The international group of researchers from Israeli Technology Institute, University of Extremadura (Spain), New Jersey Institute of Technology (USA) and National University of the Central Buenos Aires (Argentina) developed a new method of oil separation from water in slow-breaking emulsion with the help of surface acoustic waves of 20 MHz frequency. Such method does not need chemical agents, complex membranes or significant energy; hence, it may be a compact and cheap substitution for traditional technologies at small industrial sites, housing and commercial facilities.
Emulsions represent homogenous distribution of oil in water. They emerge during oil production, in food industry, healthcare and domestic sewage. Oil there is in the form of micro drops surrounded by protective skins from surfactants, so it does not mix with water. The classical methods (distillation or coagulation) work only within big plants and required a lot of energy and chemical agents, thus, they are not instrumental for local treatment.
To simplify the process, the researchers used the difference in the way water and oil wet the solid surface. Water with its high surface tension (40–70 mN/m) stays motionless, while as oil with its low surface tension (20 mN/m for silicon and circa 34 mN/m for sunflower oil) disperses under oscillation. During the experiments, the scientists put 10 micro-liters drop with oil content from 10% to 50 % on a piezoelectric plate made of lithium-niobate. The size of oil drops was circa 230 nanometers, and the emulsion itself remained stable for 12–18 months.
After exposing it to surface acoustic wave with 0.5–2.5 nanometers shift amplitude and 60–300 mm/sec oscillation, water started to evaporate. The process went faster at lower humidity: at 50% humidity oil appeared on the edge of the drop after 170–190 seconds, while as at 85% humidity — only after 465 seconds. Due to evaporation, the oil content at the drop surface was growing forming a film. When exposed to acoustic pressure, this film went outside the drop and spread on the surface in the direction opposite the direction of the wave.
The oil left the emulsion in the following way: the acoustic pressure shifted the oil drops inside the emulsion towards the backside of the drop; hence, the first thin “strips” of oil appeared on the sides and behind, not in front. After some time, they merged into continuous film with 25 micrometers length — this is in line with the acoustic wetting when ultrasound of 75 mcm flows into oil. Laser measurements showed that in addition to thick zones there were thin zones of 1–3 mcm in the film, and honeycomb relief with 0.1–0.3 mcm altitude difference and dozens of microns pitch was generated on the surface of thicker zones.
In the opinion of the researchers, weak acoustic waves may be used for local treatment of effluents and grey waters — coming from small plants and residential house, and from major industrial sites as well. This will allow for reducing the volumes of polluted effluents, decreasing the load on treatment facilities and decreasing the chemical agents and energy consumption.
