Hypothesis: In droplet microfluidics applications, flow control and mixing in a small volume without any active external devices is a challenge. Vapor-mediated solutal Marangoni flows can be effectively generated by applying the vapor of a volatile liquid, which can be possibly controlled, and can eventually be used in a mixing enhancement device.
Experiments: We investigated and controlled vapor-mediated solutal Marangoni flows by varying the local surface tension. We systematically tested the effects of different volatile liquids and their vapor concentration on the flow pattern. Furthermore, by varying the number of vapor sources, we generated and controlled multiple vortices, and analyzed them by particle image velocimetry (PIV). The proposed method was applied to a mixing enhancement application.
Findings: We show that in addition to the surface tension of the volatile liquid, the vapor concentration also influenced the local surface tension along the interface, which in turn changed the internal flow velocity. To predict the flow velocity and oscillatory frequency of the solutal Marangoni flow, we developed a theoretical model based on scaling analysis that showed a good agreement with the experimental results. We believe that the current study will motivate low-cost and portable sample flow control and mixing systems in the near future.
Keywords: Multiple vortical flows; Non-contact mixing enhancement; Surface tension; Vapor-driven solutal Marangoni flow.
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