Synopsis: Sticky water

Spray deposition processes are determined by how droplets bounce off or stick to water-repellant surfaces.
Synopsis figure
Illustration: Sami Mitra

The physics of wetting, manifest in everyday occurrences ranging from leaves in a rainfall to functional raingear, has intrigued scientists for over a century. Nonetheless, even a seemingly simple situation such as a droplet bouncing off a water-repellant (“hydrophobic”) surface is worth revisiting.

Writing in Physical Review Letters, Michael Smith (at the University of Edinburgh and the University of Nottingham, both in the UK) and Volfango Bertola (at the University of Edinburgh and the Politecnico di Torino, Italy) examine why droplets of water containing small amounts of a flexible polymer do not rebound from water-repellant surfaces. They dispute the prevalent wisdom which holds that the rebound effect is suppressed because the polymer stretches, which temporarily increases viscosity due to large extensional deformations (extensional viscosity). Instead, by measuring the fluid velocity inside the droplet using fluorescent particles, and directly visualizing fluorescent biopolymer molecules (DNA), they posit that as the line of contact between the edge of the droplet and the substrate moves, it stretches the polymer, bringing about an effective resistance that lowers the available energy below that necessary for a droplet to rebound: The receding contact line, not the increased extensional viscosity, affects the dynamics of the droplet. This work augurs well for the study of spray deposition processes ranging from small-scale (spray painting, inkjet printers, etc.) scenarios to industrial-scale ones such as spraying of agrochemicals. – Sami Mitra


Announcements

More Announcements »

Subject Areas

Fluid DynamicsSoft Matter

Previous Synopsis

Semiconductor Physics

Noisy spins

Read More »

Next Synopsis

Nuclear Physics

Exclusive pions

Read More »

Related Articles

Viewpoint: Superfluids Hit the Street
Atomic and Molecular Physics

Viewpoint: Superfluids Hit the Street

A flow pattern dubbed the von Kármán vortex street, which is renowned for its aesthetic beauty and extreme power, has been created in a superfluid. Read More »

Synopsis: Wetting Matters When Emptying a Tank
Fluid Dynamics

Synopsis: Wetting Matters When Emptying a Tank

The speed at which water drains out of a tank depends on the affinity of water with the outside surface of the orifice through which it drains.   Read More »

Viewpoint: Signs of a Gardner Transition in a Granular Glass
Soft Matter

Viewpoint: Signs of a Gardner Transition in a Granular Glass

Two-dimensional disk packings under compression and vibration display signatures of the Gardner phase transition that is thought to occur between the glass and jamming transitions. Read More »

More Articles