How to change surface energy?

High surface energy is important for solution wetting, especially in processes such as spin coating. Most solids with high surface energy do not maintain a high energy surface when exposed to atmospheric conditions. Hydrocarbon contaminants present in the air adsorb to solid surfaces, reducing the surface energy. A common approach to tune surface energy is through surface treatments, which usually aim to enhance the energy by removing these contaminants or forming high surface energy functional groups. Many of these techniques produce only temporary changes in surface energy. This is because adsorption of low surface energy molecules occurs over time, slowly reducing the average surface energy.

UV ozone treatment

UV radiation ionizes oxygen in the atmosphere to form oxygen radicals, which then go on to react and form ozone. Radiation of different wavelengths strikes organic molecules, exciting them or forming free radicals, which react with ozone. This forms volatile molecules that can be easily desorbed from the surface, freeing it from contaminants and having increased surface energy.

Plasma and Corona Treatment

Plasma treatment is also commonly used to clean surfaces and increase surface energy. Here, a gas or gas mixture is ionized by a high-frequency voltage to form a reactive gas plasma. This is a mixture of free radicals, ions, electrons and gas molecules. Plasmas will interact with surfaces in a variety of ways. Contaminants can be removed by ablation, electron and ion bombardment, or reaction with species in the plasma. This forms volatile organic molecules that desorb (and sometimes displace) contaminants with high surface energy functional groups such as C=O and H2O-. In some cases, plasma treatment can etch the surface and affect surface roughness. The exact impact of the plasma treatment will depend on the gas used, for example an oxygen plasma will react with and oxidize organic pollutants,

Corona treatment is similar to atmospheric plasma, but uses a lower plasma density. Here, ionized gas is produced by releasing a high-frequency voltage across electrodes on a grounded surface. The thin film passes under the electrodes and the surface is oxidized to remove contaminants and increase surface energy.

In addition to UV-ozone and plasma treatments, other commonly used methods to increase surface energy include flame treatment, etching, and coating high surface energy interfacial layers of different materials.

lower surface energy

In some cases, such as the generation of hydrophobic coatings, it is desirable to reduce rather than increase the surface energy. This is usually done through coatings of lower surface energy materials such as waxes or through specific surface microstructures designed to minimize solution wetting.