Picoliter viscometry using optically rotated particles

Abstract

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...

Torsional oscillations of a sphere in a Stokes flow

“…The advantage of a viscometer based on rotational motion, compared to conventional methods based on translational motion, is that a smaller volume of fluid is probed. This enables the measurement of the rheological properties of micro-to pico-litre volumes of viscous fluids [23] and in vivo testing [32]. Furthermore, non-mechanical control is essential in such viscometers, as the friction between the rotational axis of the sphere and the mechanical support often restricts the lower limit of viscosity measurement [22].…”

Engineering light-matter interaction for emerging optical manipulation applications

“…Microfabrication is also advancing developments in optically driven micromotors [28-30, 98, 99]. Optical micromotors can, among others, drive fluids in micropumps [29], establish a velocity field for applying hydrodynamic shear [30], or, upon proper calibration, work as miniature instruments for probing fluid properties [99]. Interestingly, fluid-mediated optomechanics can be achieved not only by delivering optical momentum but also by optical energy, e.g., via pulsed laser-induced cavitation bubbles [100].…”

In situ viscometry by optical trapping interferometry

“…The sphere's position is recorded with a quadrant photodiode by back-focal plane interferometry. 4,5 Several similar so-called active microrheological methods have been introduced before, [6][7][8][9][10][11] but they all rely on calibration against standards, for a preliminary determination of the detector's sensitivity or of the local laser power at the sample, employing usually reference solutions with wellknown properties. Therefore, they suffer limitations when truly quantitative in situ measurements are needed.…”