photomicrograph of surface

Direct laser interference patterning (DLIP) creates a regular array of pillar-like structures in aluminum that can be used to fashion superhydrophobic, “self-cleaning” surfaces. [Image: © Technische Universit├Ąt Dresden]

A research team from Technische Universit├Ąt (TU) Dresden and the Frauhofer Institute for Material and Beam Technology IWS, Germany, has developed a process for creating superhydrophobic, “self-cleaning” aluminum surfaces using direct laser writing (DLW) and direct laser interference patterning (DLIP), and a method for characterizing the effectiveness of these laser-written surfaces (Appl. Surface Sci., doi: 10.1016/j.apsusc.2020.146518).

Creating superhydrophobic surfaces

Superhydrophobic surfaces are those on which water droplets show virtually no adherence, making the surface extremely hard to wet. Such surfaces not only are extremely repellent to water and ice, but also raise the possibility of cleaning merely by the action of water droplets rolling along the material and picking up dirt particles along the way. Such surfaces could prove particularly useful in areas such as food processing, where the disadvantages of using caustic chemical products for surface cleaning are obvious.

The process developed by the TU Dresden–Fraunhofer team involves using DLW to create a meshlike “trench and hill” structure on the aluminum surface, followed by picosecond pulsed DLIP, which creates a regular period array of microscale lumps or pillars. Combining the two processes results in a nubby, hierarchical structure combining both morphologies.

High-speed camera tests

To characterize and test the technique’s ability to create a dynamically self-cleaning surface, the researchers first use the aluminum grade Al20204, commonly used in the aerospace industry, to create three different laser-processed samples: one using DLW alone, one using DLIP alone, and one combining the two techniques. The researchers next contaminated each surface with dirt (specifically manganese oxide and polyamide particles), and set the surfaces up on an inclined plane in front of a high-speed camera capable of grabbing 12,500 frames per second.

They then rolled water droplets down the surfaces and used the camera to assess how effectively the droplets picked up and removed the surface contamination. They found that both the surface using DLIP alone and the one combining DLW and DLIP resulted in particularly high cleaning efficiencies. Indeed, one DLIP-created surface, with a spatial period of 7.0 microns, showed a cleaning efficiency of up to 99% of the contaminating particles.

The TU Dresden–Fraunhofer team used high-speed cameras to capture images of individual water droplets lifting off micron-scale contaminant particles as they dribbled down the superhydrophobic surfaces.

The experimental setup also provided some cool views of the water droplets doing their work. “This way we can perfectly see how a water drop can remove the dirt from the aluminum surface,” noted Fraunhofer coauthor Thomas Kuntze in a press release accompanying the work.