Different Applications of Rheological Techniques in Studies of Physical Gels

Physical gels are of both great scientific and practical interest. The cytoplasm of cells, which consists of a complex physical gel of protein filaments, is important for most of the cellular processes including cell division and cell motility. Nature has developed this complex system of associating protein filaments with the characteristic function of individual filaments. The proteins enable the cell to regulate the mechanical properties of the cell by sol-gel transition and a variety of crosslinking reactions. In the food industry texture of products are regulated by addition of gel-forming biopolymers. Physical gels are also used in the paint industry to minimize sedimentation. Delayed and controlled drug release is of importance in the pharmaceutical industry, and one way to obtain this control is to hide active components in physical gels. Two excellent reviews cover many aspects of biopolymer and physical gels [1,2]. The nature of physical gels has been debated for many years. In contrast to chemically crosslinked gels physical gels are often thermoreversible and small changes in e.g. temperature, pH or ionic strength may shift the system from a gel state to a sol state. Ole Kramer [3] pointed out the importance of time scales for physical gels. According to him the characteristic property of a gel is the presence of two components and a constant elastic storage modulus on time scales of 1 s or frequencies of 1 rad/s. His definition of gels relies on their rheological properties. Rheological techniques are used extensively in studies of physical gels and gelation. In the lecture some of the common techniques used in studies of gels will be addressed. Small amplitude oscillatory measurements are the most common type of measurement performed, and such measurements allow a determination of the elastic storage modulus, G’, and loss modulus, G”, as a function of e.g. frequency, temperature, or time. Two other techniques, which can be very useful for studies of gels, are creep and relaxation measurements. These techniques, which allow determinations of the compliance and the relaxation modulus, respectively, are particularly useful for investigating slow motions in gels and long-time properties. An example of how these different techniques have been used to investigate the rheological properties of sputum [4] will be discussed. The results demonstrate that sputum is a viscoelastic material and that both nucleic acids and actin filaments contribute to the viscoelastic properties of sputum from cystic fibrosis patients. Many simple methods have been developed over the years in industry to characterize products. The SAG test is widely used in the food industry to grade pectin samples and their ability to form gels. The test, which consists of a SAG measurement due to gravity of gels with a very precisely defined shape, is highly reproducible. However, it does not give any information about fundamental rheological properties of the pectin gels. Our attempt to understand more fundamental aspects of this test combined creep and oscillatory measurements together with finite element simulation. The results show [5] that pectin gels can be modeled as nearly ideal incompressible elastic materials, and equations connecting SAG number and the elastic storage modulus have been obtained. Solutions of tri-block copolymers of ethylene oxide and propylene oxide form micelles in water with increasing temperature due to the hydrophobic nature of poly(propylene oxide) at higher temperatures. At high concentration micelles pack in bcc structures and various other gel structures are also formed. The gel properties are dominated by repulsive interactions between micelles, and oscillatory measurements allow a determination of the repulsive potential between micelles. Oscillatory bulk modulus measurements have been used to determine the dynamics of unimer-micelle motions. The strain properties of physical gels are of major importance in many applications. When a gel is deformed with increasing strain or strain amplitudes most gels eventually rupture. The yield stress denotes the maximum stress gels can withstand. Different ways of determining yield stress will be illustrated. Oscillatory measurements only allow determinations of G’ and G” in the linear range. Most physical gels are strain softening with decreasing moduli at large strains due to rupture, but several biopolymer gels consisting of stiff rod-like filaments show strain hardening, where the modulus increases with increasing strain amplitude. The blood protein fibrinogen forms fibrin clots that exhibit strain hardening, with possible important biological significance. References 1. A.H. Clarke & S.B. Ross-Murphy, Adv. Pol. Sci. 1987, 83, 57. 2. K. te Nijenhuis, Adv. Pol. Sci, 1997, 130, 1. 3. K. Almdal, J. Dyre, S. Hvidt & O. Kramer, Pol. Networks and Gels 1992, 1, 5. 4. H. Nielsen, S. Hvidt, C.A. Scheils & P.A. Janmey, Biophys. Chem. 2004, 112, 193. 5. H. Nielsen, B.U. Marr & S. Hvidt, Carbohydrate Pol. 2001, 45, 395.