Scientists invent the world’s slipperiest liquid surface

Researchers at Aalto University unveiled a new mechanism that can create the world’s most water-repellent surface. Their findings promise to revolutionize various industries, from plumbing to optics and automotive engineering, challenging our understanding of friction between solid surfaces and water and offering a fresh perspective on how water droplets behave at the molecular level.

Water's interaction with solid surfaces features frequently in our daily lives, impacting everything from cooking to transportation and industrial applications. For years, scientists and engineers have sought to understand how water adheres to or slips off surfaces and apply the same mechanism to our gain.

Game-changing liquid-like surfaces

At the crux of this discovery are "liquid-like surfaces," a unique type of droplet-repellent surface with many advantages over traditional approaches. These surfaces consist of molecular layers that exhibit high mobility while being covalently tethered to the underlying substrates. In essence, they create a "lubricant layer" between the solid surface and water droplets.

Under the leadership of Aalto University Professor Robin Ras, the research team used a specially designed reactor to craft a liquid-like layer of molecules known as "self-assembled monolayers" (SAMs) atop a silicon surface

This study marks the first time anyone has ventured into the nanometer level to engineer molecularly heterogeneous surfaces. The team carefully regulated temperature, water content, and other conditions inside the reactor to fine-tune the monolayer coverage of the silicon surface.

"The results showed more slipperiness when SAM coverage was low or high, which are also the situations when the surface is most homogeneous. At low coverage, the silicon surface is the most prevalent component, and at high, SAMs are the most prevalent," explained doctoral researcher Sakari Lepikko, lead author of the study, in a statement.

Surprisingly, even at low SAM coverage, the team observed water slipping off the surface, contradicting previous assumptions that low coverage would increase friction. 

The team observed that water flowed freely between the molecules of the SAM when coverage was low, sliding over the surface — contradictory to the conventional belief that water would become a film and increase friction.

Conversely, water remains atop the SAM layer and still easily slides off when SAM coverage is high. The team found water to adhere to the SAMs and stick to the surface only when SAM coverage fell within a specific range. 

This counter-intuitive mechanism has resulted in the team developing the world's slipperiest liquid surface, with water running off unless the surface is perfectly level.

Potential applications and future endeavors

Lepikko believes this discovery will revolutionize every field, ranging from everyday scenarios to industrial solutions, where droplet-repellent surfaces are essential. Key areas of interest include heat transfer in pipes, de-icing, anti-fogging, microfluidics, and the development of self-cleaning surfaces.

Lepikko emphasized the novelty of their approach and stated, "It will help … where droplets need to be moved around smoothly. Our counter-intuitive mechanism is a new way to increase droplet mobility anywhere it's needed." 

The research team's next steps involve further experimentation with their self-assembling monolayer setup and enhancing the durability of the SAM layer. 

SAMs, hindered by the low thickness of the coating, are particularly vulnerable to dispersion on physical contact. However, the team believes that the insights gained from this study will prove valuable in creating robust and durable applications. 

The findings of the team were published in the journal Nature Chemistry

Study Abstract:

Friction determines whether liquid droplets slide off a solid surface or stick to it. Surface heterogeneity is generally acknowledged as the major cause of increased contact angle hysteresis and contact line friction of droplets. Here we challenge this long-standing premise for chemical heterogeneity at the molecular length scale. By tuning the coverage of self-assembled monolayers (SAMs), water contact angles change gradually from about 10° to 110° yet contact angle hysteresis and contact line friction are low for the low-coverage hydrophilic SAMs as well as high-coverage hydrophobic SAMs. Their slipperiness is not expected based on the substantial chemical heterogeneity of the SAMs featuring uncoated areas of the substrate well beyond the size of a water molecule as probed by metal reactants. According to molecular dynamics simulations, the low friction of both low- and high-coverage SAMs originates from the mobility of interfacial water molecules. These findings reveal a yet unknown and counterintuitive mechanism for slipperiness, opening new avenues for enhancing the mobility of droplets.

Originally published on Interesting Engineering : Original article

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