Micro- and nanorheology

'Microrheology', the determination of viscoelastic properties of soft media from the observed motion of microscopic-sized tracer particles, has undergone a recent surge of development because it allows access to an unprecedented range of frequency response with unprecedented spatial resolution. Several complementary techniques have been developed, but discrepancies have been found among different experiments especially in complex systems, and the validity of interpretation is under debate. 'Nanorheology' after explicit confinement of samples to variable thickness, 1 nm to 1 mum, is also enjoying rapid development, particularly nanorheology integrated with spectroscopy. Developments, limitations, and opportunities are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.