Søren Hvidt is associate professor in Physical Chemistry at Roskilde University.  He holds a Masters degree in Chemistry and Physics from University of Copenhagen and a Ph.D. degree in Physical Chemistry from University of Wisconsin-Madison.  He joined the staff in Roskilde after postdoctoral positions in Madison and in Baltimore.  He was from 1985-1991 also a member of the Polymer group at Risø National Laboratory.

His main research intererest is physical chemical studies of biological and synthetic macromolecules in solutions and gels.  He has primarily used rheological techniques to characterize macromolecular structures and dynamics in a broad range of systems.  He has taught several courses in applied and basic rheology, and he has been a consultant on rheological projects for several companies.

Søren Hvidt has presented his research at several national and international meetings.  He was a co-founder of the Nordic Rheology Society (NRS), and has been the editor of its Annual Transactions.  He was awarded the NRS Carl Klason award in 1999 and has served on the editorial board of Polymer Networks and Gels.  He is a member of the Danish Chemical Society, Polymer Network Group, and the Society of Rheology.

Hydrogels are gaining wide-spread applications in many industries and have replaced less environmentally safe materials.  Hydrogels can be divided into physical and chemically crosslinked gels, with important differences in solubility.  The characteristic feature of a hydrogel is its solid nature despite high concentrations of water.  Gels are difficult to define precisely but the elastic rather than viscous behavior is a characteristic.  The special viscosity and elasticity properties of hydrogels are the main reason for their use in industry, and it is therefore of importance to be able to monitor gelation during production processes and to obtain optimal mechanical properties of the final products.

In this presentation, the use of rheology to charaterize gels and gelation will be discussed.  Modern rheometers enable a range of experimental tests, and the type of information which can be obtained from different tests will be illustrated.  It will be argued that small amplitude oscillations should be used to monitor gelation, and once the gel is formed other techniques can be used to characterize the gel.  The yield stress is an important quantity for many applications of gels, and different ways to determine yield stresses will be illustrated.  The rigidity of gels are primarily determined by the number of crosslinks in the gel.  Many standard industrial tests are basically rheological tests, and for the SAG test which is used to grade pectins the relationship to fundamental rheological properties has been established.