The evaporation of exotic non-spherical droplets studied for the first time

New research finds a universal scaling law valid for the evaporation of drops of any shape.

We come across evaporation, the process of turning liquid into vapour, everywhere in our daily lives. The underlying mechanism, however, is so complex that most studies have restricted their scope to the evaporation of spherical droplets. In a new paper, published in Nature Communications, researchers have investigated circular, square, triangular, pentagonal, as well as kidney and half-moon shaped-droplets on solid surfaces to gain a better understanding of the evaporative process. The collaborative study involving Imperial College London, Edinburgh, Maryland, MIT, and Tianjin University, considered drops of pure liquid and binary mixtures, as well as particle-laden ones.

The experimental and numerical results of this study uncovered a universal scaling law for the evaporation rate valid for sessile drops of any shape. This law, which uses the area-averaged mean interfacial curvature to account for the different droplet shapes, fits the experimental data to within an average absolute error of approximately 1%. The scaling law shows that a more rounded drop evaporates faster than a flatter one with the same perimeter and interfacial area.

The study also demonstrated that regions of higher curvature within a non-spherical drop lead to larger local particle deposition rates; this is related to the mechanism responsible for the ‘coffee ring’ effect. The interfacial geometry affects the flow structures within the drop: the currents in the bulk move towards the drop apices. For binary mixtures, the more volatile component exhibits spontaneous geometry-modulated spatial segregation within the drop.

According to Professor Omar K. Matar, co-author of the paper and PETRONAS/Royal Academy of Engineering Research Chair in Multiphase Fluid Dynamics, “our work shows that drop geometry is a powerful controlling mechanism for particle deposition, flow and mixing dynamics, and evaporation kinetics.” The results of this study can be exploited for ink-jet, and metal printing, protein and DNA sequencing, and DNA stretching through micro-flow control.

The work was partly-funded by the Engineering and Physical Sciences Research Council through the Multi-scale Exploration of MultiPhase Physics In FlowS (MEMPHIS) Programme Grant.

Reference:

P.J. Sáenz, A.W. Wray, Z. Che, O.K. Matar, P. Valluri, J. Kim, K. Sefiane. 2017. Dynamics and universal scaling law in geometrically-controlled sessile drop evaporation. Nature Communications. DOI: 10.1038/ncomms14783.

Cover image credit: Emma Gospodinova

[Article written by Dora Olah an Undergraduate student in the Department of Chemical Engineering.]