8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P9.68Tue 611:23-14:00Optical tweezers: wideband microrheologyManlio Tassieri 11 University of Glasgow, 80 Oakfield Avenue, G12 8LT, Glasgow, United KingdomMicrorheology is a branch of rheology having the same principles as conventional bulk rheology,but working on micron length scales. Therefore, a big advantage of this analytical method is the useof very small amounts of sample (∼ 10 micro-litres) compared with the sample volumes used forconventional rheology (∼ 1 ml). Amongst the most popular microrheological techniques, opticaltweezers have been successfully used with Newtonian fluids for rheological purposes such as determiningthe fluid viscosity with high accuracy, measuring the hydrodynamic interactions betweenparticles or estimating the wall effect on the Stokes drag coefficient. Conversely, when opticaltweezers are adopted for measuring the viscoelastic properties of complex fluids the results areeither limited to the material high frequency response, discarding the essential information relatedto long times scales (i. e. low frequency) material behaviour, or supported by low frequency measurementsperformed by different techniques (e. g. rotational rheometry). We present two equivalentand self-consist experimental procedures to perform broadband microrheology with opticaltweezers. Both the methods adopt a generalised Langevin equation to relate the time-dependenttrajectory of an optically trapped micron-sized particle to the frequency-dependent moduli of thecomplex fluid under study. These methods extend the range of the frequencies previously availableto optical tweezers measurements. In fact, the accessible frequency range is limited only by theexperiment length and by the maximum data acquisition speed. This allows access to the material’sterminal region enabling microrheological measurements to be performed on complex fluidswith very long relaxation times, such as those exhibiting soft glassy rheology. The methods providea simple yet concrete basis on which future viscoelastic measurements may be made on bothbiological and non-biological systems using optical tweezers.68