Thomas Gisler

Tracer microrheology in complex fluids

Recent results suggest that the motion of colloidal particles can be interpreted in terms of the viscoelasticity of the surrounding medium. New experimental techniques to extend these probe measurements and new methods for data interpretation have been developed.

Rheology of F-actin solutions determined from thermally driven tracer motion

We report measurements of the frequency-dependent complex shear modulus of semidilute F-actin solutions based on optical observations of the thermally excited motion of monodisperse tracer microspheres. Because the tracer spheres cause incident laser light to be strongly scattered, we determine their average motion using diffusing wave spectroscopy. From the measured mean square displacement, we extract the retardation spectrum of the actin solution using a regularized fit based on a discretized model involving a linear superposition of harmonically bound Brownian particles. At an actin concentration of C 1.2 mg/ml and for microspheres of radius a 0.8 m, we find that the complex modulus exhibits a dominant low frequency plateau modulus and a high frequency rise with the loss modulus dominating above a crossover frequency. Over a limited range of frequencies well above the crossover frequency, the magnitude of the high frequency storage modulus G() is consistent with the power law scaling 3/4 . The observed gradual crossover appears to be at odds with previous theoretical predictions, but it corresponds to a simple structure of the retardation spectrum.

Optical microrheology: From gels to cells

We describe a method for measuring the rheological properties of complex materials by embedding small probe particles within the material and monitoring the thermal motion of the probe using either light scattering or particle tracking. If the probe particle is large compared to the characteristic length scales of the surrounding material, then its motion will reflect the average behavior of the medium. This can be interpreted in terms of the frequency dependent elastic modulus of the medium, providing a simple, non intrusive method for measuring rheological properties. Examples are shown of the measurement of the actin networks using this method, and an unexpected scaling behavior is observed.