Microfluidics has proven to be an efficient tool for making fine and calibrated emul- sion droplets. The parallelization of drop makers is required for producing large amounts. Here, we investigate the generation of emulsion drops along a series of shallow microchannels emerging in a deep one, in other words the step-emulsification process. The dynamics of a single drop maker is first characterized as a function of interfacial tension and viscosities of both phases. The characteristic time scale of drop formation, namely, the necking time that finally sets drop size, is shown to be principally governed by the outer phase viscosity to interfacial tension ratio with a minor correction linked to the viscosity ratio. The step emulsification process experi- ences a transition of fragmentation regime where both the necking time and drop size suddenly raise. This transition, that corresponds to a critical period of drop formation and thus defines a maximum production rate of small droplets, is observed to be ruled by the viscosity ratio of the two phases. When drops are produced along an array of microchannels with a cross flow of the continuous phase, a configuration comparable to a one-dimensional membrane having rectangular pores, a drop boundary layer develops along the drop generators. In the small drop regime, the local dynamics of drop formation is shown to be independent on the emulsion cross flow. Moreover, we note that the development of the drop boundary layer is even beneficial to ho- mogenize drop size along the production line. On the other hand, in the large drop regime, drop collision can trigger fragmentation and thus lead to size polydispersity.

The capacity of microfluidic technology to fabricate monodisperse emulsion droplets is well established. Parallelization of  droplet production is a prerequisite for using such an approach for making high quality materials for either fundamental or industrial applications where product quantity matters.  Here, we investigate the emulsification efficiency  of parallelized drop generators based on a flow focusing geometry when incorporating the role of partial wetting in order to make emulsion droplets with a diameter  below 10 microns. Confinement intrinsically encountered  in microsystems intensifies the role played by interfaces between liquids and solids.  We thus take advantage of partial wetting to enhance the maximum confinement accessible due to liquid flow focusing. We  compare the performances brought by partial wetting to more established routes such as  step emulsification. We show that the step configuration and the partial wetting regime are both well suited for being parallelized and thus open the way to the production of fine and calibrated emulsions for further applications. Finally, this new route of emulsification that exploits partial wetting between the fluids and the channel walls opens possibilities to the formation of substantially smaller droplets, as required in many fields of application.

We discuss the theory of ligand receptor reactions between two freely rotating colloids in close proximity to one other. Such reactions, limited by rotational diffusion, arise in magnetic bead suspensions where the beads are driven into close contact by an applied magnetic field as they align in chainlike structures. By a combination of reaction-diffusion theory, numerical simulations, and heuristic arguments, we compute the time required for a reaction to occur in a number of experimentally relevant situations. We find in all cases that the time required for a reaction to occur is larger than the characteristic rotation time of the diffusion motion tau_rot. When the colloids carry one ligand only and a number n of receptors, we find that the reaction time is, in units of tau_rot, a function simply of n and of the relative surface alpha occupied by one reaction patch alpha = pi r_C²/(4pi r²), where r_C is the ligand receptor capture radius and r is the radius of the colloid.

When ligands and receptors are both attached on surfaces, because of the restriction of configurational freedom, their recognition kinetics may be substantially reduced as compared with freely diffusing species. In nature, this reduction may influence the efficiency of the capture and adhesion of circulating cells. Here we show that similar consequences are observed for colloids grafted with biomolecules that are used as probes for diagnostics. We exploit Brownian magnetic colloids that self-assemble into linear chains to show also that the resulting one-dimensional confinement considerably accelerates the recognition rate between grafted receptors and their ligands. We propose that because confinement significantly augments the colliding frequency, it also causes a large increase in the attempt frequency of the recognition. This work gives the basis of a rapid, homogeneous, and highly sensitive bioanalysis method.