Vesicle and cell rupture caused by large viscous stresses in ultrasonication is central to biomedical and bioprocessing applications. The flow-induced opening of lipid membranes can be exploited to deliver drugs into cells, or to recover products from cells, provided that it can be obtained in a controlled fashion. Here we demonstrate that differences in lipid membrane and vesicle properties can enable selective flow-induced vesicle break-up. We obtained vesicle populations with different membrane properties by using different lipids (SOPC, DOPC, or POPC) and lipid:cholesterol mixtures (SOPC:chol and DOPC:chol). We subjected vesicles to large deformations in the acoustic microstreaming flow generated by ultrasound-driven microbubbles. By simultaneously deforming vesicles with different properties in the same flow, we determined the conditions in which rupture is selective with respect to the membrane stretching elasticity. We also investigated the effect of vesicle radius and excess area on the threshold for rupture, and identified conditions for robust selectivity based solely on the mechanical properties of the membrane. Our work should enable new sorting mechanisms based on the difference in membrane composition and mechanical properties between different vesicles, capsules, or cells.
Ultrasound-driven microbubble dynamics are central to biomedical applications, from diagnostic imaging to drug delivery and therapy. In therapeutic applications, the bubbles are typically embedded in tissue, and their dynamics are strongly affected by the viscoelastic properties of the soft solid medium. While the behaviour of bubbles in Newtonian fluids is well characterised, a fundamental understanding of the effect on ultrasound-driven bubble dynamics of a soft viscoelastic medium is still being developed. We characterised the resonant behaviour in ultrasound of isolated microbubbles embedded in agarose gels, commonly used as tissue-mimicking phantoms. Gels with different viscoelastic properties were obtained by tuning agarose concentration, and were characterised by standard rheological tests. Isolated bubbles (100-200 µm) were excited by ultrasound (10-50 kHz) at small pressure amplitudes (< 1 kPa), to ensure that the deformation of the material and the bubble dynamics remained in the linear regime. The radial dynamics of the bubbles were recorded by high-speed video microscopy. Resonance curves were measured experimentally and fitted to a model combining the Rayleigh-Plesset equation governing bubble dynamics, with the Kelvin-Voigt model for the viscoelastic medium. The resonance frequency of the bubbles was found to increase with increasing shear modulus of the medium, with implications for optimisation of imaging and therapeutic ultrasound protocols. In addition, the viscoelastic properties inferred from ultrasound-driven bubble dynamics differ significantly from those measured at low frequency with the rheometer. Hence, rheological characterisation of biomaterials for medical ultrasound applications requires particular attention to the strain rate applied.
Surfactant multilamellar vesicles (SMLVs) play a key role in the formulation of many industrial products, such as detergents, foodstuff, and cosmetics. In this Letter, we present the first quantitative investigation of the flow behavior of single SMLVs in a shearing parallel plate apparatus. We found that SMLVs are deformed and oriented by the action of shear flow while keeping constant volume and exhibit complex dynamic modes (i.e., tumbling, breathing, and tank treading). This behavior can be explained in terms of an excess area (as compared to a sphere of the same volume) and of microstructural defects, which were observed by 3D shape reconstruction through confocal microscopy. Furthermore, the deformation and orientation of SMLVs scale with radius R in analogy with emulsion droplets and elastic capsules (instead of R(3), such as in unilamellar vesicles). A possible application of the physical insight provided by this Letter is in the rationale design of processing methods of surfactant-based systems.
Recently, optical tweezing has been
used to provide a method for
microrheology addressed to measure the rheological properties of small
volumes of samples. In this work, we corroborate this emerging field
of microrheology by using these optical methods for the characterization
of polyelectrolyte solutions with very low viscoelasticity. The influence
of polyelectrolyte (i.e., polyacrylamide, PAM) concentration, specifically
its aging, of the salt concentration is shown. The close agreement
of the technique with classical bulk rheological measurements is demonstrated,
illustrating the advantages of the technique.
Surfactant based systems are ubiquitous in everyday life products, including food, pharmaceuticals, and
detergents. In concentrated solutions, surfactants may assemble into multilamellar, onion-like vesicles,
whose size distribution is known to be affected by the action of flow during processing and usage.
However, very little is known about the dynamic behaviour and the mechanisms by which the packed
microstructure of a surfactant multilamellar vesicle (MLV) is deformed under flow. In this work, the
microstructure of surfactant MLVs is investigated both at rest by confocal microscopy and under flow by
a rheo-optical parallel plate apparatus using bright field and polarized light microscopy. Small MLVs
have a more homogeneous multilamellar structure extending to the vesicle centre and show small
deformation regimes, such as tumbling and breathing. Larger MLVs have an inner isotropic core
surrounded by a multilamellar shell and can also exhibit a tank-treading deformation mode depending
on the flow conditions. The main result of this work is that tank-treading is associated with the
formation of parabolic focal conic defects in the outer shell, which is driven by shear-induced bilayer
dilation and undulation as in smectic A liquid crystals, and with convection-induced surfactant rearrangement
at a constant shell volume. This finding is supported by the analysis of vesicle retraction
upon cessation of flow, which scales with the inverse third power of radius and thus is consistent with
the effect of a compressibility modulus
We present a new light scattering setup coupled to a commercial rheometer operated in the plateplate geometry. The apparatus allows the microscopic dynamics to be measured, discriminating between the contribution due to the affine deformation and additional mechanisms, such as plasticity. Light backscattered by the sample is collected using an imaging optical layout, thereby allowing the average flow velocity and the microscopic dynamics to be probed with both spatial and temporal resolution. We successfully test the setup by measuring the Brownian diffusion and flow velocity of diluted colloidal suspensions, both at rest and under shear. The potentiality of the apparatus are explored in the startup shear of a biogel. For small shear deformations, γ ≤ 2%, the rheological response of the gel is linear. However, striking deviations from affine flow are seen from the very onset of deformation, due to temporally and spatially heterogeneous rearrangements bearing intriguing similarities with a stick-slip process.
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