Elastoviscoplastic materials present a transition from a gel-like to a liquidlike state induced by shearing: While the first is primarily elastic, the second is predominantly viscous. The point that characterizes this transition is usually known as the yield point, which is associated to critical quantities such as yield stress and/or yield strain. Another characteristic of elastoviscoplastic materials is the transition from linear to nonlinear viscoelasticity. In the current work, a commercial hair gel, which is an elastoviscoplastic material, was tested in two rotational rheometers in order to evaluate these two transition points. Stress oscillatory amplitude sweeps at different frequencies were performed and a Fourier-Transform analysis was applied to the results in order to determine the linear viscoelastic limit. The linear viscoelastic limit stresses and strains at different frequencies were then compared to quantities that are usually associated to the yield point: The extrapolated zero-shear-rate stress obtained from the equilibrium flow curve, the minimum stress required to start up flows in creep experiments, the stress overshoot reached in constant shear rate experiments and the G′-and-G″ crossover stress determined through oscillatory amplitude sweeps. The results showed that the stresses and strains obtained as the linear viscoelastic limits were smaller than the critical quantities associated to the yield point for all evaluated cases. Although the critical quantities depend on the experimental condition, the linear viscoelastic limit strain was remarkably constant. Additionally, the linear viscoelastic limit strain was found to be in the same order of magnitude of the strain that characterizes the onset of plastic behavior in recovery experiments. This suggests that the beginning of the transition from a completely structured state of elastoviscoplastic materials to an unstructured state might be associated to the threshold of nonlinear viscoelasticity.
Impinging liquid jets are widely employed in cleaning operations to remove residual soiling layers from walls and other surfaces of process vessels. Insoluble viscoplastic soiling layers represent challenging soils to clean as removal is primarily by hydraulic forces. The rheological behaviour of a commercial petroleum jelly was investigated and shown to exhibit significant creep below its critical stress. The removal of thin (< 1 mm) layers of petroleum jelly from glass and Perspex surfaces by coherent water jets impinging normally on vertical walls were studied experimentally. The jet clears a roughly circular area, forming a berm of removed material at the cleaning front. The shape of the berm was measured and found to depend on the ratio of the height of the water film and the initial thickness of the soil layer. The data were compared with the adhesive removal of viscoplastic soils proposed by Glover et al. (J. Food Eng., 2016, 178, 95-109) with the momentum flow rate calculated using the results in Bhagat and Wilson (Chem. Eng. Sci, 2019, 152, 606-623). The asymptotic approach to a cleaning limit observed in experiments with static nozzles required modification of the model: a semiempirical term which represents the transition to a creeping regime is presented. The modified model allowed results obtained using static nozzles to predict the shape of the region cleaned by a jet from a similar nozzle moving across a soiled plate. The influence of process conditions on model parameters is discussed.
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