Direct measurement of unsteady microscale Stokes flow using optically driven microspheres
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Abstract
A growing body of work on the dynamics of eukaryotic flagella has noted that
their oscillation frequencies are sufficiently high that the viscous penetration depth
of unsteady Stokes flow is comparable to the scales over which flagella
synchronize. Incorporating these effects into theories of synchronization requires an understanding
of the global unsteady flows around oscillating bodies.
Yet, there has been no precise experimental test on the microscale of the most basic aspects of
such unsteady Stokes flow: the orbits of passive tracers and the position-dependent phase lag between the oscillating
response of the fluid at a distant point and that of the driving particle.
Here, we report the first such direct Lagrangian measurement of this unsteady flow. The method uses
an array of 30 submicron tracer particles positioned by
a time-shared optical trap at a range of distances and angular positions with respect to a
larger, central particle,
which is then driven by an oscillating optical trap at frequencies up to
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2469-990X
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Wellcome Trust (207510/Z/17/Z)
European Research Council (682754)