on vesicle deformation: pulsating, reorienting and symmetrical regimes. We construct phase diagrams characterizing the transition of vesicle shapes between pulsating, reorienting and symmetrical regimes within the two-dimensional Pipkin space defined by ${De}$ and ${Ca}$ . Contrary to observations on clean Newtonian droplets, vesicles do not reach a maximum length twice per strain rate cycle in the reorienting and pulsating regimes. The distinct dynamics observed in each regime result from a competition between the flow frequency, flow time scale and membrane deformation time scale. By calculating the particle stresslet, we quantify the nonlinear relationship between average vesicle stress and strain rate. Additionally, we present results on tubular vesicles that undergo shape transformation over several strain cycles. Broadly, our work provides new information regarding the transient dynamics of vesicles in time-dependent flows that directly informs bulk suspension rheology.

Dynamics and active mixing of a droplet in a Stokes trap

“…Lin et al. 2021), these boundary conditions should be imposed on a section of a channel branch away from the intersection to more accurately model typical experimental set-ups, either by simulating the droplet in the full microfluidic domain or by using a moving-frame construction like the one described in Roure et al. (2023) (see figure 1 c ), in which the frame changes with time as the drop moves and/or deforms.…”

“…Besides the droplet or vesicle size, the dynamics of the problem is strongly affected by changes in shape, especially when the droplets/vesicles undergo large deformations. Although some recent works tackled the deformation of vesicles (Kumar et al 2020a,b;Lin et al 2021) and control of small droplets using a Stokes trap (Narayan et al 2020a,b), the complex dynamics of drop deformation and response to different flow modes have not been simulated. However, recently, the work by Razzaghi & Ramachandran (2023) has shown experimentally that richer flow modes produced by hydrodynamic traps, such as a quadratic flow, can result in interesting drop shapes.…”

“…2020 a , b ; Kumar & Schroeder 2021; Lin et al. 2021) and control of small droplets using a Stokes trap (Narayan et al. 2020 a , b ), the complex dynamics of drop deformation and response to different flow modes have not been simulated.…”

Nonlinear Transient and Steady State Stretching of Deflated Vesicles in Flow

“…Interestingly, it was found that vesicle relaxation is governed by a double-mode process, with the first stage involving relaxation of the thin lipid nanotube connecting the two spherical bulbs in the dumbbell. , Vesicle dynamics in a time-dependent extensional flow with a single flow-reversal step was previously reported . Recently, vesicle dynamics in large-amplitude oscillatory extensional flow was studied using a combination of precision flow experiments using the Stokes trap and numerical modeling and simulations . Overall, vesicle stretching in extensional flows generally involves distributed viscous forces across the entire membrane for freely suspended bodies in the absence of external forces.…”

“…50 Recently, vesicle dynamics in large-amplitude oscillatory extensional flow was studied using a combination of precision flow experiments using the Stokes trap and numerical modeling and simulations. 51 Overall, vesicle stretching in extensional flows generally involves distributed viscous forces across the entire membrane for freely suspended bodies in the absence of external forces. From this view, vesicle deformation in extensional flow is distinct from classical methods used to deform vesicle membranes, e.g.…”