A new method uses ultraviolet light to control the flow of fluids by encouraging particles — from plastic microbeads, to bacterial spores, to pollutants — to gather and organize at a specific location within a liquid and, if desired, move to new locations.
Many applications related to sensors, drug delivery, and nanotechnology require the precise control of the flow of fluids. The new method provides a simple means to gather particles at a particular location so that they can perform a useful function and then move them to a new location to perform the function again. The method could be used to gather silica or polymer beads that carry a payload, like antibodies or drugs, at particular locations within a fluid.
The process first involves adding a small amount of titanium dioxide or gold nanoparticles to a liquid like water that also contains larger particles of interest like pollutants or beads carrying a pay-load. Shining a light at a specific point in the liquid heats up the tiny metal nanoparticles and the heat is then transferred to the fluid. The warmer liquid then rises at the point of light — just as warm air rises in a chilly room — and cooler water rushes in to fill the space that the warm water just left, bringing the larger particles with it. This causes the larger particles to collect at the point of UV light, where they form closely packed, well-organized structures called colloidal crystals. Changing the intensity of the light or the amount of titanium dioxide or gold particles alters how quickly this process occurs.
When the light is removed, the larger particles randomly diffuse through the liquid. But if the light is instead relocated, the larger particles move toward the new point of light, mostly maintaining their structure as they move. This dynamic assembly, disassembly, and movement of organized particles may have important implications for sensing and drug delivery. In addition to water, the researchers demonstrated the effectiveness of this method in hexadecane, an organic liquid, which suggests the technique could be applied in biological fluids.
The research team is currently testing the limits of this method; for example, if particles can move uphill toward the light source or if the method can be used to sort particles by size. Because ultraviolet light and titanium dioxide are easy to control, this method could be harnessed in various technologies; for example, a fluid pump that relies on this method could potentially replace the bulky and more expensive traditional pumps that require a power source or that rely on magnetics or mechanical movement to function.