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


Features

More Features »

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

Focus: Why Sediments Are So Uniform
Fluid Dynamics

Focus: Why Sediments Are So Uniform

A new theory suggests that sedimenting particles of irregular shape will drift horizontally as they fall, a result that may resolve a long-standing puzzle. Read More »

Focus: Making Rogue Waves with Wind and Water
Fluid Dynamics

Focus: Making Rogue Waves with Wind and Water

Wind-generated waves in a ring-shaped water tank can spontaneously grow into single behemoth waves, mimicking a poorly understood ocean phenomenon.   Read More »

Synopsis: Small Particles Untangle Polymer Chains
Soft Matter

Synopsis: Small Particles Untangle Polymer Chains

Adding nanoparticles to molten polymer disentangles its constituent molecular chains, allowing them to flow more easily. Read More »

More Articles