We integrate the optical elements required to generate optical orbital angular momentum into a microdevice. This allows the rotation of either naturally occuring microparticles or specially fabricated optical rotors. We use a two photon photopolymerization process to create microscopic diffractive optical elements, customized to a wavelength of choice, which are integrated with micromachines in microfluidic devices. This enables the application of high optical torques with off-the-shelf optical tweezers systems.

This paper reports on a simple synthesis and characterization of highly birefringent vaterite microspheres, which are composed of 20-30 nm sized nanocrystalls. Scanning electron microscopy shows a quite disordered assembly of nanocrystals within the microspheres. However, using optical tweezers, the effective birefringence of the microspheres was measured to be Deltan = 0.06, which compares to Deltan = 0.1 of vaterite single crystals. This suggests a very high orientation of the nanocrystals within the microspheres. A hyperbolic model of the direction of the optical axis throughout the vaterite spherulite best fits the experimental data. Results from polarized light microscopy further confirm the hyperbolic model.

We present a controlled stress microviscometer with applications to complex fluids. It generates and measures microscopic fluid velocity fields, based on dual beam optical tweezers. This allows an investigation of bulk viscous properties and local inhomogeneities at the probe particle surface. The accuracy of the method is demonstrated in water. In a complex fluid model ͑hyaluronic acid͒, we observe a strong deviation of the flow field from classical behavior. Knowledge of the deviation together with an optical torque measurement is used to determine the bulk viscosity. Furthermore, we model the observed deviation and derive microscopic parameters.

This paper reports on the synthesis of birefringent vaterite microspheres with narrow size distribution using a seeded growth method. In a post-treatment the microspheres were stabilized and functionalized through coating with a combination of organosilica and silica. The coating vastly enhanced the stability of the vaterite microspheres in biological buffers and allowed the attachment of biomolecules such as DNA or proteins. As an example, streptavidin was attached to the surface of the functionalized microspheres. These results pave the way for the use of birefringent vaterite particles for the micromanipulation of single biological molecules such as DNA or specific proteins in an optical trap capable of exerting and measuring torques. The stabilized birefringent microspheres may also find use for biosensor and biological screening applications.

Important aspects in the field of microrheology are studies of the viscosity of fluids within structures with micrometer dimensions and fluid samples where only microliter volumes are available. We have quantitatively investigated the performance and accuracy of a microviscometer based on rotating optical tweezers, which requires as little as one microliter of sample. We have characterized our microviscometer, including effects due to heating, and demonstrated its ability to perform measurements over a large dynamic range of viscosities ͑at least two orders of magnitude͒. We have also inserted a probe particle through the membrane of a cell and measured the viscosity of the intramembranous contents. Viscosity measurements of tears have also been made with our microviscometer, which demonstrate its potential use to study unstimulated eye fluid. Recently, there has been increasing interest in microrheology, the study of flows and deformations of a material or medium using probes of microscopic size. In this paper, we will concentrate on microrheological methods that probe viscosity on micrometer length scales. Suitable existing techniques are magnetic tweezers ͓1,2͔, particle tracking ͓3͔, and optical tweezers based techniques ͓4,5͔. Magnetic tweezers allow comparatively large forces to be applied to probe particles and thus the effects of high rates of shear can be studied. Particle tracking elegantly extracts the viscoelasticity of a medium over a large frequency range and allows fluidprobe coupling effects to be removed ͓6͔. Optical tweezers allow the viscoelasticity of very localized regions to be probed, which enables the investigation of picoliter volumes and micrometer structures, such as the interior of cells. The region can be further localized by studying rotational motion of the trapped particle ͓5͔, which is also true for passive techniques ͓7-9͔. These techniques have been used to study the viscoelasticity of cells ͓10,11͔ and also polymer solutions, where small volumes and high throughput are advantageous ͓12͔. Another potential application is small-volume medical samples, such as eye fluid ͓13͔.Rotating optical tweezers have been discussed in detail by Parkin et al. ͓14͔. A spherical birefringent microparticle, combined with an optical measurement of the torque applied to it, can be used to probe fluid properties ͓5,15͔. Using the light transmitted through the probe particle trapped in optical tweezers, the rotation rate of the probe particle and the change in the polarization of the light are measured. We use vaterite, which is a calcium carbonate crystal that forms spherical structures under certain growth conditions ͓5͔, as our probe particle. This particle has also been used to create and study microfluidic flows ͓16-18͔. We present the characterization of our microviscometer, based on this rotating sphere, and the application of this device to measure intramembranous liquid and tear fluid. This technique allows a flow to be generated in a very localized region, of picoliter volume, and the visc...