18. Using a rotational viscometer operating in the constant strain rate mode, the rheological characteristics of 20% aqueous graphite suspensions were obtained at various pH values of the dispersion medium:
Strain rate, , s–1 Stress P, N/m2 at pH
13 11 7 4 2
50 1.8 8.2 14.2 9.6 1.0
100 2.4 9.0 15.0 10.2 1.8
200 3.0 10.0 16.0 11.2 2.2
400 4.4 10.8 16.6 12.0 3.0
800 7.0 12.8 17.6 13.8 4.6
1200 9.6 15.0 18.8 16.0 6.6
Plot the flow curves, calculate the yield strength and plot its dependence on the pH of the suspension medium.

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