Microfluidic in-line dynamic light scattering with a commercial fibre optic system.

We report the coupling of dynamic light scattering (DLS) in microfluidics, using a contact-free fibre-optic system, enabling the under-flow characterisation of a range of solutions, dispersions, and structured fluids. The system is evaluated and validated with model systems, specifically micellar and (dilute) polymer solutions, and colloidal dispersions of different radii (∼1-100 nm). A systematic method of flow-DLS analysis is examined as a function of flow velocity (0-16 cm s-1), and considerations of the relative contribution of 'transit' and 'Brownian' terms enable the identification of regions where (i) a quiescent approximation suffices, (ii) the flow-DLS framework holds, as well as (iii) where deviations are found, until eventually (iv) the convection dominates. We investigate practically relevant, robust setups, namely that of a capillary connected to microdevice, as well as direct measurement on a glass microdevice, examining the role of capillary dimensions and challenges of optical alignment. We conclude with a demonstration of a continuous flow measurement of a binary surfactant/salt solution, whose micellar dimensions vary with composition, characterised with hundreds of data points (every ∼5 s) and adequate statistics, within a few minutes

Effect of surface energy on the removal of supported triglyceride films by a flowing surfactant solution

We investigate the role of substrate surface energy on the removal of triacylglycerol (TAG) films by a model surfactant solution under laminar flow conditions. We select a mixture of 0.5:0.3:0.2 triolein:tripalmitin:tristearin mass fraction as a model TAG, yielding a solid ‘fat’ film, and expose it to a micellar aqueous surfactant solution, flowing at rate 250 μLmin−1 (corresponding to a velocity of 1.4 cms−1 ), within a microfluidic channel. We employ a combination of optical and atomic force microscopy, profilometry, X-ray photoelectron and infrared spectroscopy, and contact angle measurements to characterise a range of model substrates, the TAG films, and their removal over time. Our experimental data show a clear dependence of the TAG removal time with surface energy and, in particular, with the polar component of the surface energy, . TAG films on very hydrophilic surfaces, with high (> 20 mJ/m2 ), are generally found to delaminate rapidly, while those on lower surfaces are progressively eroded over longer timescales. An approximately inverse linear relation is found between and removal time , which holds for a large range of surfaces including glass, silicon and plastics, and various surface treatments