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, Journal of Fluid Mechanics, 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, Physics Review Letters, 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.

Measuring Cell Viscoelastic Properties Using a Microfluidic Extensional Flow Device

“…In our study, although we did observe rolling of cells with visible defects in the entrance channels of the cross-slot device (shear dominant Poiseuille flow), we never observed tumbling or rolling in the stagnation-point region (pure extensional flow), only stretching. The existence of one deformation mode in extensional flow-stretching-is expected from cell-mimetic vesicle simulations (60,61) and experiments (37,62). Thus, the differences in our observation of one deformation mode compared to the previous four modes of deformation are a result of the different flow fields.…”

Conformational dynamics and phase behavior of lipid vesicles in a precisely controlled extensional flow

“…We use the method proposed by Pécréaux et al [57] to determine vesicle bending modulus. In this way, we follow a rigorous selection criteria outlined in prior work [32,58] that provides an unbiased procedure for rejecting unsuitable vesicles that do not fluctuate according to an analytical fluctuation spectrum given by the Helfrich model [59]. Vesicle contours are first detected in each image with high precision using a custom MATLAB program that relies on intensity gradient maxima values to locate the edges ( ESI †, Fig.…”

“…For these experiments, the membrane contour is located with a high precision using the edge detection method discussed in Section 2.4. To determine reduced volume, we follow the approach by Dahl et al [32]. In brief, the surface area and volume of a vesicle are estimated by revolution of the observed 2D membrane contour along the vesicle's short axis (ESI †, Fig.…”

Fluctuation spectroscopy of giant unilamellar vesicles using confocal and phase contrast microscopy

“…The fluctuating amplitudes have variance dependent on the membrane bending rigidity and tension. Imaging is most commonly done by phase contrast microscopy [1,7,15,[17][18][19][20][21][22] but other methods such as confocal [23][24][25] and light sheet microscopy [26] have also been employed. The increased variety of imaging methods raises the question if they all yield the same results.…”

“…It is known that many factors such as sugars (and gravity), salt, buffers, solution asymmetry, concentration of fluorescent lipids, preparation method or type of bilayer configuration (stacked or free-floating), influence the measured mechanical properties of bilayer membranes [1,18,[27][28][29][30][31]. For example, even measurements with the same method can give wide range of values, e.g., the bending rigidity of a DOPC bilayer measured with flickering spectroscopy has been reported form 15 k B T [24] to 27 k B T [32], where k B T is the thermal energy; see Table 1 in the Supporting Information (SI) for a comprehensive list.…”