Journal Name Droplet microfluidic SANS † Marco Adamo, a,b Andreas S. Poulos, a Carlos G. Lopez, a‡ Anne Martel, b Lionel Porcar, b and João T. Cabral ⇤a The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H 2 O/D 2 O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are eval- uated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dis- persion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging im- posed by large (⇠1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters. 1 Introduction Small Angle Neutron Scattering (SANS) plays a major role in the elucidation of the structure, interactions and kinetic processes in soft matter. 1–4 Typical incident neutron wavelengths range from 4 to 20 Å and resolve lengthscales between 1-500 nm, charac- teristic of molecular and mesoscopic assemblies and processes. Neutrons interact primarily with the atomic nucleus and thus iso- topic labelling enables the precise, non-invasive, spatio-temporal mapping at these scales, in particular for soft and biological mat- ter which comprise largely of low atomic number elements (and thus exhibit limited X-ray contrast). The measurement of the con- formation of a polymer in its own melt 5 or the measurement of exchange kinetics in block-copolymer micelles 6 illustrates SANS key capabilities in this field. SANS therefore underpins our under- standing of surfactants, polymers, colloids, and their mixtures, as well as their response to external stimuli, including temperature, pressure, and flow. 7 a Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK. E-mail: j.cabral@imperial.ac.uk b Institute Laue-Langevin, 71 avenue des Martyrs - CS 20156 - 38042 Grenoble CEDEX 9, France. † Electronic Supplementary Information (ESI) available. See DOI: 10.1039/b000000x/ ‡ Present address: Institute of Physical Chemistry, RWTH Aachen University, Lan- doltweg 2, 52056 Aachen, Germany. Conventional SANS experiments are carried out by sequentially measuring individually-prepared samples, pre-loaded into stan- dard cells, and moved into position by a sample changer (with tens of slots). Continued improvement in neutron guides, optics, and detection routinely provides SANS fluxes at the sample in the range of 10 6 to 10 8 neutrons cm -2 s -1 , depending on spec- trometer configuration, and will increase further with next gen- eration neutron sources, such as the European Spallation Source. As SANS measurement times decrease from min to s and sub-s timescales, novel sample formulation and preparation approaches are thus needed to remove this bottleneck. Small Angle X-ray scattering (SAXS) faced this challenge ear- lier given the higher brilliance of synchrotron sources, and au- tomated liquid handlers are already routinely employed at se- lected beamlines. 8 Microfluidic approaches for sample prepara- tion and reaction kinetics have been explored in laboratory 9 and in synchrotron SAXS. 10–13 Further, we and others have explored microflow-SAXS to examine complex fluids under flow. 14–18 Re- quirements for microfluidic-SANS are, however, distinct since neutron beams are typically ⇠1 cm footprint and, while smaller beamsizes (⇠1 mm) are attainable, the flux decreases approxi- mately in proportion to the area. Moreover, microdevices must exhibit low SANS background, high transmission, and limited neutron-induced radioactivity, 19 in addition to usual microflu- idic requirements of chemical, pressure, thermal compatibility 1–12 | 1 Page 1 of 13 Soft Matter Soft Matter Accepted Manuscript Published on 16 January 2018. Downloaded by Rheinisch Westfalische Technische Hochschule Aachen on 16/01/2018 13:45:59. View Article Online DOI: 10.1039/C7SM02433A