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Omnidirectional droplet propulsion on surfaces with a Pac-Man coalescence mechanism

Jana Chaaban, Patrick Galliker, Thomas M. Schutzius*, and Dimos Poulikakos†

  • Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland

  • *Present address: ETH Zurich, Laboratory of Multiphase Thermofluidics and Surface Nanoengineering, Department of Mechanical and Process Engineering, CH-8092 Zurich, Switzerland; Corresponding author: thomschu@ethz.ch
  • †Corresponding author: dpoulikakos@ethz.ch
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Phys. Rev. Fluids 5, 123602 – Published 11 December, 2020

DOI: https://doi.org/10.1103/PhysRevFluids.5.123602

Abstract

Dispensing, transporting, and manipulating minute liquid volumes is important to a wide range of scientific and application areas, spanning from biology and medicine to chemistry and materials. Although digital microfluidics are emerging as a popular methodology in the lab-on-a-chip field, important obstacles to its broad adoption, such as those related to reusability, reconfigurability, and susceptibility to fouling, persist. In addition, as liquid volumes decrease in size, their sustenance in an open atmosphere becomes practically impossible due to high volatility, leading rapidly to complete evaporation. Here we introduce and demonstrate a microfluidic platform based on the coalescence-induced self-propulsion of sessile microdroplets on unpatterned substrates. We employ controlled dropwise liquid ejection by electrohydrodynamic printing to create, actively sustain (despite high volatility), and freely translate femtoliter-sized droplets (∼2–5 μm radius) under open-atmosphere conditions. The omnidirectional planar movement of the droplets is achieved by a precisely controlled, on-demand sequence of coalescence events, where the directed motion of an already deposited droplet is dictated by the positioning of the subsequent droplet printed adjacent to it. We studied the transport mechanism experimentally and theoretically and found that, for short time scales (relative to the substrate translation and droplet evaporation), the radial growth of the new smaller droplet being printed can greatly exceed the retraction of the neighboring, already deposited droplet. This rapid growth is exploited to create a liquid meniscus bridging the so generated droplet doublet, and merging it into a single droplet by the action of capillary pressure differences. Evaporative losses ensure that the moving droplets are kept at a constant size after merging, in each coalescence cycle. We observe this coalescence whenever the interspacing between two sequentially printed droplets is below a critical value, and show that we can control this with the underlying substrate velocity. Armed with this transport mechanism, we then demonstrate the utility of this approach by tasking the coalescing self-propelled droplet doublet to perform microfluidic operations such as collecting, transporting, and merging solid residues on a surface, in an on-demand, Pac-Man type motion, exemplifying capabilities for digital microfluidic applications.

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“Pac-Man” Mechanism for Moving Tiny Droplets

Published 11 December, 2020

Movies of a new technique for moving tiny droplets across a surface are reminiscent of an iconic maze-based video game.

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Outline

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Phys. Rev. Fluids 5, 123602– Published 11 December, 2020
  • Received 2 July 2020
  • Accepted 20 October 2020
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