Lutz Lab
Department of Bioengineering, University of
Washington
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Oscillating flow microfluidics for single cell trapping (prior work with Dan Schwartz, UW)
Steady streaming is a secondary flow created when oscillating fluid is required to turn, such as around an obstacle or a bend. In contrast to the great work using acoustic forces in microfluidics for mixing and cell manipulation, the physics here is entirely different – there are no acoustic effects whatsoever, rather the flow is driven by a simple back and forth motion of the fluid (it is sometimes referred to as “acoustic streaming”, but I prefer the more accurate term “steady streaming” given by N. Riley). Steady streaming was first described in the 1800’s by Lord Rayleigh, and since then has mostly been an intellectual curiosity written up in fluid dynamics journals. The flow is often characterized by recirculating eddies, and the speed of rotation and size of the eddies are controlled by the amplitude and frequency of the oscillation, respectively.
Imagine
eddies created behind a bridge pillar in a river – now, if you reversed the
flow of the river back and forth, you might expect to see eddies formed on both
sides of the pillar (and the direction of rotation is what you would expect). This
flow is governed by two dimensionless parameters, amplitude and frequency, thus
it can be created and predicted at size scales ranging from bridge pilings in
low-frequency ocean wave action (sub-Hertz) down to the microfluidic
length scales and audible frequencies (e.g., 100 Hz) that we explored.
In
my PhD work with Dan Schwartz, we showed for the first time that steady
streaming eddies can be used to trap particles or cells without any contact
with solid surfaces. The trapping force is so strong that it can trap swimming
cells (we tested swimming plankton) and can hold objects even in the presence
of flow (up to 1 cm/sec, which is huge for microfluidics).
The well-known non-contact trapping methods are optical tweezers and dielectrophoresis – the intense light and electric fields
can alter cell behavior or even kill sensitive cells. Our approach, which we
call hydrodynamic tweezers, creates comparable trapping forces (piconewtons) without these unnatural perturbations, and the
shear stress created is within the natural range of typical suspension cells
(e.g., cells that circulate in the blood).
References:
· Lutz, B.R.; Chen, J.; & Schwartz, D.T. “Hydrodynamic tweezers: non-contact trapping of single cells using steady streaming microeddies.” Analytical Chemistry, 78, 5429-5435 (2006). [pdf
]