Description
Droplet microfluidics has emerged as a powerful platform for biological applications[1], enabling the generation of highly controlled aqueous emulsions dispersed in an oil phase with surfactants. The presence of the droplet-oil interface alters the droplet internal hydrodynamics, leading to modified recirculation patterns and stagnation points compared with monophasic flows. This has important implications for the internal mixing of liquids confined in droplets, as has been deeply investigated in the literature[2]. However, if, instead of liquid-liquid mixing, droplets contain micrometric particles, commonly used in biochemistry for isolation purposes, the recirculation is less obvious. In fact, depending on their size and density compared to those of the surrounding medium, these microparticles may exhibit different behaviours. For example, according to the size and speed, particles are confined in specific regions[3,4]. Despite these preliminary results, a systematic study of the heavy-bead recirculation dependencies from droplet volume, flow rate, channel dimension and the related consequences on specific applications is still missing. Here, we present a methodical analysis of the accumulation of beads denser than the medium (about 2-fold), for different capillary numbers (between 3e-4 and 5e-3), droplet volumes (30nL-3uL), and microchannel size (diameter from 300um to 800um). We observed that, as the droplet length-to-channel width ratio (L/W) of the droplet gets smaller, the mixing is improved. Moreover, an increase in the flow rate causes the beads to accumulate at the rear of the droplet. This behaviour contrasts with what is typically observed for liquids inside droplets. This raises the question whether it is preferable to operate at high flow rates, leading to bead accumulation but strong recirculation of the analytes within the droplet, or at lower flow rates, where fluid motion is less pronounced but the beads remain more uniformly dispersed throughout the droplet. To address this question, we performed an enzymatic reaction inside droplets, in which the enzyme was immobilised on the beads while the analyte was freely dispersed in the droplet. When the temporal increase in fluorescence signal arising from the enzymatic reaction was measured, we found that the overall reaction efficiency is higher when the beads are well dispersed, even though fluid mixing is weaker under these conditions. These results indicate that bead dispersion and thus the conditions to maximise it, rather than fluid mixing alone, define the optimal operating conditions in bead-based droplet microfluidics.
[1] Sackmann E. et al., Nature 507, 2014
[2] Baroud C. N. et al., Lab on a Chip 10, 2010
[3] Hein M. et al., Lab on a Chip 15, 2015
[4] King C. et al., Microfluid Nanofluid 3, 2007