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An isolated charge-neutral droplet in a uniform electric field experiences no net force. However, a droplet pair can move in response to field-induced dipolar and hydrodynamic interactions. If the droplets are identical, the centre of mass of the pair remains fixed. Here, we show that if the droplets have different properties, the pair experiences a net motion due to non-reciprocal electrohydrodynamic interactions. We analyse the three-dimensional droplet trajectories using asymptotic theory, assuming spherical droplets and large separations, and numerical simulations based on a boundary integral method. The dynamics can be quite intricate depending on the initial orientation of the droplets line-of-centres relative to the applied field direction. Drops tend to migrate towards a configuration with line-of-centres either parallel or perpendicular to the applied field direction, while either coming into contact or indefinitely separating. We elucidate the conditions under which these different interaction scenarios take place. Intriguingly, we find that in some cases droplets can form a pair (tandem) that translates either parallel or perpendicular to the applied field direction.
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Mesoscopic models for electrohydrodynamic interactions of polyelectrolytes
Two mean-field models for polyelectrolytes in simultaneous electric field and pressure-driven flow field were developed and compared. The models predict migration perpendicular to the anti-parallel or parallel fields, where the migration is caused by electrohydrodynamic interactions calculated using either a short- or long-range approximation. Inputs for the mean-field models were determined from Brownian dynamics simulations in a simple shear flow. Both models qualitatively reproduce experimental observations of DNA focusing as reported in previous publications. Specifically, it is observed that combination of the shear and electric fields leads to polyelectrolyte motion in the direction transverse to the flow and electric field direction, which in turn leads to concentration of the polyelectrolyte in the centre of a microfluidic channel. Furthermore, both models predict that there is an optimal strength of electric field that leads to the narrowest distribution profile of the polyelectrolyte in the centre of the channel. The analysis suggests that this is due to dispersion induced by the electrohydrodynamic interactions. However, quantitative disagreement between the model predictions and the experimental data indicates that further progress in the model development is needed.
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- Award ID(s):
- 1804302
- PAR ID:
- 10233007
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 915
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2021.11
