Traditional white-light and fluorescent imaging techniques provide powerful methods to extract high-resolution information from two-dimensional (2-D) sections, but to retrieve information from a three-dimensional (3-D) volume they require relatively slow scanning methods that result in increased acquisition time. Using an ultra-high speed liquid lens, we circumvent this problem by simultaneously acquiring images from multiple focal planes. We demonstrate this method by imaging microparticles and cells flowing in 3-D microfluidic channels.
The video stream captured by an in-line holographic microscope can be analyzed on a frame-by-frame basis to track
individual colloidal particles' three-dimensional motions with nanometer resolution, and simultaneously to measure their
sizes and refractive indexes. An efficient particle-tracking algorithm automates initial position estimation with sufficient
accuracy to enable unattended holographic particle tracking and characterization. In this work, we demonstrated this
approach to flow visualization in a microfluidic channel and also to flow cytometry of micrometer-scale colloidal
spheres.
Mechanical equilibrium at zero temperature does not necessarily imply thermodynamic equilibrium at finite
temperature for a particle confined by a static, but non-conservative force field. Instead, the diffusing particle
can enter into a steady state characterized by toroidal circulation in the probability flux, which we call a Brownian
vortex. The circulatory bias in the particle's thermally-driven trajectory is not simply a deterministic response
to the solenoidal component of the force, but rather reflects an interplay between advection and diffusion in
which thermal fluctuations extract work from the non-conservative force field. As an example of this previously
unrecognized class of stochastic machines, we consider a colloidal sphere diffusing in a conventional optical
tweezer. We demonstrate both theoretically and experimentally that non-conservative optical forces bias the
particle's fluctuations into toroidal vortexes whose circulation can reverse direction with temperature or laser
power.
Optical traps use forces exerted by specially structured beams of light to localize microscopic objects in three
dimensions. In the case of single-beam optical traps, such as optical tweezers, trapping is due entirely to gradients
in the light's intensity. Gradients in the light field's phase also control optical forces, however, and their quite
general influence on trapped particles' dynamics has only recently been explored in detail. We demonstrate
both theoretically and experimentally how phase gradients give rise to forces in optical traps and explore the
sometimes surprising influence of phase-gradient forces on trapped objects' motions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.