When liquids meet solid surfaces

When liquids meet solid surfaces

TU Darmstadt scientists research what happens at the interfaces between solid surfaces and fluids. Image: Katrin Binner
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TU Darmstadt scientists research what happens at the interfaces between solid surfaces and fluids. Image: Katrin Binner

Whether rain drops roll off or seep into your anorak has a big impact on your comfort level. Whether a car's frictional resistance to the air is large or small makes a difference to its fuel consumption. Whether water boils intensively or moderately has an effect on a cook's time management. These three examples have one thing in common: they deal with the interaction between a gas or a liquid and a solid surface. Water interacts with the fabric or the inside of the saucepan; air interacts with the car body.

Scientists at TU Darmstadt have the know-how to understand precisely the interaction of solid surfaces with fluids, that is, gases and liquids. They research what happens at the interfaces right down to the molecular and atomic level. This knowledge allows them to design surfaces that maximise user utility, for instance surfaces that will spread water evenly, or an aircraft wing that will minimise air resistance.

To achieve this goal, the “Smart Interfaces” Cluster of Excellence bundles expertise from five TU Darmstadt departments and four non-university research institutions.

Avoiding turbulence with plasma actuators

Plasma actuators on aircraft wings improve uplift. Image: Katrin Binner
Plasma actuators on aircraft wings improve uplift. Image: Katrin Binner

Even though it is only a few millimetres thick, the boundary layer, a sheet of air directly on the aircraft wing, plays a vital role. It is a rather busy place, too: air molecules that touch the aerofoil stick to it and are swept along with the moving aircraft. The air molecules above are somewhat slower, and at the upper end of the layer, they have the same speed as the surrounding air, which means they are usually much slower than the air molecules at the bottom. So within this thin layer, air molecules fly at widely varying speeds.

Air molecules do not always move evenly on parallel tracks, but often in a chaotic jumble. Depending on the airspeed, turbulence occurs at different points downstream from the front edge of the wing and needs to be avoided because it increases the aircraft's air drag and thus its fuel consumption.

Researchers at TU Darmstadt have found a way to partially suppress turbulence with a plasma actuator which uses high voltage to ionise neutral air molecules. The electric field accelerates the charged particles, creating a force directly in the air. Precise pulsation of this force can smooth out existing boundary layer disruptions, which could otherwise quickly turn into turbulence. The result of this is that the transition to turbulence is pushed towards the back edge of the wing. Turbulent air flow around the wing decreases, as does air drag.

Essentially, plasma actuators can be used on any body that moves through air – such as gas turbines.

Nothing will stick to these surfaces

The lotus leaf is the most famous example of a self-cleaning surface. Image: Center of Smart Interfaces
The lotus leaf is the most famous example of a self-cleaning surface. Image: Center of Smart Interfaces

Save yourself a trip to the car wash – the next rain shower will leave your car squeaky clean. Nature shows us how this could work: the surface of the lotus plant repels rain drops, which take dirt particles along with them. The surface of the lotus plant cleans itself. Humans have copied this principle from nature and come up with self-cleaning products such as exterior paint or clothing that repels coffee or ketchup stains.

But a lotus-effect car is still not in sight, because self-cleaning surface coatings are not scratch-resistant and durable enough yet. Scientists in the “Smart Interfaces” Cluster of Excellence want to change all that.

The self-cleaning lotus effect rests on two pillars. Firstly, the surface must repel water so that water drops will not adhere to it. Instead of forming a dome, they largely retain their sphericity, offering only a small contact surface, like a very slightly deflated beach ball lying on the ground. Secondly, the surface has microscopic irregularities, much like the studs on a football boot, on which water drops will perch as though they were on a bed of nails. This decreases the contact with the surface yet further. The drops easily roll off the surface and wash away dirt particles which also have little contact with the coating.

The raspberry model

Stages of a drop striking the surface of a raspberry drupelet (top), a lotus leaf (middle) and a glass surface (bottom). Image: Center of Smart Interfaces
Stages of a drop striking the surface of a raspberry drupelet (top), a lotus leaf (middle) and a glass surface (bottom). Image: Center of Smart Interfaces

In order to make coatings more durable, researchers have designed microscopic beads reminiscent of raspberries. They consist of an outer shell of hard silicon dioxide (silica), dotted with tiny nodules. This unevenness minimises the contact surface with the water drops. Inside the beads is a polystyrene core.

The surface that is to be treated is coated with the small beads and then steamed in a solvent which easily dissolves the polystyrene. The plastic then penetrates the silicon mantle through pores and spreads out between the beads on the outside, forming plastic bridges that bind the beads together permanently.

Boiling processors

Pressure chamber to measure evaporative cooling. Image: Katrin Binner
Pressure chamber to measure evaporative cooling. Image: Katrin Binner

CPU performance is constantly increasing while processors are getting ever smaller. As performance increases, heat build-up becomes a problem. Fans will no longer be able to deal with the waste heat, because air cannot discharge sufficient heat. A phenomenon better known in the kitchen than in the office might help here: boiling. Steam bubbles transport huge amounts of energy in short periods of time from the bottom of the pot into the liquid itself. Boiling a liquid could therefore cool processors noiselessly and efficiently.

But there is boiling and boiling. Depending on the form of the hot surface, a larger or smaller number of steam bubbles will appear in the liquid above, detaching themselves from the surface at varying speeds. Researchers are trying to generate steam bubbles that detach themselves as fast as possible so that heat can be transported with maximum efficiency.

They thus designed a surface resembling a bed of nails with microscopic needles that generate lots of bubbles that detach themselves very rapidly. The “bed of nails” transports heat ten times faster than a smooth surface.

Research for greater efficiency

Boiling experiment to cool a computer chip. Image: Center of Smart Interfaces
Boiling experiment to cool a computer chip. Image: Center of Smart Interfaces

But this is not a universal solution. Industry uses various liquids with various boiling points for their cooling systems. In order to design customised surfaces, researchers must understand exactly how the bubbles are generated, what their local temperature is, and how they finally detach themselves.

To do so, they conduct boiling experiments in the laboratory and on research planes flying on so-called parabolic flight paths that cause weightlessness inside the aircraft. Weightlessness is of interest to researchers because the bubbles increase in size and break away more slowly; the whole procedure can be observed in slow motion. Researchers also use computer simulations that help them gain a quantitative understanding of heat transport during the boiling process.

The knowledge they acquire can be applied in many ways. Car engines are also achieving ever higher performance in ever smaller spaces. If the heat produced in the process could be discharged faster with the help of intelligent surfaces, engines could become even smaller and even more efficient.