Artist rendering of particle-induced convection in a microchannel. |
From driftwood traveling down a river to a blood cell flowing
through your artery, objects moving in a stream of fluid are mostly thought to
passively go with the flow but not disturb it in controllable ways.
Researchers at the University of California, Los Angeles
(UCLA) Henry Samueli School of Engineering and Applied Science have
recently found that objects within a confined stream create controllable
disturbances that can be used to move mass or heat at high rates, potentially
providing simple solutions to performing chemical reactions on particles or cooling
microelectronic chips.
Bioengineers at the UCLA’s Microfluidic Biotechnology Laboratory
have been studying the behavior of small objects flowing in microfluidic
systems—small-scale pipes with dimensions similar to that of a human hair. They
have previously demonstrated the existence of several non-intuitive
behaviors, including the lateral movement and alignment of randomly distributed
particles entering a channel and the spontaneous ordering of these suspended
objects into trains with consistent spacing.
In addition to studying how the momentum of the fluid acted on
the objects, the researchers investigated how objects themselves affect the
flow. They found that suspended and self-aligned spheres in a microchannel
about five to 10 times larger in diameter than the spheres within induced
an additional lateral motion of fluid that helped move surrounding fluid
towards the aligned stream.
The researchers identified that this effect depends on the
combined effect of fluid inertia and the nearby walls around the
particles. Conversely, if the objects were not initially aligned while
flowing downstream, the effect becomes destructive, leading to less fluid motion.
The new discovery has implications for enhancing mass and heat
transfer by the simple addition of small particles to a flow. The
transport of fluid, and therefore heat, uniquely operates at an improved
efficiency as flow rate increases, which is normally not expected.
The research, published online in the Proceedings of the National Academy of
Sciences, was led by UCLA biomedical engineering doctoral candidate Hamed
Amini and UCLA associate professor of bioengineering Dino Di Carlo. It was
funded by the National Science Foundation.
“We first became aware of this effect by observing
fluorescent streaks of fluid move across the channel towards the suspended and
aligned particles,” said Di Carlo, who is also a member of the California
NanoSystems Institute at UCLA. “It was quite exciting to see this unexpected and
beautiful behavior for the first time!”
This discovery could enable applications such as the cooling of
electronics to increase their performance, or enhanced catalytic reactions on
the surfaces of suspended microparticles for speedier chemical synthesis.
