## 1 Introduction
Passive mixing using microfluidics has been widely implemented in order to investigate the kinetics of biochemical reactions owing to the fast (milli- and microsecond) mixing times achievable, comparatively low sample consumption rates, and ease of integration with different experimental setups [27]. Employing droplet microfluidics in combination with passive micro-mixing further decreases sample consumption whilst allowing for a wide range of possible mixing times. For example, [PERSON] et al. [28] used two-photon fluorescence lifetime imaging microscopy combined with FEM simulations (using COMSOL) to generate 2D maps of chaotic mixing patterns inside of microdroplets travelling through a serpentine mixer. Their experimental and simulation results showed that their micro-mixer could achieve mixing efficiencies of up to 80% after just 18 ms. Of particular relevance to the current work is the fact that by forcing the droplets through sharp turns within a serpentine mixing channel, the authors were able to demonstrate the introduction of chaotic mixing within the droplets, which in turn enhanced the mixing efficiency. Another study of chaotic mixing in serpentine channels was carried out by [PERSON] and [PERSON] [29], who examined the millisecond kinetics of an enzymatic reaction by measuring the fluorescence intensities using nanolitre droplets. Micromixers have also been used for crystallisation studies; for example, [PERSON] et al. [30] utilised a passive micromixer consisting of multiple 3D crossing channels in order to optimise the production of silver azide (SA) crystals. In addition to achieving high mixing efficiencies and low reagent consumption, the micro-mixer allowed for the optimisation of the size and shape of crystals which was not possible using macroscale crystallisation approaches. By employing micromixers, researchers have the means to trigger dynamic changes in proteins either in solution or as nanocrystals on sub-millisecond time scales. These timescales are suitable for studying large-scale conformal changes in proteins, such as folding and unfolding [31], which can be probed using a range of laboratory and synchrotron-based techniques including small-angle and wide-angle X-ray scattering (SAXS/WAXS). An example of a system that could benefit from the availability of sub-millisecond sample mixing and delivery is human ubiquitin, a key protein found in almost all human cellular tissue, whose conformational dynamics are still not well understood [32]. The structural dynamics of the four-helix bundle within the FF Domain (where FF indicates the presence of two phenylalanine), which is found in a large number of nuclear proteins, from Human HYPA/FBP11 is also a suitable target. This has previously been studied using Nuclear Magnetic Resonance (NMR) spectroscopy [33]. A particularly promising avenue for studying molecular dynamics at the atomic scale is via either serial femtosecond nanocrystallography (SFX) [34] or SAXS/WAXS. [PERSON] et al. and [PERSON] et al. [35] previously used conventional capillary injectors to deliver protein solution to the X-ray beam in the form of a micro-jet for time-resolved SFX studies of G protein-coupled protein receptors. In order to investigate the dynamics of proteins, a reaction must be initiated prior to measurement. To achieve the necessary sub-millisecond time-resolution, rapid, homogeneous mixing of the reactants is required immediately prior to measurement. Microfluidic devices incorporating micromixers and injector nozzles into one integrated structure (i.e., mix-and-inject) serve as the ideal platform for initiating and measuring molecular dynamics for time-resolved structural biology. Recently, [PERSON] et al. [36] have reported the development of an ultra-compact 3D microfluidic mix-and-inject device for performing time-resolved serial femtosecond crystallography (SFX) measurements. For their device, they used a two-photon additive manufacturing technique to produce an integrated 3D helical mixer. [PERSON] et al.