Direct evidence of electric double layer (EDL) repulsive force controlling the time-dependent behaviour of clay gels in structural rejuventation: Annual European Rheology Conference 2024 or AERC2024 abstract book

We postulated in 2018 that the electric double layer (EDL) repulsive force controls the structural development process during ageing and was therefore responsible for the time-dependent behaviour of charge anisotropic clay gels. The same explanation goes with structural rejuvenation process during the stepdown in the shear rate. Now we have direct evidence that this is indeed the case. A strong EDL repulsive force was needed to accentuate the time-dependent behaviour of charge and shape anisotropic clay gels at the stepdown shear rate. This force was strengthened by P2O74- adsorption increasing the negative charge density of the clay particles. At the stepdown shear rate of 10 s-1 it is strong enough to disrupt the flow aligned structure attained at 1000 s-1 and orient the particles to form more bonds. The resultant outcome is a stepdown shear stress increasing with time until these structure disruption and bond forming processes reached an equilibrium state. The number of lower energy approach configurations (-ve face - +ve edge) for bonding is reduced by the strengthened EDL repulsive force slowing down the bonding process. The time to reach the equilibrium stepdown shear stress value increased initially and then decreased and became zero at high negative charge density where charge anisotropy of the particles no longer exists. The need of a sufficiently strong EDL repulsive force for the display of time dependent behaviour is true for all clay gels; Laponite, hectorite, NaMnt, sepiolite and kaolin gels. The untreated NaMnt gel displayed time-dependent behaviour as the EDL repulsive force is already strong enough. The same EDL-control time dependent behaviour was obtained if pH was used to vary the negative charge density of the clay particles. This mechanism is similar to the reaction mechanism proposed by the collision theory