Rapid contrast matching by microfluidic SANS

Abstract

We report a microfluidic approach to perform small angle neutron scattering (SANS) measurements of contrast variation and matching, extensively employed in soft and biological matter research. We integrate a low scattering background microfluidic mixer and serpentine channel in a SANS beamline to yield a single phase, continuous flow, reconfigurable liquid cell. By contrast with conventional, sequential measurements of discrete (typically 4-6) solutions of varying isotopic solvent composition, our approach continually varies solution composition during SANS acquisition. We experimentally and computationally determine the effects of flow dispersion and neutron beam overillumination of microchannels in terms of the composition resolution and precision. The approach is demonstrated with model systems: HO/DO mixtures, a surfactant (sodium dodecyl sulfate, SDS), a triblock copolymer (pluronic F127), and silica nanoparticles (Ludox) in isotopic aqueous mixtures. The system is able to zoom into a composition window to refine contrast matching conditions, and robustly resolve solute structure and form factors by simultaneous fitting of scattering data with continuously varying contrast. We conclude by benchmarking our microflow-SANS with the discrete approach, in terms of volume required, composition resolution and (preparation and measurement) time required, proposing a leap forward in equilibrium, liquid solution phase mapping and contrast variation by SANS.

Droplet microfluidic SANS

“…SDS at this concentration forms ellipsoidal micelles that have been extensively characterised by SANS in both continuous flow microfluidics and bulk measurements. 21,41 Suitable flowrates for the aqueous input were found to be 3 mLh −1 and 3 mLh −1 fluorinated oil or 0.8 bar air, and 4.2 mLh −1 (aqueous) with 4.8 mLh −1 for toluene-d. The resulting segmented flows are shown in Fig.…”

“…Given the quadratic ∆ρ 2 , the minimum or vanishing point provides the composition for which the solute becomes "contrast-matched" and thus enables selective labelling to structurally resolve complex fluids and biological mixtures. 26,27 The experimental protocol seems well-suited for automation: we have previously demonstrated continuous flow CV in microfluidic-SANS, 21 and now seek to benchmark its performance with a segmented flow approach.…”

“…The stage was adjusted such that the beam centre was placed at 2670 mm from the double T-junction, for comparison with previous work. 21 The illuminated sample area was found to cover 16 full channels and 2 partial channel widths, reducing the illuminated beam footprint from 100 mm 2 to 66 mm 2 , thus illuminating a sample volume of 16.7 µL, including both the sample and carrier fluid (∼ 30% of that volume). Simultaneous measurements of scattering and transmission were made possible by using a semitransparent beamstop.…”

“…The data were corrected by empty cell and electronic background subtraction, and the intensity calibrated to absolute units [cm −1 ] by the direct neutron beam flux, taking into account the illuminated sample volume and the non-uniform cross-section of the microdevice (as described previously 21 ). GRASP 7.15 was employed for data reduction, and IgorPro 6.37 31 and SASView 4.1.2 32 for data fitting.…”

“…In order to enable rapid phase mapping in SANS, we have recently reported a microfluidic formulator capable of the sequential preparation of liquid samples, compatible with SANS. 21 Such single-phase, continuous flow formulator is needed to rapidly mix input solutions (i.e. radial diffusion), but simultaneously limit dispersion along the microchannel (i.e.…”