E​RCOFTAC PC UK

Liquid Droplets in Bloom

Teng Dong, Kristo Kotsi, Takeshi Kobayashi, Alexander Moriarty,

Panagiota Angeli, Ian McRobbie, Alberto Striolo

(ThAMeS Multiphase, Department of Chemical Engineering, University College London, UK; Innospec Ltd., UK;
School of Sustainable Chemical, Biological and Materials Engineering, The University of Oklahoma, USA)

As an emulsion droplet evaporates, a hidden choreography begins. Thin-film interference reveals that an ultrathin liquid film spreads across the solid surface, leaving behind a ring of perfectly ordered beads and unfolding into a two-layer structure. Driven by contact-line instabilities, the liquid appears to bloom like a living flower—an illusion of magic born from physics. The video is presented in accelerated time.

Abstract

The submitted video originates from an experimental study on the spreading dynamics of emulsion liquid droplets on solid substrates. This research is motivated by challenges in agricultural pesticide applications. In practice, a large fraction of sprayed pesticides is lost due to droplet bouncing, rolling, or being washed away from leaf surfaces, resulting in poor deposition efficiency and unintended soil and water pollution. The overarching aim of this work is to enhance droplet wetting and adhesion on plant leaves by optimising pesticide formulations and spray strategies.

Many commercial pesticides are formulated as oil-in-water emulsions, in which the active ingredients are dissolved in an organic phase and dispersed as micron-sized droplets within water. When such an emulsion droplet is deposited on a solid surface, a sequence of coupled physical processes unfolds. Initially, evaporation of the aqueous phase induces internal convection, transporting the dispersed oil droplets toward the droplet edge. These oil droplets then coalesce, forming a continuous organic film that begins to spread outwards across the surface. While the evaporation of the water phase is completed within approximately one minute, the subsequent spreading of the thin organic film can persist for tens of minutes.

A striking and unexpected feature revealed by thin-film interference imaging is that the spreading film develops a transient two-layer structure. Together with instabilities at the contact line, this produces a visual effect reminiscent of a liquid “blooming” across the surface-behaviour that is absent in the spreading of single-component organic liquids. This phenomenon is attributed to the presence of surfactants (Span 80 in the present study), which are essential for emulsion stability and are present at concentrations above the critical micelle concentration. The resulting micelles are believed to self-organise within the spreading film, preferentially populating a secondary layer and giving rise to the observed flower-like spreading dynamics. These observations suggest a spreading law that goes beyond the classical Tanner’s law for wetting liquids.

The visualisation presented here was obtained during the initial stage of the research. Future work will focus on quantitatively elucidating how micelle size, shape, and organisation govern the layered structure of the spreading film. In addition, the evaporation and spreading of complex droplets on biomimetic leaf surfaces will be investigated as part of a Marie Skłodowska-Curie–funded research project over the coming years.

Publications related to the research:

Dong, Teng, et al. "Flowerlike spreading of micellar films during emulsion drop evaporation." Physical Review Letters 133.17 (2024): 174001.