A heatshield for ‘never-wet’ surfaces: Rice engineering team repels even near-boiling water with low-cost, scalable coating

(Press-News.org) Superhydrophobic surfaces — those famously “never-wet” materials that make water bead up and roll away — have a stubborn weakness: hot water. Once temperatures climb above roughly 40 degrees Celsius, many superhydrophobic coatings abruptly lose their magic. Instead of skittering off, hot droplets start sticking, soaking into the surface texture and leaving behind wet patches and residue.

A new study from mechanical engineers at Rice University describes a surprisingly straightforward fix: Instead of just engineering the surface’s chemistry and texture, they focused on engineering its heat flow. By placing a thin, thermally insulating layer beneath an off-the-shelf superhydrophobic spray coating, the researchers created what they call a multilayered insulated superhydrophobic (MISH) coating that keeps repelling water even when droplets approach their boiling point — up to 90 C, which is far beyond the point where conventional superhydrophobic coatings typically fail. The research was recently published in ACS Applied Materials & Interfaces.

“In the past, this task has been a challenge and has required up to 4,000 times the cost of our approach to achieve,” said Daniel J. Preston, assistant professor of mechanical engineering at Rice and corresponding author of the study. “We also showed that this works outside the lab in real-world situations across both large and curved surfaces, from pipes to bowls to industrial equipment.”

Classic superhydrophobicity depends on a delicate trick: Micro- and nanoscale surface textures trap a thin layer of air, so droplets rest on an “air cushion” rather than fully touching the solid. This low-contact state reduces adhesion and lets water roll away. But when a hot droplet hits a relatively cooler textured surface, some of the droplet evaporates then recondenses inside the surface texture, forming tiny liquid bridges that replace the trapped air. Those bridges pin the droplet in place and drive a transition toward a stickier wetting state. In practice, that means hot water that should bounce or slide instead clings, spreads and leaves behind residue.

For industries that constantly deal with warm or hot liquids, such as food processing, desalination, chemical manufacturing and medical sterilization workflows, a coating that works well at room temperature can fail quickly under real operating conditions.

“Instead of relying on expensive clean room nanofabrication or highly specialized surface chemistry, our approach is tackling the root cause of failure, which is the heat moving from the droplet and into the surface,” said Zhen Liu, co-lead author of the paper. Liu performed this research as a doctoral student in Preston’s lab at Rice and continued it as an assistant professor of mechanical engineering at the University of Texas at Dallas.

The MISH design is a two-layer system composed of an insulating underlayer (often a spray-on polyurethane foam, though the team also tested alternatives like acrylic foam tape) and a microtextured superhydrophobic topcoat (the researchers used a commercially available spray-on coating).

“The insulation layer reduces the cooling of the hot droplet at the interface, which in turn reduces evaporation and recondensation cycles that normally flood the surface texture with condensate,” Preston said. “Less condensate in the surface texture means fewer liquid bridges, which keeps repellency intact.”

To demonstrate the coating’s performance, the researchers put it through a series of experiments designed to mimic real-world conditions.

First, they tilted coated samples slightly and released heated water droplets onto them. As each droplet grew, gravity eventually pulled it off the surface. The hotter the surface, the more stubborn the droplet became — and the larger it had to grow before sliding. Compared with conventional superhydrophobic coatings, the insulated MISH coatings stayed far less sticky as temperatures rose. This showed that the added insulation was blocking the condensation that normally causes hot water to cling to surfaces.

To explain the results, the team built a heat-transfer model showing how much condensation forms inside the surface texture. Because all samples used the same surface chemistry and texture, the model isolated the effect of insulation alone.

“Once we scaled the data correctly, the results from different insulation thickness all followed the same pattern,” Preston said. “This means that a surface’s performance can be predictably tuned simply by adjusting the insulation without redesigning the surface each time, making this approach easier to scale.”

The researchers also fired hot water jets at the coatings to mimic splashes and continuous exposure. Traditional coatings quickly lost their ability to repel water as temperatures climbed, but the MISH coatings, especially thicker ones, continued to bounce and deflect hot jets.

Taking the tests even further, the team blasted the surfaces with hot droplets for a full week, totaling nearly 2 million impacts. The standard coatings failed almost immediately, but the MISH coatings kept repelling hot droplets for more than 80 hours (about 1 million impacts) before gradually degrading. Upon examination, the researchers discovered that the weak link in the MISH coatings was the commercial top layer and not the insulation concept itself, suggesting future versions could last longer.

To demonstrate that the coating could work outside the lab, the researchers also experimented on larger plates, the inside of pipes and even used hot milk, coffee and split pea soup to test coated surfaces. The results? The hot liquids left less than 1% residue on MISH-coated surfaces compared with about 31% or more on typical superhydrophobic coatings.

“We’re excited about the potential applications of this approach, but there is also room for further improvement,” Preston said. “Long-term performance really comes down to how durable and chemically stable the outermost layer is, especially at higher temperatures or in harsher environments, so we’re now looking at more insulating top layers, new coating architectures and manufacturing approaches that go beyond simple spray coatings.”

Notably, because this method utilizes widely available materials and a spray-on process, implementing it would be far less expensive and more scalable than the clean room-fabricated options currently available. And its applications are wide-ranging, from helping factories run cleaner and more efficiently to cutting waste in food and chemical systems.

“As soon as you can keep hot liquids from sticking, a lot of downstream problems start to disappear,” Preston said. “That’s what makes this method exciting; it opens the door to surfaces that behave the way we designed them to, even under harsh conditions.”

Co-lead author on the study and Rice doctoral alum Rawand Rasheed, now working as CEO of the Preston lab spinout company Helix Earth in Houston, added: “Our work shows that understanding fundamental science and combining that with practical engineering principals can unlock order-of-magnitude improvements in performance, cost and simplicity.”

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