High School

A concentrated solution of sugar dissolved in water is flowing through a capillary tube with an inside diameter of 2 mm. The capillary tube will be used to find the viscosity of this solution.

- The length of the capillary tube is 10 cm.
- The density of the solution is 1200 kg/m\(^3\).
- For a flow rate of 60 cm/min, the pressure drop per length of the tube was found to be 1.0 mmHg/cm.

What is the viscosity (Pa·s) of the fluid?

Answer :

The viscosity of the fluid is approximately 0.0000003927 Pa s.

To find the viscosity of the fluid, we can use Poiseuille's law, which relates the flow rate, pressure drop, viscosity, and dimensions of the capillary tube.

Poiseuille's law states:

[tex]\[ Q = \frac{\pi r^4 \Delta P}{8 \mu L} \][/tex]

Where:

- Q is the flow rate,

- r is the radius of the capillary tube,

- [tex]\( \Delta P \)[/tex] is the pressure drop per length of the tube,

- [tex]\( \mu \)[/tex] is the viscosity of the fluid, and

- L is the length of the capillary tube.

Given that the inside diameter of the capillary tube is 2 mm, we have [tex]\( r = \frac{2}{2} = 1 \) mm = \( 0.001 \) m, and \( L = 0.1 \) m.[/tex]

We are also given that [tex]\( Q = 60 \) cm/min = \( 0.01 \) m/min, and \( \Delta P = 1.0 \) mmHg cm\(^{-1}\).[/tex]

We can rearrange Poiseuille's law to solve for viscosity [tex](\( \mu \))[/tex]:

[tex]\[ \mu = \frac{\pi r^4 \Delta P}{8 Q L} \][/tex]

[tex]\[ \mu = \frac{\pi (0.001)^4 \times 1.0}{8 \times 0.01 \times 0.1} \][/tex]

[tex]\[ \mu \approx \frac{\pi \times 0.000000001 \times 1.0}{0.008} \][/tex]

[tex]\[ \mu \approx \frac{3.1416 \times 0.000000001}{0.008} \][/tex]

[tex]\[ \mu \approx \frac{0.0000000031416}{0.008} \][/tex]

[tex]\[ \mu \approx 0.0000003927 \, \text{Pa s} \][/tex]

Therefore, the viscosity of the fluid is approximately 0.0000003927 Pa s.