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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Kaye et al. (1995, Wear, 186-187, 413-420) conducted an experimental investigation of the cleaning of thin soil layers by water jets generated from stationary nozzles of diameter 0.25-2 35 mm and velocities ranging from 6-40 m s -1 . The soil layers were prepared by drying a commercial cleaning suspension on polymethylmethacrylate plates. Several of their data sets have been re-analysed and are shown to fit the 'strong soil' model for cleaning by peeling reported by Bhagat et al. (2017, Food Bioproduct Proc., 102, 31-54). The model is demonstrated to be able to explain the trends in cleaning efficiency observed by Kaye et al. 40 quantitatively, including the transition in efficiency observed at longer times. Kaye et al. also investigated the case of cleaning by a moving nozzle for one set of jet conditions, and reported an increase in cleared area with nozzle velocity. This behaviour is not predicted for the 'strong soil' and the model was modified to incorporate a contribution from the milling action of the jet in its impingent zone. This 'very strong soil' result gave good agreement with the data for 45 the area cleared and the shape of the cleared region. NomenclatureRoman 50 a Radius of cleaned area, [m] ai Radius when cleaning front is first seen, [m] ai* Dimensionless radius when cleaning front is first seen, [-] ax Distance to cleaning front on jet path, [m] A Area of the cleared region, [m 2 ] 55 c1 Lumped parameters, 1 = 10 o 2 3 2 , [m -3 ] c2 Parameter, Eq. [8], [-] c3 Parameter, Eq. [9], [m -3 ] dN Nozzle diameter, [m] E Efficiency factor, [m -1 ] 60 EU Prefactor efficiency, [m -1 ] k' Lumped cleaning rate constant, [m s kg -1 ] L Distance from nozzle to target, [m] ṁ Mass flow rate, [kg s -1 ] M Momentum flow per unit width, [kg s -2 ] 65 p Radial distance to cleaning front, [m] p* Dimensionless radial distance to cleaning front, [-] Q Flow rate leaving nozzle, [m 3 s -1 ] r Radial co-ordinate, [m] rb Boundary layer radius, [m] 70 ro Jet radius, o = 1 2 N [m] Re Reynolds number, defined Re = UodN/µ t Time, [s] ti Induction time, [s] Uo Jet velocity, [m s -1 ] 75 V Jet's traversal velocity, [m s -1 ] WeG Weber number gaseous, defined We = GUo 2 dN/γ WeL Weber number liquid, defined We = LUo 2 dN/γ 80 4 Greek  Angle to direction of nozzle motion, [-] γ Surface tension, [N m -1 ]    Density, [kg m -3 ] µ  Dynamic viscosity, [Pa s] 85 σ Cleaning rate parameter, [m 2 s -1 ] Acronyms PMMA Polymethylmethacrylate RFZ Radial flow zone 90

Thixotropic yield stress materials show a shear-induced solid-liquid transition at the yielding point, characterized by yield stress and yield strain. It is well known in the literature that the elastic modulus and the yield stress of thixotropic materials increase with aging time. In the current work, we propose a discussion on the brittleness of a suspension of swollen bentonite in water, focusing mainly on the role of aging times on the yield strain and on the critical strain at the linear to nonlinear viscoelastic transition of the material. The yield strain was measured in creep and constant shear rate start-up experiments, whereas the linear to nonlinear viscoelastic transition was evaluated from Fourier transforms on transient data in oscillatory shear stress amplitude sweeps. We show that aging increases material brittleness since the yield strain decreases with the resting time. On the other hand, the linear to nonlinear viscoelastic transition strain is surprisingly unaffected by the aging process. Other thixotropic systems were also investigated: 8 and 10 wt. % suspensions of bentonite in water and a 2 wt. % suspension of Laponite® in tap water. These lead to similar observations, showing constant linear to nonlinear viscoelastic strains and decreasing yield strains over increasing aging times. These findings bring relevant information to the intricate open-discussion issue on how to describe the behavior of thixotropic materials below the yield stress.

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