Shear-free chemotaxis: nanoparticle tracking showed significant flow reduction conferred by the membrane

To assess how shear-free our [ “shear-free” chemotaxis system] is, we tracked the motion of nanoparticles in the shear-free compartment as we input a known flow rate in the flow compartment.  The nanoparticles we use are the 210 nm dragon green fluorescent beads.

==================================================================Following is a 6-min long movie of the nanoparticles flown at a flow rate of 0.01 uL/min in the shear-free compartment.  This movie serves as our reference: [Movie] bottom 0.01 uLmin (6 min at a playback of 5 fps)

Tracking of the nanoparticles: [Image] bead tracking bottom 0.01 uLmin (2012-11-14)

The average speed of the ensemble of particles is 845 um/min.

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To push the limit our system without breaking the membrane, we input a flow rate of 10 uL/min in the flow compartment on top and imaged the motion of the nanoparticles in the shear-free compartment in the bottom: [Movie] top_10_uLmin (3 min at playback of 5 fps)

Tracking of the nanoparticles: [Image] bead tracking top 10 uLmin (2012-11-14) .  The red number is the ID number assigned to each particle and is positioned at the starting point of each particle trajectory.

We performed a linear curve-fitting to the x and the y position of each particle to determine the underlying drift speed component in the x and the y direction:

[Particle Tracking] baseline drift in x

[Particle Tracking] baseline drift in y

We then assessed the baseline drift speed of each particle to see if the speed is normally distributed with a mean speed of 0.  If it is, then the particle motion that we observed is essentially just diffusion.  We used a statistical technique known as the quantile-quantile plot (Q-Q plot) to determine how closely matching is the distribution of particle drift speed to a normal distribution.  In a Q-Q plot, essentially the area under the cumulative distribution of the drift speed is compared to that of a corresponding normal distribution.  The more closely matching the two are, the more likely that the drift speed is normally distributed.

[QQ-plot] drift speed in the x-direction

[QQ-plot] drift speed in the y-direction

If we insist that the particle motion is convective in nature, then the drift speed of the ensemble of particles is 1.38 um/min.  This is determined by taking the square root of the sum of the square of the x and the y component of the average drift speed of the whole population of particles.  If we assume the particlemotion to be purely diffusive, the mean diffusion coefficient of the whole population is 112 um/min, comparable to the diffusion coefficient of 86 um/min for a 210 nm particle.

NOTE: For gradient generation only an input flow rate of 0.50 uL/min is required in the flow compartment, so the 10 uL/min is 20 times more than needed.

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CONCLUSION: The motion of the nanoparticles appears to be diffusive in nature even with the relatively high input flow rate (10 uL/min) in the flow compartment.  As far as chemotaxis is concerned the system is indeed “shear-free”.