Multifunctional surfaces
Interfacial phenomena are the occurrences that take place when two different phases come into contact (e.g. solid-gas, solid-liquid, liquid-gas). Often the consequences of these phenomena are undesirable or even detrimental, and fixing the issue imparts significant costs across many sectors. The way in which water, the most ubiquitous liquid, interacts with a solid surface for example, presents extensive issues from condensation and icing which deteriorate the efficiency of heat exchange systems and the safety of aircraft, to corrosion and staining. Not only does liquid interaction with solid surfaces have detrimental effects, but contact with living organisms results in numerous problems such as biofilm formation (promotes the spread of microbes) and biofouling (decreases the efficiency of marine vessels). Just as with physical matter, light can interact with surfaces to cause unwanted reflections resulting in reduced efficiency of solar cells for example.
It is therefore of paramount importance that these phenomena are controlled, and one way in which this can be achieved is through micro- and nano-structuring of the surface. In Pi-lab we fabricate precisely designed structures with dimensions ranging from sub-100 nm to >1 mm (see micro-/nano-fabrication capabilities for further details) to tackle these issues. Our key projects include wettability (superhydrophobicity, slippery surfaces, anti-condensation), antibacterial surfaces and targeting light interaction (reflectance, transmittance, plasmonics), however these are often overlapping in nature.
Wettability
The study of how a liquid spreads on a solid or liquid is referred to as ‘wetting’, where there exist two regimes; total wetting and partial wetting. This depends on the surface energy at the solid, liquid and air interfaces as well as the roughness of the solid.
Superhydrophobicity
Figure 1
Figure 1 - Schematics demonstrating; (a) a superhydrophobic surface in the Cassie-Baxter state, (b) a superhydrophobic surface in the Wenzel state and (c) a superhydrophilic surface in the Wenzel state. SEM images (d) and (e) are two example surfaces fabricated in Pi-lab with insets of a water droplet atop the surface1.
A superhydrophobic surface is characterised by water droplets assuming near perfect spherical shape and exhibiting a water contact angle >150° (in contrast to superhydrophilicity where the water totally wets the surfaces with a contact angle < 10° as seen in Figure 1c). Superhydrophobicity is achieved through a combination of surface roughness and a low surface energy material/coating. In most cases the aim is to occupy the Cassie-Baxter state (Figure 1a) where the water droplet sits on a composite solid-air interface without penetrating into the structure, in this case the contact angle hysteresis (difference between the advancing and receding angle) is <10°. On the other hand, if the surface exhibits a hysteresis of >10°, the surface is thought to be occupying the Wenzel state (Figure 1b), whereby water penetrates into the structure and follows the roughness. SEM images of two conical nanostructures fabricated in Pi-lab can be seen in Figures 1d and 1e, with the inset showing the water contact angle of a droplet on the surface after functionalisation with a hydrophobic silane; due to the sharp tips of the structures in Figure 1d (163°), the contact angle is greater than that for Figure 1e (152°).
The video below shows a droplet bouncing on one of our surfaces 19 times before rolling off, indicating excellent water repellency.
Anticondensation
Condensation or fogging is a problem as it results in poor visibility due to the nucleation and subsequent growth of water droplets causing light scattering. To prevent this, superhydrophobic surfaces with structures of certain dimensions can be used to combat fogging by preventing large droplets from forming on the surface as is illustrated in Figure 2 below.
Figure 2
Figure 2 – Top view schematic illustrating the difference in the maximum droplet radius between a nanostructured surface and a flat surface.
Slippery surfaces
Lubricant infused surfaces (LIS) comprise a functionalised micro- or nano-structured surface infused with a lubricant. They exhibit very low sliding angles for liquids of high and low surface tension, which can be ascribed to the thin lubricant film above the structure. The video below shows one of our LIS tilted at 45° with a constant stream of water droplets sliding down the surface.
Antibacterial
Bacteria tend to colonize on many surfaces, often forming biofilms – organized bacterial communities – that can be resistant to conventional antiseptic agents. Not only is this colonization potentially detrimental to our health (implants, medical equipment) but it is also problematic in steam condensers, water filtration membranes or pumping systems. It has been recently discovered that some natural surfaces (e.g.: cicada wings) are able to repel or mechanically damage bacterial cells through nanopillars.
We demonstrate how our synthetic analogous can efficiently kill bacteria upon their contact with the nanostructures.
Figure 3
Figure 3 - Bacteria (Staphylococcus aureus) interacting with a flat or a nanostructured surface (SEM image in inset); (A) showing the reduced number of viable cells (bacterial colonies) after the interaction with the pillars, (B and C) images obtained via confocal microscopy indicating increased numbers of dead cells on the nanostructured surface.
Light interaction
It is well known that nanostructures can accurately control such light properties as transmission, reflection, absorption and scattering. In our lab, we have developed moth-eye structures that almost completely eliminate reflections while also combining unique self-cleaning properties. Such structures can be applied in Si to produce highly absorbing solar cells or in glass to produce anti-glare products [1]
Figure 4
Figure 4 – Photos demonstrating some of the nanostructured surfaces produced to manipulate light interaction; (A) a bare 6’’ silicon wafer (top) and a nanostructure silicon wafer (bottom) indicating the suppression of reflections. (B) a nanostructured glass surface which exhibits high diffraction to produce the effect shown in (C) when light is directed through the sample
Representative publications
M. Michalska, S. K. Laney, T. Li, M. K. Tiwari, I. P. Parkin, and I. Papakonstantinou, "A route to engineered high aspect-ratio silicon nanostructures through regenerative secondary mask lithography," Nanoscale 14, 1847–1854 (2021).
P. Lecointre, S. Laney, M. Michalska, T. Li, A. Tanguy, I. Papakonstantinou, and D. Quéré, "Unique and universal dew-repellency of nanocones," Nat. Commun. 12, 4–12 (2021).
M. Michalska, S. K. Laney, T. Li, M. Portnoi, N. Mordan, E. Allan, M. K. Tiwari, I. P. Parkin, and I. Papakonstantinou, "Bioinspired Multifunctional Glass Surfaces through Regenerative Secondary Mask Lithography," Adv. Mater. (2021).
S. K. Laney, M. Michalska, T. Li, F. V Ramirez, M. Portnoi, J. Oh, I. G. Thayne, I. P. Parkin, M. K. Tiwari, and I. Papakonstantinou, "Delayed Lubricant Depletion of Slippery Liquid Infused Porous Surfaces Using Precision Nanostructures," Langmuir 37, 10071–10078 (2021).
Funding
1. Self-cleaning coatings for targeting solar energy and water supply mismatch in India and Brazil, Royal Academy of Engineering Frontiers Research Grant, FF1920182 (2020-2023).
2. IntelGlazing, Intelligent functional coating with self-cleaning properties to improve the energy efficiency of the built environment. European Research Council, ERC-StG GA 679891 (2016-2022).
3. Nanoengineered Smart Surfaces, Lloyd’s Register Foundation International Consortium in Nanotechnology (2017-2021).
4. EPSRC Doctoral Training Program PhD studentship EP/N509577/1 (2017-2021).
5. Low-energy drinking water filtration system with nanopore membranes. Royal Academy of Engineering, Global Research Challenges proof of concept grant, FOESF1617/1/8 (2017).
6. Biologically Inspired Nanostructures for Smart Windows with Antireflection and Self-Cleaning Properties, UK EPSRC, Grant No. EP K015354/1 (2013-2017).
