Microrheology is the study of fluid flows and material deformations on a microscopic scale. The study of
viscoelasticity of microscopic structures, such as cells, is one application of microrheometry. Another application
is to study biological and medical samples where only a limited volume (microlitres) of fluid is available. This
second application is the focus of our work and we present a suitable microrheometer based on optical tweezers.
Optical tweezers are an optical trap created by a tightly focused laser beam. The gradient force at this focus
acts to trap transparent micron sized particles, which can be manipulated within the surrounding environment.
We use the polarisation of the incident field to transfer angular momentum to a trapped spherical birefringent
particle. This causes the particle to rotate and measuring the polarisation of the forward scattered light allows
the optical torque applied to the sphere to be calculated. From the torque, the viscosity of the surrounding
liquid can be found. We present a technique that allows us to perform these measurements on microlitre volumes
of fluid. By applying a time-dependent torque to the particle, the frequency response of the liquid can also be
determined, which allows viscoelasticity to be measured. This is left as a future direction for this project.
We describe two methods to optically measure the torque applied by the orbital angular momentum of the
trapping beam in an optical tweezers setup. The first decomposes the beam into orbital angular momentum
carrying modes and measures the power in each mode to determine the change in angular momentum of the
beam. The second method is based on a measuring the torque transfer due to spin angular momentum and the
linear relationship between rotation rate and applied torque to determine the orbital angular momentum transfer.
The second method is applied to measuring the transfer efficency for different particle-mode combinations. We
present the results of these experiments and discuss some of the difficulties encountered.
Manipulation of micrometer sized particles with optical tweezers can be precisely modeled with electro dynamic theory using Mie's solution for spherical particles or the T-matrix method for more complex objects. We model optical tweezers for a wide range of parameters including size, relative refractive index and objective numerical aperture. We present the resulting landscapes of the trap stiffness and maximum applicable trapping force in the parameter space. These landscapes give a detailed insight into the requirements and possibilities of optical trapping and provide detailed information on trapping of nanometer sized particles or trapping of high index particles like diamond.
The ability to exert optical torques to rotationally manipulate microparticles has developed from an interesting curiosity to seeing deployment in practical applications. Is the next step to genuine optically-driven micromachines feasible or possible? We review the progress made towards this goal, and future prospects.
Exposure of optically curing resin with highly focussed femtosecond laser pulses provides excellent means to produce high resolution micron sized structures. We use the process to fabricate micromechanical components for lab-on-a-chip applications. We present here our experimental realization of the microscope system used for
photopolymerization and detail the advantage of our fabrication process. We characterize our structures using scanning electron microscopy, and compare the results with available data. We demonstrate the technique by manufacturing a movable joint and a free floating cross which is three dimensionally trapped. Future applications of this technique will focus on developing optically driven motors and an all optical measurement of applied torques.
We use passive and active techniques to study microrheology of a biopolymer solution. The passive technique is video tracking of tracer particles in the biopolymer, a technique which is well established. The active technique is based on rotating optical tweezers, which is used to study viscosity. A method to actively measure viscoelascity using time varying rotation of a particle trapped in optical tweezers is also presented.
We investigate the dynamics of microscopic flow vortices. We create flow vortices by rotation of birefringent particles in optical tweezers. We then use either highly sensitive drag force measurements or video tracking to map the fluid velocity around that particle. The results obtained from these different methods are compared. Velocity profiles obtained for water agree very well with theoretical predictions. In contrast, we find a strong deviation of velocity profiles in a complex fluid from those predicted by simple theory.
We present a technique to measure the viscosity of microscopic
volumes of liquid using rotating optical tweezers. The technique
can be used when only microlitre (or less) sample volumes are
available, for example biological or medical samples, or to make
local measurements in complicated micro-structures such as cells.
The rotation of the optical tweezers is achieved using the
polarisation of the trapping light to rotate a trapped
birefringent spherical crystal, called vaterite. Transfer of
angular momentum from a circularly polarised beam to the particle
causes the rotation. The transmitted light can then be analysed to
determine the applied torque to the particle and its rotation
rate. The applied torque is determined from the change in the
circular polarisation of the beam caused by the vaterite and the
rotation rate is used to find the viscous drag on the rotating
spherical particle. The viscosity of the surrounding liquid can
then be determined. Using this technique we measured the viscosity
of liquids at room temperature, which agree well with tabulated
values. We also study the local heating effects due to absorption
of the trapping laser beam. We report heating of 50-70 K/W in the
region of liquid surrounding the particle.
We are investigating the formation of a tissue capsule around a foreign body. This tissue capsule can be used as an autologous graft for the replacement of diseased blood vessels or for bypass surgery. The graft is grown in the peritoneal cavity of the recipient and the formation starts with the adhesion of cells to the foreign body. We identify the cell type and measure the adhesion of these cells to foreign materials using optical tweezers. Cell adhesion to macroscopic samples and microspheres is investigated. No difference in the adhesion force was measurable for polyethylene, silicon and Tygon on a scale accessible to optical tweezers. The density of adherent cells was found to vary strongly, being highest on polyethylene. The mean rupture forces for cell-microsphere adhesion ranged from 24 to 39 pN and changed upon preadsorption of bovine serum albumin. For plain microspheres, the highest mean rupture force was found for PMMA, which also showed the highest adhesion probability for the cell-microsphere interaction.