Visualization of micro-scale inhomogeneities in acrylic thickener solutions: A multiple particle tracking study

Abstract

Multiple Particle Tracking (MPT) microrheology in combination with bulk mechanical rheometry has been used to characterize the structural and viscoelastic microheterogeneity of commercial acrylic thickeners Carbopol EDT2050, Viscalex HV30, and Sterocoll D in aqueous solutions at technically relevant polymer concentrations. According to bulk rheology Carbopol solutions exhibit a much higher elasticity than Sterocoll and Viscalex solutions at low polymer concentration whereas all thickeners are composed of similar main monomers. MPT experiments confirm that the latter two systems form homogeneous predominately elastic polymer networks with a mesh size <200 nm. In contrast, Carbopol solutions are highly heterogeneous and the degree of heterogeneity strongly increases with increasing polymer concentration. Predominantly elastic and viscous regions within the solution can be identified based on the slope of the mean squared displacement of individual particle trajectories. This heterogeneity is directly imaged here for the first time using Voronoi diagrams and characteristic length scales varying from 5 to 20 μm are found. Additionally, variation of the probe size reveals that the elastic regions themselves are heterogeneous including areas with mesh size <200 nm and a larger fraction with mesh size between 200 nm and 500 nm.