Focused optical beams for driving and sensing helical and biological microobjects

A novel and interesting approach to detect microfluidic dynamics at
a very small scale is given by optically trapped particles that are
used as optofluidic sensors for microfluidic flows. These flows are
generated by artificial as well as living microobjects, which
possess their own dynamics at the nanoscale. Optical forces acting
on a small particle in a laser beam can evoke a three dimensional
trapping of the particle. This phenomenon is called optical
tweezing and is a consequence of the momentum transfer from
incident photons to the confined object. An optically confined
particle shows Brownian motion in an optical tweezer, but is
prevented from long term diffusion. A careful analysis of the
motion of the confined particle allows a precise detection of
microfluidic flows generated by an artificial or living source in
the close vicinity of the particle. Thus, the particle can be used
as a sensitive optofluidic detector. For this aim, several optical
tweezers at different wavelengths are integrated into a dark-field
microscope, combined with a high speed camera, to achieve a precise
detection of the motion of the center-of-mass of the trapped
particle. With this unique experimental system, a gold sphere is
used as an optofluidic nanosensor to analyze for the first time the
microfluidic oscillations generated by a biological sample. Here, a
freely swimming larva of Copepods serves as the living source of
flow. However, even if the trapping laser wavelength is
off-resonant to the plasmon resonance of the flow detector, a
finite heating of the gold nanoparticle occurs which reduces the
sensitivity of detection. To increase the sensitivity of the
optofluidic detection, a non-absorbing, dielectric microparticle is
introduced as the optofluidic sensor for the microflows. It enables
a quantitative, two dimensional mapping of the vectorial velocity
field around a microscale oscillator in an aqueous environment.
This paves the way for an alternative and sensitive detection
approach for the microfluidic dynamics of artificial and living
objects at a very small scale. To this aim and as a first step, an
optically trapped microhelix serves as a model system for the
mechanical and dynamical properties of a living microorganism. An
optical tweezer is implemented for initiating a light-driven
rotation of the chiral microobject in an aqueous environment and
the optofluidic detection of its flow field is established. The
method is then adopted for the measurement of the microfluidic flow
generated by a biological system with similar dynamics, in this
case a bacterium. The experimental approach is used to quantify the
time-dependent changes of the flow generated by the flagella bundle
rotation at a single cell level. This is achieved by observing the
hydrodynamic interaction between a dielectric particle and a
bacterium that are both trapped next to each other in a dual beam
optical tweezer. This novel experimental technique allows the
extraction of quantitative information on bacterial motility
without the necessity of observing the bacterium directly. These
findings can be of great relevance for an understanding of the
response of different strains of bacteria to environmental changes
and to discriminate between different states of bacterial activity.