Determining a membrane's shear modulus, independent of its area-dilatation modulus, via capsule flow in a converging micro-capillary

Deformation and rupture of microcapsules flowing through constricted capillary

The dynamics of deformable microcapsules flowing through constricted channels is relevant in target delivery of chemicals in physiological systems, porous media, microfluidic medical diagnostic devices and many other applications. In some situations, the microcapsules need to sustain the stress they are subjected to as they flow through constricted channels and in others, the stress may be the rupture trigger used to release the internal content. We experimentally investigate the flow of monodispersed gellan gum microcapsules through a constricted capillary tube by measuring the evolution of the pressure difference and flow visualization. The maximum pressure difference and capsule deformation is obtained for capsules with different diameter and shell thickness. We map the conditions, e.g. diameter and shell thickness, at which the capsule membrane ruptures during the flow, releasing its internal phase.

Capsule Migration and Deformation in a Converging Micro-Capillary

The lateral migration of elastic capsules towards a microchannel centerline plays a major role in industrial and physiological processes. Via our computational investigation, we show that a constriction connecting two straight microchannels facilitates the lateral capsule migration considerably, which is relatively slow in straight channels. Our work reveals that the significant cross-streamline migration inside the constriction is dominated by the strong hydrodynamic forces due to the capsule size. However, in the downstream straight channel, the increased interfacial deformation at higher capillary numbers or a lower viscosity ratio and lower membrane hardness results in increased lateral cross-streamline migration. Thus, our work highlights the different migration mechanisms occurring over curved and straight streamlines.

Motion of an Elastic Capsule in a Trapezoidal Microchannel Under Stokes Flow Conditions

Even though the research interest in the last decades has been mainly focused on the capsule dynamics in cylindrical or rectangular ducts, channels with asymmetric cross-sections may also be desirable especially for capsule migration and sorting. Therefore, in the present study we investigate computationally the motion of an elastic spherical capsule in an isosceles trapezoidal microchannel at low and moderate flow rates under the Stokes regime. The steady-state capsule location is quite close to the location where the single-phase velocity of the surrounding fluid is maximized. Owing to the asymmetry of the trapezoidal channel, the capsule's steady-state shape is asymmetric while its membrane slowly tank-treads. In addition, our investigation reveals that tall trapezoidal channels with low base ratios produce significant off-center migration for large capsules compared to that for smaller capsules for a given channel length. Thus, we propose a microdevice for the sorting of artificial and physiological capsules based on their size, by utilizing tall trapezoidal microchannels with low base ratios. The proposed sorting microdevice can be readily produced via glass fabrication or as a microfluidic device via micromilling, while the required flow conditions do not cause membrane rupture.

Cooperative size sorting of deformable particles in porous media

Diverse applications-ranging from enhanced oil recovery, filtration, and lab on a chip sorting-rely on the flow-induced transport of deformable particles in porous media. However, how fluid flow can force such particles to squeeze through pore constrictions of complex geometries is poorly understood. Here, we study the transport of model deformable particles in millifluidic porous media with constrictions of tunable aspect ratio. We find that multiple particles can unexpectedly squeeze through large-aspect ratio constrictions, even when isolated particles cannot. This phenomenon arises from pairwise flow-mediated interactions between the particles: when one particle is trapped at a constriction, the increased fluid flow around it enables a second to squeeze past due to locally increased hydrodynamic stresses. This cooperative mechanism causes the particles to ultimately sort themselves by size through the pore space. By revealing a new mode of deformable particle transport in porous media, our work helps to inform real-world applications and provides a straightforward way to sort particles based on size.

Elastic particle deformation in rectangular channel flow as a measure of particle stiffness

In this study, we experimentally observed and characterized soft elastic particle deformation in confined flow in a microchannel with a rectangular cross-section. Hydrogel microparticles of pNIPAM were produced using two different concentrations of crosslinker. This resulted in particles with two different shear moduli of 13.3 ± 5.5 Pa and 32.5 ± 15.7 Pa and compressive moduli of 66 ± 10 Pa and 79 ± 15 Pa, respectively, as measured by capillary micromechanics. Under flow, the particle shapes transitioned from circular to egg, triangular, arrowhead, and ultimately parachute shaped with increasing shear rate. The shape changes were reversible, and deformed particles relaxed back to circular/spherical in the absence of flow. The thresholds for each shape transition were quantified using a non-dimensional radius of curvature at the tip, particle deformation, circularity, and the depth of the concave dimple at the trailing edge. Several of the observed shapes were distinct from those previously reported in the literature for vesicles and capsules; the elastic particles had a narrower leading tip and a lower circularity. Due to variations in the shear moduli between particles within a batch of particles, each flow rate corresponded to a small but finite range of capillary number (Ca) and resulted in a series of shapes. By arranging the images on a plot of Ca versus circularity, a direct correlation was developed between shape and Ca and thus between particle deformation and shear modulus. As the shape was very sensitive to differences in shear modulus, particle deformation in confined flow may allow for better differentiation of microparticle shear modulus than other methods.

Off-center motion of a trapped elastic capsule in a microfluidic channel with a narrow constriction

Owing to their significance in capsule-related engineering and biomedical applications, a number of studies have considered the dynamics of elastic capsules flowing in constricted microchannels. However, these studies have focused on capsules moving along the channel centerline. In the present study, we numerically investigate the transient motion of an elastic capsule in a microfluidic channel with a rectangular constriction, which is initially trapped at the constriction inlet while off the channel centerline (i.e., on the channel bottom-wall). Under the push of the surrounding flow, the capsule can squeeze into the constriction, but only if the capsule deformability or the constriction size is sufficiently large. We find that the critical capillary number leading to the penetration of the capsule into the constriction is larger for off-centerline capsules compared to centered capsules. The centered capsule is stationary at the steady state when it remains stuck at the constriction; in contrast, the off-centerline capsule is not stationary but exhibits a tank-treading motion, i.e., its overall shape maintains a nonspherical shape with a protrusion into the constriction while its membrane exhibits a continuous rotation. Further, we examine the dependence of the capsule motion type, capsule deformation degree and membrane tension distribution on the capillary number (measuring the effects of flow strength and membrane mechanics) and constriction geometries (including the constriction height and width). Finally, we discuss the mechanism governing the capsule motion by analyzing the hydrodynamic forces acting on the capsule. The shear force acting on the capsule top owing to the fluid flow in the gap between the capsule top and the channel top-wall is the main source inducing the membrane tank-treading rotation.

Deformation of double emulsions under conditions of flow cytometry hydrodynamic focusing

Water-in-oil-in-water (w/o/w) microfluidics double emulsions offer a new route to compartmentalise reagents into isolated aqueous microenvironments while maintaining an aqueous carrier fluid phase; this enables compatibility with commercial flow cytometry systems such as fluorescence-activated cell sorting (FACS). Double emulsion (inner core) deformation under hydrodynamic focusing conditions that mimic the environment double emulsions experience in flow cytometry applications is of particular importance for droplet stability and cell viability. This paper reports on an experimental study of the dynamic deformation of aqueous cores of w/o/w double emulsions under hydrodynamic focusing, with the sheath flow directed at 45° to the sample flow. A number of factors affecting the inner core deformation and recovery were examined. Deformation was found to depend significantly on the core or shell viscosity, the droplet-to-sheath flow velocity ratio, and core and shell sizes. Core deformation was found to depend more on the type of surfactant rather concentration with high molecular weight surfactant exhibiting a negligible effect on deformation whereas low molecular weight surfactant enhancing deformation at low concentrations due to their lateral mobility at the interface.