We describe a microfluidic device for studying the orientational dynamics of microrods. The device enables us to
experimentally investigate the tumbling of microrods immersed in the shear flow in a microfluidic channel with
a depth of 400 μm and a width of 2.5 mm. The orientational dynamics was recorded using a 20X microscopic
objective and a CCD camera. The microrods were produced by shearing microdroplets of photocurable epoxy
resin. We show different examples of empirically observed tumbling. On the one hand we find that short stretches
of the experimentally determined time series are well described by fits to solutions of Jeffery's approximate
equation of motion [Jeffery, Proc. R. Soc. London. 102 (1922), 161-179]. On the other hand we find that
the empirically observed trajectories drift between different solutions of Jeffery's equation. We discuss possible
causes of this orbit drift.
Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest. We report on the development of single fiber ultrafast optical tweezers and its use in simultaneous two-photon fluorescence (TPF) excitation of trapped fluorescent microscopic objects. Using this method, trapping depth of a few centimeters was achieved inside a colloidal sample with TPF from the trapped particle being visible to the naked eye. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber tip, a relatively longer streak of fluorescence was observed along the microsphere length. The cone angle of the axicon was engineered so as to provide better trapping stability and high axial confinement of TPF. Trapping of the floating objects led to stable fluorescence emission intensity over a long period of time, suitable for spectroscopic measurements. Furthermore, the stability of the fiber optic trapping was confirmed by holding and maneuvering the fiber by hand so as to move the trapped fluorescent particle in three dimensions. Apart from miniaturization capability into lab-on-a-chip microfluidic devices, the proposed noninvasive microaxicon tipped optical fiber can be used in multifunctional mode for in-depth trapping, rotation, sorting, and ablation, as well as for two-photon fluorescence excitation of a motile sample.
High throughput analysis of trapped samples requires effective loading and unloading into the trap in a microfluidic
environment. We demonstrate development of a hybrid optical transport trap (HOTT) which combines a tapered fiberoptic
2D trap for transport of microscopic objects into and out of the optical tweezers trap in an orthogonal geometry.
For small cone angle of the tip, the microscopic objects (polystyrene and red blood cells) were found to be trapped in
two-dimensions and pushed along the axial direction by domination of scattering force. This was found to be in
consistence with the estimated axial forces caused by the beam profiles emerging from the small-cone tapered fiber tip.
While for loading of the microscopic objects into the optical tweezers trap, the fiber tip was placed ~ 30μm away from
the tweezers trap, unloading was carried out in presence of the tip close (<15 μm) to the tweezers trap. Further, for a
fixed fiber trap and tweezers separation (~ 30 μm), both loading and unloading could be achieved by reducing the
tweezers trap power so that the scattering force exerted by the fiber trap exceeded the transverse gradient force of
tweezers trap. Since the tapered tip can be easily integrated onto a microfluidic channel, the proposed configuration can
find potential applications in lab-on-a-chip devices. We demonstrate analysis of transported microscopic objects using
digital holographic microscopy integrated with the HOTT.
Two photon polymerization (TPP) has enabled three-dimensional microfabrication with sub-diffraction limited
spatial resolution. However, depth at which TPP could be achieved, has been limited due to the high numerical
aperture microscope objective, used to focus the ultrafast laser beam. Here, we report fiber-optic two photon
polymerization (FTP) for in-depth fabrication of microstructures from a photopolymerizable resin. A cleaved single
mode optical fiber coupled with tunable femtosecond laser could achieve TPP, forming extended waveguide on the
fiber itself. The length of the FTP tip was found to depend on the laser power and exposure duration. Microfabricated
fiber tip using FTP was employed to deliver continuous wave laser beam on to polystyrene microspheres
in order to transport and manipulate selected particles by scattering force and 2D trapping. Such microstructures
formed by TPP on tip of the fiber will also enable puncture and micro-surgery of cellular structures. With use of a
cleaved fiber or axicon tip, FTP structures were fabricated on curved surfaces at large depth. The required Power for
FTP and the polymerization rate was faster while using an axicon tip optical fiber. This enabled fabrication of
complex octopus-like microstructures.
Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest.
We report on the development of single fiber femto second optical tweezers and its use in two-photon fluorescence
(TPF) excitation of trapped fluorescent particles. Trapping of the floating objects led to stable fluorescence emission
intensity over a long period of time, suitable for spectroscopic measurements. Trapping depth of few cm was achieved
inside colloidal sample with TPF from the trapped particle being visible to the naked eye. Furthermore, the fiber optic
trapping was so stable that the trapped particle could be moved in 3D even by holding the fiber in hand and slow
maneuvering of the same. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber
tip, a relatively longer streak of fluorescence was observed along the microsphere length. The cone angle of axicon was
engineered so as to provide better trapping stability and high axial confinement of TPF. The theoretical simulation of
fiber optical microbeam profiles emerging from the axicon tip and trapping force estimations was found to be in good
agreement with the experimentally observed stiffness and TPF patterns. Apart from miniaturization capability into lab-on-
a-chip micro-fluidic devices, the proposed non-invasive micro axicon tipped optical fiber can be used in multifunctional
mode for in-depth trapping, rotation, sorting and ablation as well as for two-photon fluorescence excitation of
motile sample which will revolutionize biophysics and research in material science.
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