<p>Physical transport and thermal evaporation of liquid microdroplets are two essential phenomena in various scenarios such as bio/chemical analyses and colloidal materials assembly. On one hand, inspired by the genius design of Mother nature, several strategies such as introducing Laplace pressure, wettability gradient and external force fields have been adopted to achieve predefined manipulations of microdroplets. On the other hand, droplet evaporation has been widely used in the colloidal micro/nanoparticles assembly and in the sampling matrix preparation for biochemical sensing. In general, these two techniques are developed separately and applied independently in various applications. As such, less attention has been paid to combine these two mechanisms together and to achieve multifunctions on the platform with a much compact design. In this paper, we report the asymmetric evaporation of liquid microdroplets that are confined between two non-parallel hydrophobic surfaces, i.e., in a V-shaped or beak-like groove. Similar to droplet evaporation between two parallel surfaces, the evaporation of droplets between two non-parallel hydrophobic surfaces is also greatly suppressed. However, due to the geometric confinement of the V-shaped groove on the droplet, the contact line motion of the droplet is not symmetric and during evaporation the droplet &nbsp;moves in a self-driven fashion towards the cusp (or apex) of the groove. Intriguingly, the droplet locomotion is not always in the same direction, which is dependent on the local position of the droplet in the groove. To explore this phenomenon, we used the open access software Surface Evolver to obtain the equilibrium or quasistatic shapes of water droplets with various volumes at multiple positions in the groove. Then we conducted force analysis and found that the direction of motion is mainly determined by the competence between the Laplace pressure-induced force and the adhesion force on the contact line of the droplet. Once the dihedral angle of the groove and the droplet volume are fixed, a metastable state at a local position will prevail when the Laplace pressure-induced force&nbsp;is balanced by the adhesion force. As the droplet volume continues decreasing during evaporation, the local equilibrium position gradually moves towards the cusp, which could explain the self-driven mode of the lateral motion of the evaporating droplet. This work unveils that evaporation and transport of liquid droplets could be achieved simultaneously in a confined space with pore size gradient. We believe this study could deepen our understanding of the confined liquid droplet evaporation in a complex geometry, and provide us an effective method to manipulate evaporating droplet in various scenarios and applications.</p>

Self-Driven Transport of Liquid Droplets in V-Shaped Grooves Due to Asymmetric Evaporation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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