viscosity

Question 1

• what is viscosity and what is it dependent on?

Answer 1

viscosity is the resistance of fluid to flow or change in shape that is caused by friction within the fluid that is dependent on the concentration and the conformation of the polysachride

Question 2

• how is viscosity measured?

Answer 2

By appartuses like:

- bostwick consistometer used in food sci 2
- capillary viscometer
- brookfield rotational viscometer which has a probe that rotates and measures viscosity based on rotation
- rheometer that measures the resistance based on it pressing down

Question 3

• and what is shear rate and stress and how does that translate to viscosity?

Answer 3

shear rate=rate of change

shear stress=intensity of the force applied to deform

viscosity=unit force/unit flow=**shear stress/shear rate=** applied force/flow force

Question 4

draw the viscosity graph to explain newtonian flow and non-newtonian flow

Question 5

define newtonian

Answer 5

Newtonian= is viscosity that is independent of stress and time so it flows smoothly no matter how fast it’s stirred or poured and the stress will be directyl propotional to shear rate so increasing the stress by 2 increases the shear rate in the same way. like in the case of low molecular weight carbohydrate solutions.

Question 6

define non-newtonian (shear-thickening)

Answer 6

behavior of certain fluids that become thicker or more viscous when they are stirred or shaken more vigorously. So the shear stress increases creating a more viscous material and the shear rate will first increase then not it will stay stagnanent as its harder to move

Question 7

define non-newtonian (shear thining-pseudoplastic):

Answer 7

viscosity decreases with increase shear rate because there is an increased shear stress which will cause an increased shear rate. This is independent of time as the viscosity will immediatly decrease with an increase in shear stress which is then reversible when the force is reduced

Question 8

• explain the implaction of non-newtonian flow (pseudplastic flow) in foods

Answer 8

common for linear polymers, and in food this means at rest the product is at its max viscosity to avoid separtation during storage and when increased shear the viscosity will decrease to allow for easy mixing and pumping

Question 9

• explain bingham plastics and their relation to pseudoplastics

Answer 9

they behave like solids until enough stress is applied so when they reach their yield stress then they are like pseudoplasric fluids like ketchup for example

Question 10

• what is yield stress

Answer 10

the minimum stress that must be applied for the dispersion to start flowing

Question 11

• describe thixotropic flow

Answer 11

is a special type of shear thining that is time dependent. It has pseudoplastic behaviour in that the viscosity decreases with increased shear stress and therefore increased shear rate, but this doesnt happen instanenousely instead it will take time. and with decreased shear it will increase its viscosity. So what normally happens is gel→solution→ gel, so a weak gel at rest will become a liquid under stress

Question 12

• what is the relationship between depolymerization, molecular weight and viscosity

Answer 12

increasing depolymeration will decrease the molecular weight which decreases the viscosity. If MW is halved the viscosity decreases 4-fold

Question 13

• what are the positives and negatives of viscosity decreasing very quickjly with hydrolysis

Answer 13

+: can use hydrolyzed polysachrides for water control without increasing the viscosity like in freeezing and thawing stability

-: under high shear or acidic conditions the loss of viscosity that occurs may not be desired.

Question 14

• what are the main effects of temperature on viscosity?

Answer 14

1. change in rheological properties→ viscosity decreases with increasing temperature
   
   2. degree of hydrocolloid stability→ as there is a risk of depolymerization which will decrease molecular weight and viscosity