Biomedical subjects

Timo A Nieminen

Publications and source records attributed to Timo A Nieminen.

5 recordsLinked to original sources

PubMed2006

Measurement of the index of refraction of single microparticles.

The refractive index of single microparticles is derived from precise measurement and rigorous modeling of the stiffness of a laser trap. We demonstrate the method for particles of four different materials with diameters from 1.6 to 5.2 microm and achieve an accuracy of better than 1%. The method greatly contributes as a new characterization technique because it works best under conditions (small particle size, polydispersion) where other methods, such as absorption spectroscopy, start to fail. Particles need not be transferred to a particular fluid, which prevents particle degradation or alteration common in index matching techniques. Our results also show that advanced modeling of laser traps accurately reproduces experimental reality.

Models, Biological↗

PubMed2006

Optical force field mapping in microdevices.

We present a method for characterizing microscopic optical force fields. Two dimensional vector force maps are generated by measuring the optical force applied to a probe particle for a grid of particle positions. The method is used to map out the force field created by the beam from a lensed fiber inside a liquid filled microdevice. We find transverse gradient forces and axial scattering forces on the order of 2 pN per 10 mW laser power which are constant over a considerable axial range (>35 microm). These findings suggest future useful applications of lensed fibers for particle guiding/sorting. The propulsion of a small particle at a constant velocity of 200 microm s(-1) is shown.

Lasers↗

PubMed2006

Orientation of optically trapped nonspherical birefringent particles.

While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected--the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis.

Birefringence↗

PubMed2004

Optical microrheology using rotating laser-trapped particles.

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.

Calcium Carbonate↗

PubMed2003

Anomalous absorption of bulk shear sagittal acoustic waves in a layered structure with viscous fluid.

It is demonstrated theoretically that the absorptivity of bulk shear sagittal waves by an ultra-thin layer of viscous fluid between two different elastic media has a strong maximum (in some cases as good as 100%) at an optimal layer thickness. This thickness is usually much smaller than the penetration depths and lengths of transverse and longitudinal waves in the fluid. The angular dependencies of the absorptivity are demonstrated to have significant and unusual structure near critical angles of incidence. The effect of non-Newtonian properties and non-uniformities of the fluid layer on the absorptivity is also investigated. In particular, it is shown that the absorption in a thin layer of viscous fluid is much more sensitive to non-zero relaxation time(s) in the fluid layer than the absorption at an isolated solid-fluid interface.

Journal Article↗