Swimming bacteria exploit viscous drag forces to generate propulsion in low Reynolds number environments. A rotating helical flagellar bundle can propel the cell body at typical speeds of ten body lengths per second. Not surprisingly, this ability to efficiently swim is preserved even in confining micro-environments which constitute their typical habitat. Quantitative studies would require the ability of fabricating complex environments with controlled geometrical properties. Experimental studies so far were limited to large diameter micro capillaries or 2D confinement. In this last case, E.coli has been shown to swim with an unaltered speed even when the gap size is slightly larger than the cell body thickness. The case of tight 1D confinement is however more challenging requiring 3D fabrication capabilities. Using two-photon polymerization we fabricate 3D microstructures that can confine swimming bacteria in quasi 1D geometries.
We observe individual E.coli cells swimming through a sequence of micro-tunnels with progressively decreasing diameters. We demonstrate that E.coli motility is preserved also in tight 1D confinement. Moreover we find that there's an optimal channel diameter for which the increase in flagellar thrust due to 1D confinement can even overcome the increased drag on the cell body resulting in swimming speeds that can be up to 15% larger then the bulk speed.
Lorenz-Mie scattering theory allows to predict the field scattered by spherical objects illuminated by coherent light. By fitting the fringe pattern resulting from the interference of incident and scattered light, it is possible to track and size colloidal particles with a few nanometer precision.
Using digital holographic microscopy (DHM) we extend the applications of Lorenz-Mie theory to hollow spherical structures and to extremely high pressure conditions.
On the one hand, we geometrically and optically characterize complex colloids as polymer-shelled microbubbles, with high precision, low costs and short acquisition time. These microbubbles are likely to be unique tools for targeted drug delivery and are currently used as contrast agents for sonography. We measured size, shell thickness and refractive index for hundreds of polymeric microbubbles showing that shell thickness displays a large variation that is strongly correlated with its refractive index and thus with its composition.
On the other hand we demonstrate that DHM can be used for accurate 3D tracking and sizing of a holographically trapped colloidal probe in a diamond anvil cell (DAC). Polystyrene beads were trapped in water up to Gigapascal pressures while simultaneously recording in-line holograms at 1 KHz frame rate. This technique may potentially provide a new method for spatially resolved pressure measurements inside a DAC.
Diamond anvil cells can be used to study the behavior of materials at high pressure by compressing small samples
up to hundreds of GigaPascals. There is no mechanical access to the sample once the cell is pressurized but it
is possible to observe the sample through the diamond windows. Optical tweezers can be used to measure the
mechanical properties of fluids, such as viscosity, by trapping and monitoring micron sized spheres suspended in
the fluid. We use a diamond anvil cell within a modified optical tweezers instrument to measure the viscosity
of water as a function of pressure up to 1:3GPa. Development of this technique will allow investigations of the
mechanical changes in biological cells and other soft materials placed under high pressure.
Diamond anvil cells allow us to study the behaviour of materials at pressures up to hundreds of gigaPascals in a small and convenient instrument, however physical access to the sample is impossible once it is pressurised. Optical tweezers use tightly focussed lasers to trap and hold microscopic objects, and their ability to measure nanometric displacements and femtonewton forces makes them ubiquitous across the nano and bio sciences. We show that optical tweezers can be used to hold and manipulate particles in such a cell, in the ``macro tweezers'' geometry allowing us to use objective lenses with a higher working distance. Traps are structured to overcome the limitations imposed by the sample cell. Wedemonstrate the effectiveness of the technique by measuring water's viscosity up to 1.2 GPa. The maximum pressure reached was limited by the water crystallising under pressure.
We present experimental evidence of hydrodynamic assisted escape from a potential well. Holographic optical
tweezers are used to landscape a bistable system composed of two optical traps, separated by 400nm as seen
by a Si colloid of radius 400nm. We observe thermally activated transitions between the two metastable states
in the system with transition rates that are in agreement with Kramers theory. Introducing a second bistable
system into our experiment allows us to study the behaviour of thermally activated transitions in the presence of
hydrodynamic interactions. The two bistable systems are placed in a line separated by a few micrometers. Using
camera tracking technologies we track each of the two beads as they hop back and forth within their respective
system. The escape events are recorded and any correlation between the two systems are then computed. We
consistently find that the number of observed correlations are as expected and that the number of correlations
having a positive coefficient are greater than the number of correlations having a negative coefficient. The
hydrodynamic interactions assist in the escape from a metastable potential. Our results are particularly relevant
in the context of concentrated colloidal suspensions where hydrodynamic interactions could lead to the formation
of higher mobility paths along which it is easier to overcome barriers to structural rearrangement.
The Brownian dynamics of an optically trapped water droplet is investigated across the transition from over to
under-damped oscillations. The spectrum of position fluctuations evolves from a Lorentzian shape typical of overdamped
systems (beads in liquid solvents), to a damped harmonic oscillator spectrum showing a resonance peak.
In this later under-damped regime, we excite parametric resonance by periodically modulating the trapping
power at twice the resonant frequency. We also derive from Langevin dynamics an explicit numerical recipe
for the fast computation of the power spectra of a Brownian parametric oscillator. The obtained numerical
predictions are in excellent agreement with the experimental data.
Holographic or diffractive optical components, such as a spatial
light modulator (SLM), can be used in optical tweezers for the
creation of multiple and modified optical traps. In addition to
this, SLMs can also be used to correct for aberrations within the
optical train resulting in an improved trapping performance.
Typically an electrically addressed SLM may deviate from flatness
by up to 4λ, dominated by astigmatism due to the overall
curvature of the SLM surface. This astigmatism may be corrected by
adding the appropriate hologram to the SLM display resulting in a
dramatic improvement in the fidelity of the focussed spot. The
impact that this correction has on the performance of the optical
trap is most noticeable for small particles. For the SLM used in
this study, the improvement in trap performance for a 0.8 μm
diameter particles can be in excess of 25%. However, for 5 μm
diameter particles our results show an improvement of less than
0.5%. This dependence upon particle size is most probably
associated with the relative size of the PSF and the trapped
particle. Once the PSF is significantly smaller than the particle
diameter, further reduction brings little improvement in trap
Central to the success of microfluidic systems has been the development of innovative methods for the
manipulation of fluids within microchannels. We demonstrate a method for generating flow within a
microfluidic channel using an optically driven pump. The pump consists of two counter rotating birefringent
vaterite particles trapped within a microfluidic channel and driven using optical tweezers. The transfer of spin
angular momentum from a circularly polarised laser beam rotates the particles at up to 10 Hz. We show the
that the pump is able to displace fluid in microchannels, with flow rates of up to 200 μm<sup>3</sup> s<sup>-1</sup> (200 fL s<sup>-1</sup>). The direction of fluid pumping can be reversed by altering the sense of the rotation of the vaterite beads. We also
incorporate a novel optical sensing method, based upon an additional probe particle, trapped within separate
optical tweezers, enabling us to map the magnitude and direction of fluid flow within the channel. The
techniques described in the paper have potential to be extended to drive an integrated lab-on-chip device,
where pumping, flow measurement and optical sensing could all be achieved by structuring a single laser
We demonstrate a technique for the multi-point measurement of fluid flow in microscopic geometries. The
technique consists of an array of microprobes can be simultaneously trapped and used to map out the fluid flow
in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are
displaced by the flow of the surrounding fluid and then re-trapped. The particles' displacements are monitored by
digital video microscopy and directly converted into velocity field values. The techniques described have potential
to be extended to drive an integrated lab-on-chip device, where pumping, flow measurement and optical sensing
could all be achieved by structuring a single laser beam.
We use holographic optical tweezers to create and monitor the liquid flow within a micro-fluidic device. Using the tweezers to both trap and spin micron-sized beads within a 10-20 micron wide channel creates a fluid flow of the order of 200 cubic microns/sec. We also use the optical tweezers to measure the fluid flow by trapping and releasing probe particles that are imaged with high temporal and spatial resolution. Using the multi-trap capability of the holographic optical tweezers we measure the transverse fluid velocity at many positions simultaneously with an accuracy of better than 1 micron/sec. Such studies are highly pertinent to lab-on-chip systems for various applications and studies within the biosciences.