A smart surface capable of offering fast and reversible surface wetting properties, from superhydrophobic to superhydrophilic, has been developed by Researchers at the University of British Columbia (UBC) – and it uses such a small voltage it could be powered by conventional batteries.
A unique type of surface able to both repel and absorb liquids, and whose ability to do so – its wetting behavior – can be swiftly and precisely controlled has be sought after for many years. Such technology would have numerous applications ranging from water filtration and lab-on-a-chip systems to biomedical devices and liquid optical lenses.
Such a smart surface has now been created by Researchers at UBC; it is inexpensive, scalable and powered by a conventional electric battery. The copper-based surface changes from being very water-repellent (superhydrophobic) to being very water-absorbent (superhydrophilic) as an electric potential is applied.
When tiny voltages are applied to the surface, water droplets that initially roll off stick to it more and more tightly. By changing the magnitude of the voltage and how long it is applied, we can easily control the angle that each droplet forms with the surface and how quickly this happens.
Ben Zahiri, Co-lead Author of the Study Published in Advanced Material Interfaces
The mechanism underlying the wetting alteration is based on the Faradaic phase transformation at the surface. The oxidation state of the copper surface – revealed to be a mixture of hydrophilic CuO and hydrophobic Cu2O by surface composition analysis - is changed as the electric potential is applied. When the electric potential is removed, the droplet maintains its shape and remains pinned in place.
This switchable wettability allows for simple and precise control over the surface wettability, ranging from superhydrophobic to superhydrophilic with a short response time. The rate of wetting transition and the desired contact angle - the angle where a liquid-vapor interface meets a solid surface and thus quantifies wettability using the Young equation – can be precisely controlled by controlling the magnitude of the applied potential and how long for.
The process is completely reversible when the sample is dried at ambient temperature, or heat dried at 100°C. The latter does not affect the surface composition when compared with drying at room temperature.
The Researchers chose copper because it is cheap, abundant and one of the most commonly used metals in the world. Many groups have modified the behavior of copper surfaces using other stimuli like heat, UV radiation and X-rays. However, to achieve this, temperatures of up to 300°C are required, and exposure times range from tens of minutes to days, which make them impractical for consumer and industrial purposes.
In contrast, the electric stimulus the UBC team used modifies the wetting behavior rapidly – from a few seconds to a few minutes – and reversibly at voltages less than 1.5V, commonly found in everyday batteries.
Although the Researchers initially worked with copper, Zahiri believes that similar result might occur with the electrochemical manipulation of other metals, metal oxides and mixed oxides. Other conductive fluids – such as blood – could also be used.
These findings could open up a new area of exploration for smart surfaces.
Walter Mérida, Mechanical Engineering Professor, UBC
The ability to control surface wettability could find applications wherever droplets, or solid particles absorbed by droplets, need to be manipulated, including microfluidic devices and hazardous material handling systems. It also offers advanced self-cleaning capabilities by enabling the controlled roll-off of fluids and could be useful for self-cleaning windows and solar cells.
Image Credit: University of British Columbia