Search

A rheometrical investigation of incipient clots formed in fibrin-thrombin gels is reported in which the Gel Point (GP) is characterised by frequency independence of the loss tangent in small amplitude oscillatory shear measurements over a wide range of thrombin concentration. Values of the fractal dimension (Df) of the GP network calculated from measurements are consistent with those reported in simulations of diffusion limited cluster–cluster aggregation (DLCCA) and reaction limited cluster–cluster aggregation (RLCCA), but differ insofar as the values of Df calculated from the present experiments increase progressively with a reduction in gel formation time. A molecular dynamics simulation (MDS) of systems of rod-like particles was designed to (i) test the hypothesis that the presence of an activation profile in a cluster aggregation model could account for the trend of Df as a function of gel formation time observed experimentally in fibrin-thrombin gels and whole heparinised blood without recourse to the inclusion of fibrinogen-specific interactions; and (ii) to explore the effect of monomer activation kinetics on the microstructure of fractal clusters formed in systems of rigid rod-like particles. The results identify two possible mechanisms for the increase in Df as the gel formation time decreases, both being a consequence of altering the evolution of the clustering dynamics by a process referred to herein as activation limited aggregation (ALA). This ALA-based MDS substantiates the experimental findings by confirming the trend evident in the formation of incipient clots in fibrin-thrombin gels and in whole heparinised blood. A mechanism for ALA involving the aggregation of pre-GP sub-clusters is proposed.

Shear modulus data are presented for a polyurethane system at different stages of the gelation process. Data were collected over a wide frequency domain covering both the α relaxation and the internal mode relaxation. At the gel point, both the loss modulus G″ and the storage modulus G′ have a power law frequency dependence over four decades with an exponent 0.70 close to the value predicted by percolation theory assuming Rouse dynamics. After the gel point G′ exhibits a gel modulus which increases with increasing reaction extent, while G″ continues to have a power law behaviour at low frequencies with an exponent that decreases initially to 0.5 and then increases to 0.7 for the fully grown gels. The presence of slow dynamical processes in gels is visible in most reported dynamic mechanical measurements on different types of gels and is not expected by the standard model of polymeric gels. A number of possible explanations are mentioned, but we conclude that the slow dynamics in gels are at present an unexplained experimental observation.

Rheological Gel Point measurements may incur errors in the case of rapid gelling systems due to the limitations of multiple frequency oscillatory shear techniques such as frequency sweeps and Fourier Transform Mechanical Spectroscopy, FTMS. These limitations are associated with sample mutation and data interpolation. In the present paper we consider how an alternative rapid characterisation technique known as Optimal Fourier Rheometry, OFR, can be used to study a rapidly gelling material, namely collagen at near physiological temperatures. The OFR technique is validated using a model reference gelling system whose GP characteristics have been widely reported. An analysis of the susceptibility of OFR measurements to rheometrical artefacts is made prior to its use in the study of rapid gelling collagen gels formed over a range of physiologically relevant collagen concentrations. The results of this OFR study are the first measurements of the stress relaxation characteristics of collagen gels performed in a single rheological experiment.

Rheological Gel Point measurements may incur errors in the case of rapid gelling systems due to the limitations of multiple frequency oscillatory shear techniques such as frequency sweeps and Fourier Transform Mechanical Spectroscopy, FTMS. These limitations are associated with sample mutation and data interpolation. In the present paper we consider how an alternative rapid characterisation technique known as Optimal Fourier Rheometry, OFR, can be used to study a rapidly gelling material, namely collagen at near physiological temperatures. The OFR technique is validated using a model reference gelling system whose GP characteristics have been widely reported. An analysis of the susceptibility of OFR measurements to rheometrical artefacts is made prior to its use in the study of rapid gelling collagen gels formed over a range of physiologically relevant collagen concentrations. The results of this OFR study are the first measurements of the stress relaxation characteristics of collagen gels performed in a single rheological experiment.

Processes involving a unidirectional shear flow component are widespread in industrial manufacturing techniques such as printing and coating, or in physiological events such as blood coagulation. Standard rheometric techniques are usually employed under quiescent conditions and as such are inappropriate for the study of microstructural modification induced by the presence of a unidirectional shear flow. We demonstrate how controlled stress parallel superposition (CSPS) may be exploited to enable accurate detection of the Gel Point and analysis of Gel Point parameters for systems undergoing a viscoelastic liquid (VEL) to viscoelastic solid (VES) transition in the presence of a unidirectional flow field. Specifically, we note that certain features of the CSPS experiment, when performed near the Gel Point, may obviate previously reported concerns regarding the experiment. A biopolymer system (gelatin) which forms gels by thermoreversible gelation is employed as a model gelling material to confirm the ability of CSPS to characterise the stress relaxation characteristics of critical-gels in the presence of (a) progressively decreasing and (b) progressively increasing unidirectional strain rate and oscillatory strain amplitude. Additional validation of CSPS results is reported for a silicone dielectric gel used in the industrial production of printed electronic products. Finally, CSPS is used to investigate microstructural modification of fibrin–thrombin gels as a consequence of clot formation under a unidirectional shear stress. The results confirm the validity of the CSPS technique in gelation studies and the technique is used, for the first time, to directly record the thermally induced VES to VEL transition in aqueous gelatin systems.