LIQUID FOUNDATION:
THIXOTROPIC HYSTERESIS LOOP
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LIQUID FOUNDATION: THIXOTROPIC HYSTERESIS LOOP |
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A thixotropic fluid is one whose resting "network" structure degrades with shear but can restructure at rest over a period of time[1]. The time required to completely reform network structure is long enough that the material can appear to be a liquid for an appreciable time interval. Yet in due course it will reacquire the properties of a solid or liquid with a highly developed network structure. Thus, a thixotropic fluid has time-dependent rheological properties. Because structured liquids are viscoelastic, these time effects are best observed by measuring both the viscosity and elasticity in oscillatory flow. A cosmetic liquid foundation was selected to illustrate time effects in a thixotropic fluid. A sequence of measurements was made starting with the liquid in its quiescent state. Stepping through increasing shear rate and then back through decreasing shear rate shows whether there is a time delay before the liquid regains its quiescent viscoelastic structure. Using oscillatory flow at 2 Hz and at 22°C in a 1 mm diameter tube, measurements were initiated at a shear rate of 0.03/sec and then increased stepwise to a maximum of 28/sec before returning to 0.04/sec. The total time for this sweep in shear rate was 4 minutes. Note that as the shear rate increased, the viscosity and elasticity diminished. Increasing shear rates progressively degraded the quiescent structures to form new structures that dissipated and stored less energy. As the shear rate returned to the minimum level, the viscosity and elasticity increased but did not return to the previous low shear values. The new structure created by the high shear rates failed to return completely to the initial quiescent structure in the time duration of the measurements. For comparing materials, testing must be done under identical time sequences. The wider the gap between the starting and ending values of viscosity and elasticity, the longer the time required to attain equilibrium.
[1] Nae, H. N., "Introduction to rheology", in Rheological Properties of Cosmetics and Toiletries, edited by D. Laba (Marcel Dekker, Inc., New York) 9-34 (1993). *Ingredients: water, diisocetyl dodecanedioate, cyclomethicone, octyl palmitate, glyceryl isostearate, propylene glycol, steric acid, mica, glycerin, glyceryl stearate, sodium hyaluronate, silica, sodium lauryl sarcosinate, cellulose gum, triethanolamine, imidazolidinyl urea, perfluoropolymethylisopropyl ether, methylparaben, magnesium aluminum silicate, propylparaben
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