Product Viscosity Versus Shear

Chemical and Process Engineering Resources

Feb 11 2013 06:50 PM | Steve Hall in Fluid Flow

Viscosity Measurements

Pseudoplastic compounds follow a power curve, with viscosity decreasing with shear rate. Viscosity measurements were performed with a Brookfield Viscometer, Model LVDV-II+. Using the #31 spindle and Sample Adapter 13R cup, shear rate is determined by multiplying the rotational speed by 0.34ⁱⁱⁱ. This gives the following raw data for the placebo product made for the tests:

Table 3: Viscosity Measurements
RPM	Viscosity (cP)	Torque (%)	Shear Rate, sec^-1
0.5	8338	13.9	0.17
1.0	5159	17.2	0.34
1.5	3919	19.6	0.51
3.0	2449	24.6	1.02
6.0	1540	30.8	2.04
12.0	977.3	39.1	4.08
20.0	706.3	47.1	6.8
30.0	551.9	55.2	10.2
45.0	435.2	65.3	15.3
60.0	370.4	74.1	20.4
100.0	283.7	94.6	34
150.0	EEEE	EEEE

Measurements made at 25°C

Here are the data charted in Excel with the trend line and trend line equation superimposed:

Figure 1: Graph of Viscosity Data with Trend Line

The power law equation for apparent viscosity is:

μ_a = K γ^n-1

Eq. (6)

K = flow consistency index, Pa-sn = flow behavior index, dimensionless

Viscosity is in units Pa-s. Since our data is in cP (=mPa-s), the values of K and n are 2.53 and 0.356.

The viscometer’s measurement range is limited, but extrapolation is acceptable with decreased accuracy.

Using the trend line equation, the viscosity at any shear rate is estimated. But this is for a temperature of 25°C. Since data at only one temperature is given, the Lewis and Squires temperature correlation is used to estimate the viscosity at the actual flowing condition.