Development of millifluidic devices for structural characterisation of viscoelastic materials

Authors: Cate T. O'Brien (University of Sheffield)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: July 2021

Abstract: The work reported in this thesis involves the design and fabrication of millifluidic devices suitable for the investigation of viscoelastic materials by small-angle X-ray scattering (SAXS). It was found that existing designs of micro- or millifluidic devices presented in literature were unsuitable for the study of viscous materials due to the larger pressures exerted on the walls of devices by the fluid. Leakage was observed in prototypes similar to those observed in the literature, specifically at the inlet and window areas. A range of design criteria necessary for the fabrication of successful millifluidic devices for viscoelastic materials were developed. The features of such a device, created as a result of the prototypes trialled, included the use of Luer lock inlets, enclosed channels and a threaded sealed window port providing access to the sample using scattering and visualisation techniques. Stereolithography (SLA) three-dimensional (3D) printing was utilised to provide an inexpensive, rapid and straight-forward fabrication method, using a commercially available printer. Devices could be produced with very few fabrication and assembly steps providing good print resolution and leading to a smooth internal surface non-interfering with flow. This fabrication method was found to be a viable alternative to the traditional, time-consuming and expensive, soft lithography techniques often used for microfluidic manufacture. The design criteria formulated in this work could be employed to fabricate a range of millifluidic geometries, with straight channel and cross slot geometries demonstrated in this thesis as the most representative examples for shear and extensional flow, respectively. The devices produced could be combined with a variety of techniques, in particular optical microscopy and X-ray scattering were utilised extensively in this work. Both geometries were shown to possess a stable and reproducible laminar flow field for the materials tested at all Q values. This was confirmed by Reynolds number (Re) values estimated from finite element analysis (FEA) simulations as well as by experimental techniques such as particle tracing. Two types of polymeric materials forming anisotropic morphologies under flow conditions were utilised as model materials within the millifluidic devices; an aqueous solution of modified cellulose and water dispersions of worm-like micelles formed by self-assembled block copolymers. The straight channel millifluidic device shows flow behaviour analogous to a slit rheometer, with polarised optical microscopy (POM) and SAXS measurements indicating alignment of the worm-like micelles under flow. Despite the fact that the cellulosic materials demonstrated optical birefringence under flow, suggesting the formation of an orientated morphologies, this morphology was not detectable by SAXS possibly because of a small volume of oriented material. The cross-slot millifluidic device has strong extensional forces along the outlet plane as seen by POM and SAXS. Extensive mapping of the cross-slot region using a synchrotron SAXS beamline in a microfocus configuration identifies regions of orientation of the worm-like micelles where extensional flow is present, as well as remarkable stability in the flow field over time.

Subject Areas: Materials, Chemistry

Instruments: I22-Small angle scattering & Diffraction

Discipline Tags:

Material Sciences Polymer Science Soft condensed matter physics Chemistry

Technical Tags:

Scattering Small Angle X-ray Scattering (SAXS)