Abstract
The lamellar-to-multilamellar vesicle (MLV) transformation in a model surfactant system, sodium dodecyl sulfate (SDS), octanol and brine, is investigated under continuous and oscillatory microfluidic contraction-expansion flows, employing polarised optical microscopy and small angle neutron scattering (SANS), with sample volume probed down to ≃20 nL. We determine the lamellar-to-MLV transition requirements at varying flow velocity, oscillation amplitude, frequency, and number of oscillatory cycles. The spatio-temporal evolution of the hierarchical fluid structure is elucidated: lamellar sheets initially align with flow direction upon entering a constriction and then perpendicularly upon exiting; the formation of MLVs at the nanoscale is first observed by SANS within a few (<5) oscillatory cycles, followed by the gradual appearance of a regular (albeit not crystalline) MLV arrangement, at the micronscale, by optical microscopy after tens of cycles, under the conditions investigated. Once MLVs form under flow, these remain metastable for several days.
Highlights
Surfactants in solution can self-assemble into ordered mesophases, known as lyotropic liquid crystalline phases,[1] which are generally responsive to flow, changing macro- and microscale properties upon application of shear
The spatio-temporal evolution of the hierarchical fluid structure is elucidated: lamellar sheets initially align with flow direction upon entering a constriction and perpendicularly upon exiting; the formation of multilamellar vesicle (MLV) at the nanoscale is first observed by small angle neutron scattering (SANS) within a few (o5) oscillatory cycles, followed by the gradual appearance of a regular MLV arrangement, at the micronscale, by optical microscopy after tens of cycles, under the conditions investigated
While the effect of microflow on La phases has been previously investigated by SANS/SAXS,[46,50,51] here we focus on the lamellar to MLV transformation in model microdevice geometries
Summary
Surfactants in solution can self-assemble into ordered mesophases, known as lyotropic liquid crystalline phases,[1] which are generally responsive to flow, changing macro- and microscale properties upon application of shear. In order to precisely control flow type, magnitude and enable the spatiotemporal investigation of flow-induced transformations, we employ microfluidics, coupled with SANS and polarised optical microscopy (and ancillary characterisation approaches). Flow induced self-assembly in microfluidics has been employed to investigate, for example, the formation of wormlike micelles from micellar solutions under extensional flows[41,42,43] and the formation of non-spherical block copolymer nanostructure.[44] The coupling of microfluidics with a range of analytical techniques such as rheology,[45] SANS/SAXS46–48 and NMR49 has enabled the precise, in situ characterisation of minute (o0.1 mL) samples in exceptionally well defined environments. We demonstrate that a single contraction–expansion device can induce flow transformations on a short timescale, with lamellar phase reorientation orthogonal to the flow direction, and over longer time periods, achieve size control in an isotropic MLV phase
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