title: Experimental observation of the asymmetric instability of intermediate-reduced-volume vesicles in extensional flow.
abstract: Vesicles provide an attractive model system to understand the deformation of living cells in response to mechanical forces. These simple, enclosed lipid bilayer membranes are suitable for complementary theoretical, numerical, and experimental analysis. A recent study [Narsimhan, Spann, Shaqfeh, J. Fluid Mech., 2014, 750, 144] predicted that intermediate-aspect-ratio vesicles extend asymmetrically in extensional flow. Upon infinitesimal perturbation to the vesicle shape, the vesicle stretches into an asymmetric dumbbell with a cylindrical thread separating the two ends. While the symmetric stretching of high-aspect-ratio vesicles in extensional flow has been observed and characterized [Kantsler, Segre, Steinberg, Phys. Rev. Lett., 2008, 101, 048101] as well as recapitulated in numerical simulations by Narsimhan et al., experimental observation of the asymmetric stretching has not been reported. In this work, we present results from microfluidic cross-slot experiments observing this instability, along with careful characterization of the flow field, vesicle shape, and vesicle bending modulus. The onset of this shape transition depends on two non-dimensional parameters: reduced volume (a measure of vesicle asphericity) and capillary number (ratio of viscous to bending forces). We observed that every intermediate-reduced-volume vesicle that extends forms a dumbbell shape that is indeed asymmetric. For the subset of the intermediate-reduced-volume regime we could capture experimentally, we present an experimental phase diagram for asymmetric vesicle stretching that is consistent with the predictions of Narsimhan et al.
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA 94720-1460, USA. muller2@berkeley.edu.
authors.forename: Joanna B
authors.initials: JB
authors.lastname: Dahl
authors.affiliation: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Vivek
authors.initials: V
authors.lastname: Narsimhan
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA 94720-1460, USA. muller2@berkeley.edu.
authors.forename: Bernardo
authors.initials: B
authors.lastname: Gouveia
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA 94720-1460, USA. muller2@berkeley.edu and Department of Bioengineering, University of California, Berkeley, Berkeley, CA 94720-1762, USA.
authors.forename: Sanjay
authors.initials: S
authors.lastname: Kumar
authors.affiliation: Department of Chemical Engineering, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA and Institute of Computational and Mathematical Engineering, Stanford University, Stanford, CA 94305, USA.
authors.forename: Eric S G
authors.initials: ES
authors.lastname: Shaqfeh
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA 94720-1460, USA. muller2@berkeley.edu.
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Muller
mesh_terms: D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry*; D008956:Models, Chemical
chemical_list: D008051:Lipid Bilayers; D008081:Liposomes