Dynamic flow behaviour of surfactant vesicles under shear flow: role of a multilamellar microstructure

Abstract

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

Microfluidic interactions between red blood cells and drug carriers by image analysis techniques

“…Thus, RBC velocity is lower than the velocity of the suspending fluid in absence of RBCs (continuous line). Similar results are found for other deformable objects, such as liquid vesicles (Misbah, 2012;Pommella et al, 2013) and capsules (Bagchi and Kalluri, 2009;Misbah, 2012), which are considered as model systems of actual RBCs. It can be noticed that -Ps velocity in presence of RBCs is lower than velocity of the solution made by -Ps only (Fig.…”

Dynamic behaviour of multilamellar vesicles under Poiseuille flow

“…MLVs) with random orientation. 50 This type of microstructures was already observed through polarized microscopy and transmission electron microscopy in several non-ionic surfactants water solutions 51 and we previously 18 showed the same polycrystalline microstructure for the HLAS surfactant as well. Interestingly, an internal microstructure made by dispersed L domains has been reported at a concentration even lower than 5%.…”

“…This is likely due to the presence of defects in the structure of MLVs (e.g., images c, d, g, h in Figure 6); these defects have also been reported in a previous paper. 18 It is worth mentioning that the presence of these internal defects does not allow MLVs to assume an axial-symmetric shape even at high value of the confinement degree (Figure6 c, f, i). In any event, MLV shapes are apparently more axialsymmetric at increasing values of Ca.…”

“…This could be due to the lower deformability of the MLV structure, which is made of a stack of several bilayers, as compared to the single bilayer structure of unilamellar vesicles. 18 The formation of filaments coming out of MLVs suggests a breakup mechanism similar to droplet tip-streaming (image g in Figure 6). A quantitative investigation of the dynamic behaviour of MLVs under Poiseuille flow is carried out by analysing the MLV velocity U as shown in Figure 7.…”

“…16 We recently reported an experimental investigation on the dynamic behaviour of single MLVs under simple shear flow. 17,18 In this flow field, MLVs dynamics presents analogies with unilamellar vesicles and emulsion droplets. MLVs deform at constant volume under flow with a rich dynamic behaviour (i.e.…”