lotus effect - lotos effect - de-icing

De-icing: Using the lotus effect to fly more safely and save CO2

Martina Roters
30.03.2020
4 minutes

The Fraunhofer Institute for Material and Beam Technology (IWS) issued a press release at the beginning of March describing promising attempts (in cooperation with TU Dresden and Airbus) to functionalize the surface of an aluminium alloy that is widely used in aircraft construction by means of special structures produced by laser pulses. In this way, the surface becomes extremely water and ice repellent. WingMag has taken a closer look.

De-icing – Safety, ecology, economy

These are the three aspects at the centre of the topic of de-icing of aircraft.

WingMag reported in detail about the “why” (= safety) in an earlier article about flying in winter. We also reported about a new approach: the application of special multicomponent surfaces, which store the energy of sunlight and can thus contribute to environmentally friendlier de-icing.

Now there is a brand-new approach, which does not require sunlight and is already being tested in flight trials on an A350 with a specially prepared test area on one wing. If the technique gains acceptance, then the ecologically harmful chemical spraying treatments before take-off can be dispensed with. What is more, the energy expended to heat the wings during flight can be reduced by 90 percent.

How does the lotus effect work?

In nature, water simply rolls off many plants. Now, humankind is taking advantage of this patented effect. Maybe you already have a washbasin with lotus effect? Other than perhaps assumed by a layperson, the surface should not be as smooth as possible in order for the drops to roll off…

lotus effect - de-icing
The lotus effect is now being used for de-icing / © Pixabay Sweetaholic 1452667

Whether a surface is water repellent depends on the contact angle (= wetting angle) of the drops landing on the surface. The angle is small for hydrophilic surfaces, which are easily wetted, and large for hydrophobic surfaces.

If a surface is finely structured, there are two possibilities, which are named after researchers:

Lotus effect - de-icing
The Cassie–Baxter or fakir state

It is certainly not difficult to imagine how the complete wetting in the Wenzel state is detrimental to the slipping off of the ice from the surface, because of the increased contact surface area – clearly illustrated here by the blue dentate (“toothed”) surface. Thus a fakir state (Cassie–Baxter state) is desirable, where the water drop effectively balances on many elevations.

A laser-sharp solution

In cooperation with the University of Dresden, the Fraunhofer IWS produced various surface structures on a high-performance titanium alloy (Ti–6Al–4V). These surface structures represented the most-promising compromise between sufficiently large air pockets and sufficiently numerous elevations, which make a fakir state, and thus a rolling-off of the drops, possible in the first place.

These were then placed on an airfoil (NACA) that was subsequently submerged in a chemical solution to enhance the water-repellent characteristics. The surface produced in this way has a contact angle of over 160° and is thus superhydrophobic.

Numerous tests in the wind tunnel simulated the conditions that a real aeroplane wing has to withstand in operation, for example with parameters such as humidity, wind temperature (e.g. 120 m/s), angle of attack, temperature (e.g. minus 20°C) …

The best laser process for applying the structures turned out to be Direct Laser Interference Patterning, DLIP for short, in which the main laser beam is split into two or more that overlap and which superimpose on the surface at an exactly defined angle. Material is removed at a speed of up to 100 cm2/s. The patterns are determined by pulse energy and wavelength, resulting in three-dimensional structures in the micro- and submicrometre range.

On the Fraunhofer IWS website, researchers of the Fraunhofer IWF and Airbus explain their approach. At the bottom of the page you will find an animation illustrating the innovation.

Now this approach has to prove itself in the A350 flight lab at Airbus. We are eagerly awaiting news!

by Martina Roters

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