PH2107 - suspension, colloids & nanoparticles 2.1

Question 1

How do particles behave at equilibrium?

Answer 1

Random arrangement

• no structure
• some molecular slipping

Question 2

What does shear force introduce into a particle structure?

Answer 2

Linearity with adjacent chains

Question 3

What happens to particles during shear thinning?

Answer 3

Particles in plane alignment - facilitates easier slipping past each other easier

Question 4

What happens to particles during shear thickening?

Answer 4

Particles in our of plane alignment or aggregation, preventing slipping

Question 5

What is thixotropy?

Answer 5

Reversible, time-dependent change in viscosity in some non-Newtonian fluids

Question 6

How are particles arranged in a structure?

Answer 6

The particles form a loose 3-dimensional gel structure in the formulation

Question 7

What effect does shear force (shaking) have on the 3-dimensional structure?

Answer 7

Shear force disrupts the bonds and therefore the structure, allowing the particles to line up and the liquid to flow

Question 8

What happens to the particles after the shear action stops?

Answer 8

The particles revert back to their original structure and original viscosity restored

Question 9

Give an example of a use for thixotropy?

Answer 9

Gel-sol-gel transition

Question 10

What does the curve being displaced to the left of the up curve indicate?

Answer 10

That the breakdown of gel structure does not reform immediately

Question 11

What does a thixotropic flow curve show?

Answer 11

For a given shear stress (e.g. x) the measured shear rate is lower (thicker) on the upward curve than the downward curve (thinner)

Question 12

How can rheological properties be measured?

Answer 12

1. Test how fast the sample flows - capillary methods
2. Test how hard it is to stir the sample - rotating methods
3. Test how long it takes for something to fall through the sample - falling methods

Question 13

How does a sphere viscometer work?

Answer 13

A sphere of known size and density is introduced into the viscometer at the top and allowed to fall through a glass tube containing the stationary liquid

The sphere should reach its terminal velocity before it reaches the top mark and the time taken for the ball to pass between the two marks is noted

Repeated with several spheres of different densities and diameters

Question 14

How does a rotational viscometer work?

Answer 14

Brookfield use the idea that the torque required to turn an object in a fluid is a function of the viscosity of that fluid

They measure the torque required to rotate a disk or bob in a fluid at known speed

Question 15

What type of fluids are used in a rotational viscometer?

Answer 15

Newtonian fluids only

Question 16

How does a cone and plate rheometer work?

Answer 16

The apex of the cone is almost in contact with the plate and the teat fluid the gap in between

The cone is rotated at a known angular velocity (w) and the resulting torque (T) and rheological parameters determined

Set temperature or temperature gradient