Wick-based air fresheners and insect repellents are multi-billion-dollar markets, and yet the products could work better: Their effects often fade before the liquid within them is depleted.
The problem, says mechanical engineering professor Krishna Pillai, is that heavier molecules clog the wick, slowing evaporation.
“Lighter molecules evaporate first, leaving the heavier ones behind,” said Pillai, who encountered this problem with working with SC Johnson years ago. “It’s like a traffic jam where there are no small cars that can quickly move through small spaces, only big trucks that clog the road.”
Pillai and doctoral student Abul Borkot Md Rafiqul Hasan have developed a next-generation device to solve this problem. Their innovation, which improves the evaporation rate of volatile liquids, has earned a provisional patent – one of six filed from the college in the past year (see sidebar).
The science of wicking
Wicking is a common way of moving a liquid without a power source using a porous channel, Pillai said. The process works by “capillary action” – liquid is drawn upward into the porous material through the forces of adhesion and surface tension. But the lighter molecules evaporate more quickly, leaving the heavier molecules to accumulate and clog.
To fix this, the researchers created a device with three rotating wicks. As they move in and out of a liquid reservoir, they pass in front of a fan, ensuring better dispersion and circumventing the logjam.
The concept could work for dispersal in room-sized areas or for entire buildings if integrated with HVAC systems, Pillai said. Another application could be insect control in outdoor areas, such as arenas.
Testing various scenarios
With help from two undergraduates, the team tested five different designs using three substances similar to fragrances. Each of the substances – Hexadecane, Dodecane and Decane – are hydrocarbons with different-sized molecules. Hexadecane is the heaviest and Decane is the lightest.
The team recorded evaporation rates for multiple designs of dispensing devices over 24 hours, said Hasan, while also analyzing surface area, airflow velocity, and molecular concentration. The final prototype achieved the highest evaporation with the least remaining concentration of Decane in the reservoir.
“Bridging the gap between theoretical and experimental, and then seeing the device function as intended, was an incredibly fulfilling experience,” said Hasan, whose doctoral research focuses on how temperature variations influence liquid transport in porous structures.
Research with the prototype device has just been published in the International Journal of Heat and Mass Transfer.