Formation and shearing of drops with applications in biophysics and bioprocessing

Abstract

Drops are self-contained systems which do not need solid walls as surface tension provides containment. Solid walls can have associated issues such as sorption, chemical and electrostatic effects which can complicate the study of scientific phenomenon such as amyloid fibril formation. Amyloid fibrils are aggregates of protein that are generally associated with numerous neurodegenerative diseases, including Alzheimer's and Parkinson's. The containerless nature of drops motivated the ring-sheared drop (RSD) which is a platform for shearing of constrained drops through the action of surface shear viscosity. The ring-sheared drop will be used to study amyloidogenesis by utilizing the microgravity environment aboard the international space station (ISS) since microgravity enables testing with large-scale drops. Recently, the ring-sheared drop was also considered or analyzed for mixing within drops.

Further, the numerical results also demonstrated that the secondary motion effectively causes mixing within the drop. Such a surface shear viscosity based droplet mixing was found to be faster by at-least an order of magnitude as compared to the mixing produced in a diffusion-only (quiescent) case. Mixing produced by three configurations of the ring-sheared drop was assessed, namely steadily-driven single ring, oscillatory-driven single ring and steadily-driven counter rotating rings. Steadily-driven single ring produced the fastest mixing among the three configurations for the same Reynolds number (Re). However, at Re1 = 50, a range of oscillation frequencies 1.4 < omega < 9 was discovered for which the oscillatory-driven single ring case resulted in a faster mixing than the steadily-driven ring case. Hence, oscillatory driving can be tuned to achieve faster mixing than the steady driving. Steadily-driven counter rotating rings produced a slower mixing than one ring rotating steadily. Overall, it was concluded that RSD is an effective droplet mixer suitable for containerless applications in biophysics and bioprocessing.

This work presents a study of formation and shearing of constrained drops which are key scientific phenomena associated with the RSD. The formation of constrained drops as in the case of RSD was investigated both experimentally and computationally. Microgravity experiments performed aboard parabolic flights demonstrated successful formation and pinning of 10 mm diameter water drops. The computational model developed to simulate drop growth was validated against Earth-based (1 g) experiments. Also, it was found that the same computational model was able to predict the drop formation in microgravity. The shearing of constrained drops and resulting mixing within the drop were studied through numerical simulations of a 2.5 mm diameter drop. For benchmarking purposes, the numerical method used here was implemented on a knife-edge surface viscometer previously reported and the results were reproduced. The numerical results for the RSD showed that interfacial shear created by the differential rotation of the contact rings can produce azimuthal (primary) flow and meridional (secondary) flow through the action of surface shear viscosity.