Superslippery Liquid-Repellent Surfaces

Project Information

SuperRepel

Grant agreement ID: 725513

DOI

10.3030/725513

Project closed

EC signature date 17 January 2017

Start date 1 June 2017

End date 31 May 2022

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 999 467,64

EU contribution

€ 1 999 467,64

Coordinated by

AALTO KORKEAKOULUSAATIO SR
Finland

Periodic Reporting for period 4 - SuperRepel (Superslippery Liquid-Repellent Surfaces)

Reporting period: 2021-12-01 to 2022-05-31

Summary of the context and overall objectives of the project

Repellent surfaces —surfaces that do not get wet— are extremely important in society because they stay dry and clean and prevent fog and ice formation. On such surfaces, water droplets bead up, are highly mobile, and experience low friction. Despite these attractive properties, the application potential of repellent surfaces is usually hindered by the current state of the art - these surfaces remain fragile and are prone to wetting defects. Thus, there is significant potential for their improvement by making them durable, defect-free, and more repellent. To improve the development of repellent surfaces, there is an urgent need to understand the droplet dissipation dynamics on such surfaces (e.g. how they get pinned on wetting defects), which has been hindered due to a lack of accurate and precise characterization techniques.

The SuperRepel project overall goal was to address these complex and challenging scientific issues, to progress substantially the understanding and applications of extremely repellent surfaces, tying together basic research and attractive technological advancements. SuperRepel provided novel repellent surfaces, new methodologies for their characterization and applications. Its outcome significantly impacts superrepellency research and opens new horizons for science and technology.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During SuperRepel project, we have made major achievements that enabled us to publish in journals of the highest visibility, including Science, Nature, Advanced Materials, Science Advances, Advanced Science, Nature Communications, and Nature Protocols. We have established a protocol for the current state-of-the-art wetting characterization technique, contact angle measurements, to facilitate standardization of the measurements (Nature Protocols, 2018). Further, we have described the community-wide problem of contact angle inaccuracy (Science, 2019; Soft Matter, 2019). As a solution for current limitations in wetting characterization, we have proposed and developed the following cutting-edge force-based characterization techniques:

* Oscillation Droplet Tribometer (ODT) for friction measurements of dynamic droplets on repellent surfaces (Droplet, 2022),
* Micropipette Force Sensor (MFS) for friction measurements and assessment of droplet dissipation dynamics (Communications Materials, 2020),
* Scanning Droplet Tribometer (SDT) for scanning surface wetting and wetting defects by scanning friction forces across the sample (ERC PoC project), and
* Scanning Droplet Adhesion Microscope (SDAM) for mapping surface wetting and wetting defects by measuring droplet adhesion forces (Nature Communications, 2017). The SDAM was recognized by Anton Paar company with the Research Award for Instrumental Analytics and Characterization in 2019.

We have also highlighted the importance and advantage of force-based characterization techniques for surface wetting characterization (Advanced Materials, 2021). With these developments, we have established fundamental understandings and have developed a next generation of highly repellent and slippery (very low friction) materials with unprecedented robustness and durability (Nature, 2020). Besides fundamental development, we explored various types of applications, including droplet transport in capillaries for microfluidics (Science Advances, 2020) and 3D printing of repellent surfaces of complex shapes (Advanced Materials, 2021).

These scientific achievements resulted in a set of commercialization projects. Business Finland TUTL SDAM (2018) has been granted to prepare commercialization of SDAM, while ERC PoC (2019) for commercialization exploration of SDT. We also have established Droplet Instruments, the state-of-the-art technologies for wetting characterization (http://droplet-instruments.com/). Furthermore, Business Finland Aalto-R2B-Armor (2021) has been granted for developing dry and durable repellent surfaces (https://www.dry-and-durable.com) and a startup company is planned to be established.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During SuperRepel, there has been made progress beyond state of the art on different fronts:
Cutting-edge wetting characterization techniques. Novel and unique force-based methods have been developed that can measure friction and adhesion forces of the droplets on repellent surfaces down to a couple of nano-newtons. This is an order of magnitude better force resolution than those of commercially available wetting characterization techniques. Besides this, MFS can also study flow inside the droplets (Communications Materials, 2020), SDT can rapidly scan wetting properties and map wetting inhomogeneity even inside the tubes, while SDAM can map wetting properties with ten-micrometer spatial resolution on curved and nontransparent surfaces that have never been probed before (Nature Communications, 2017; Advanced Materials, 2021). With the development of SDAM novel concept of visualization of wetting properties in the form of wetting force maps has been introduced (Nature Communications, 2017). These techniques present powerful tools in surface wetting characterization and will guide new standards for developing super repellent and super slippery surfaces in the future.

Groundbreaking superhydrophobic durability. The state-of-the-art repellent surfaces have extreme repellency but lack longevity and mechanical durability. In SuperRepel, extremely repellent surfaces with unforeseen resistance to mechanical abrasion have been developed (Nature, 2020), and thus the new standard for durability and robustness of these surfaces has been set. This opens a new perspective for repellent surfaces and brings them to real-world applications.

Novel fundamental knowledge. From the fundamental point of view, it has been shown that super repellent surfaces allow for unique droplet dynamics. Thus, a new phenomenon in physics where high viscous liquids flow faster than low viscous counterparts have been discovered (Science Advances, 2020). This could help to guide the future design of microfluidic devices where viscous loss should be avoided.

Droplets rolling on a superhydrophobic surface (Photo by Mika Latikka).