Wednesday, October 12, 2016

From Urinals to Printers: Enough with the Splashing

My local beaches and swimming pools are closed until next year, but in bathrooms, kitchens, and operating rooms worldwide, it’s always splashing season. Whether it’s a spray of liquid from raw meat thrown hastily on the cutting board or body fluids from a surgical tray going airborne, splashes aren’t just annoying—in some cases they can cause real damage. They can compromise health, safety, and the effectiveness of pesticides, along with printing techniques, forensic interpretations of events, manufacturing processes, and more.

When a drop of liquid hits a surface, it can spread out smoothly or respond by splashing, sometimes violently and with far-reaching results. In new work about to be published in the journal Physical Review Letters, a team of European researchers demonstrates that coating a surface with a simple layer of soft material can be an effective way to reduce and even eliminate splashes. This might sound obvious, but the team also figured out the why and uncovered a mathematical relationship that scientists and engineers can use to design splash-proof surfaces.

Image Credit: Ben Oswald (CC BY 2.0)
Currently, you can prevent a droplet from splashing with techniques like reducing the air pressure, tilting the surface, slowing down the drop, or using a surface with a special texture. The problem is, none of these solutions work in a wide variety of applications and situations.

To address this need, scientists at the University of Oxford tried another approach. Alfonso Castrejon-Pita, an expert on droplets in the Department of Engineering Science, and Robert Style, an expert on soft solids in the Mathematical Institute, joined forces to explore how the stiffness of the surface affects splashing. They brought in collaborators from France and the UK and recruited Chris Howland, an undergraduate mathematics student, who carried out many of the experiments during a summer program.

The experiments involved releasing ethanol drops onto gel substrates. The substrates were all the same thickness (10mm), but varied in stiffness. The stiffness of a material describes how easily it deforms. In other words, it takes a lot more force to change the shape of a stiff material than a soft, or flexible, material.

By dropping the ethanol from different heights, the researchers changed the speed at which the drops hit the surface. They filmed the impacts with a high speed camera, and then extracted data on drop size and speed, and details of the splash (if there was one) from the images. By comparing the splashes created by similar drops hitting different substrates, they found that softer surfaces can significantly reduce and even eliminate splashes.

A droplet impacting a stiff acrylic surface. Credit: Howland et al.
A droplet impacting a soft silicone surface. Credit: Howland et al.

To find out why, they released ethanol drops onto a glass slide coated with a soft gel. This time the soft layer was much thinner, 0.003 mm instead of 10 mm. Although the gel was flexible, the results closely matched the results of the stiffest substrate in the first set of experiments. This indicates that splashing is not affected so much by the surface it lands on, but by how easily the surface deforms under pressure. In other words, when a drop hits a solid that easily deforms, some of the energy that would otherwise go into splashing goes into deforming the material instead. The 0.003 mm layer was soft, but was not able to deform very much because its movement was restricted by the stiff glass slide underneath.

When a drop hits a surface, the drop either spreads out smoothly or shoots a thin sheet of liquid outward. If this sheet is ejected at high enough speeds, it lifts up off the surface and breaks into droplets—a splash. Their results showed that the softer the substrate, the slower the ejection sheet moves and the less likely it is to break apart into a splash.

Simulations and a mathematical analysis shed further light on this. A soft surface doesn’t absorb much more energy from a drop than a stiff surface does, but the extra energy comes primarily from the splash-producing sheet. The key to controlling splashing lies in the relationship between the maximum pressure the drop exerts on the surface and how easily the surface deforms, a mathematical relationship derived in this work.

By coating surfaces with appropriate materials, their work shows that you can make something significantly more splash-proof. This could have wide ranging impacts in medicine, science research, manufacturing, kitchens, and yes, even bathrooms. With two little boys in the house, that’s a cause I definitely support.

Kendra Redmond

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