Particle Tracking Microrheology of Complex Fluids

Abstract

We present a new method for measuring the linear viscoelastic shear moduli of complex fluids. Using photodiode detection of laser light scattered from a thermally excited colloidal probe sphere, we track its trajectory and extract the moduli using a frequency-dependent Stokes-Einstein equation. Spectra obtained for polyethylene oxide in water are in excellent agreement with those found mechanically and using diffusing wave spectroscopy. Since only minute sample volumes are required, this method is well suited for biomaterials of high purity, as we demonstrate with a concentrated DNA solution. [S0031-9007(97)04234-8]

Colloidal dynamics in polymer solutions: Optical two-point microrheology measurements

“…In order to achieve good statistics on two-particle correlations we have used a significantly higher-powered laser beam than previous (single-particle) optical tracking studies [1,3,4].…”

“…By analysing the thermal motion it is possible to obtain quantitative information about the rheological properties of the polymer matrix over an extended range of frequencies not accessible to conventional rheometers. Optical microrheology experiments typically monitor the thermal fluctuations of a single probe particle using a combination of video microscopy, optical interferometry [3,4], or diffusing wave spectroscopy [2]. While microrheological techniques enjoy significant potential advantages over traditional rheological methods complications in interpretation have limited their application [5].…”

“…To achieve adequate signal-to-noise levels we have been forced to use a more intense laser beam than usually employed in particle tracking experiments [1,3,4]. We consider explicitly the effects of optical-gradient forces and assume that each particle is held within a fixed harmonic potential.…”

An historical perspective on cell mechanics

“…c Modern immunofluorescence staining of actin with rhodamine-phalloidin, over a century later, also reveals striated structures in rat cardiomyocytes granules could also be used as markers for viscosity measurements [3,4]. This represents some of the earliest uses of particle tracking in cell mechanics and is essentially a predecessor of modern-particle tracking and microrheology measurements [100,101]. Although this early work was carried out in the 1920s and suffers from an obvious lack of appropriate experimental and theoretical considerations, some of the same issues were being discussed as they are today, such as the influence of the size of the granule, the mesh size of the protoplasm, damage to the cell and the influence of temperature [3,4,102].…”

“…Similarly, an early magnetic microscope developed in 1923 [103] was used to oscillate nickel particles (~16 μm in diameter) inserted into living cells. Aside from the similarities to modern particle micro-rheology [19,20,100,101], this approach is similar in concept to magnetic bead-twisting cytometry [104][105][106]. An early example of magnetic manipulation also involved injecting iron particles into bacteria and observing how fast they were attracted to an electromagnet [4].…”