Week 2 - Rheology

Question 1

What is surface tension

Answer 1

the energy required to increase the surface area of a liquid (J/m^2)

Molecules at the surface are less stable than the ones underneath. Hence the liquid tries to minimize the surface area

Question 2

What are the units for surface tension

Answer 2

Jm-2

Question 3

Why does water have a high surface tension

Answer 3

Water has a high surface tension - as it forms multiple Hydrogen bonds, and for the surface area to be increased these hydrogen bonds (intermolecular forces) need to be broken, which requires a considerable amount of energy.

Question 4

Why are particles at the surface of a liquid less stable/ have a higher energy state

Answer 4

Molecules/atoms at the surface of a liquid are at a higher energy state (not as sable) as those below because they are interacting with less molecules/atoms and hence experience less intermolecular forces.

Question 5

What is rheology

Answer 5

the study of deformation and flow of matter against variables (temp, pressure, time, applied stresses) - used to describe behavior of materials that classic theories do not describe successfully

Question 6

What is a surfactant

Answer 6

molecules which lowers surface tension in liquids - aka surface active agents

Surfactants consist of a hydrophobic tail and hydrophilic head which congregates and align at surfaces of liquids forming a micelle. This lowers surface tension with increasing concentration.

Question 7

What is capillary action

Answer 7

the spontaneous rising of a liquid in a liquid in a narrow tube due to surface tension which arises as a result of high cohesive (intermolecular) and high adhesive (liquid/wall) forces.

Question 8

How does capillary action work

Answer 8

Due to the forces cohesion and adhesion
Adhesion - the attraction between the liquid molecule and the surface material - the liquid molecules are attracted to the walls of the tube
Cohesion - attraction between molecules of the same substance - water molecules are attracted to each other

When a liquid, such as water, is placed in a narrow tube, the adhesive forces between the liquid and the walls of the tube cause the liquid to spread along the surface. At the same time, cohesive forces within the liquid try to pull the molecules together. In very thin tubes, the adhesive forces are stronger than the cohesive forces, which pulls the liquid up the walls of the tube, against gravity. The narrower the tube, the higher the liquid can rise due to capillary action.

Question 9

What is viscosity

Answer 9

a measure of a fluids resistance to flow, it used to assess flow character of a liquid, semi solid or gel
units for viscosity is centipoise (cP)

Question 10

What are the units for viscosity

Answer 10

centipoise (cP)
Water 1 cP = 1mPa per s (SI)

Question 11

What is surface energy (solids)

Answer 11

Solids only use this term:
energy difference between the surface molecules/atoms and the bulk

Question 12

What is surface energy (liquids)

Answer 12

work/energy required by the force to overcome intermolecular forces and create a new surface (same as surface tension)
Units for surface energy = Jm^-2 = Nm-1

Question 13

What impact does an increased temperature have on surface energy

Answer 13

*As temperature increases surface energy decreases - in a solid as it is heated the particles vibrate faster due to

Question 14

What is shear stress force

Answer 14

force exerted parallel to the surface of a material (such as shaking, stirring or squeezing)

a force which causes deformation by slippage along a plane/s parallel to an imposed stress - used to assess flow character such as viscosity

Question 15

What is shear thinning

Answer 15

A fluids property where viscosity decreases under increased shear stress

Question 16

What is thixotropy

Answer 16

a property of certain fluids and gels that exhibit a time dependent change in viscosity: they become less viscous when subjected to shear stress (such as shaking, stirring or squeezing) and return to a more viscous state when the stress is removed.

Question 17

What influences viscosity

Answer 17

Viscosity is influenced by the molecule size and intermolecular forces as they impact how easily molecules may move past each other.

Question 18

What increases surface energy (tension)

Answer 18

As the number of carbons in a alkane chain increases
because size of the molecule increases and hence the dispersion forces increase between molecules. This makes it harder for molecules to past each other, hence increasing surface energy.

Question 19

What influences surface energy

Answer 19

• Intermolecular forces
• functional groups
• crystal planes presenting at surface (for metals only)

Question 20

What are the 2 ways which a substance adheres to a surface

Answer 20

Physisorption - loose dispersion forces
Chemisorption - bonded covalently or ionically

Question 21

What is surface wetting

Answer 21

ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the 2 are brought into contact

Question 22

How does a contaminated and cleaned surface influence wettability

Answer 22

Surface contamination lowers sold surface energy, decreasing wettability

cleaning a surface raises surface energy of solid, improving wettability.

Question 23

What is the influence of surface energy on surface wetting

Answer 23

High surface energy - surfaces with high surface energy (e.g. metals, glass) attract liquids, leading to better wetting. Liquids spread out, forming low contact angles.

Low Surface Energy: Surfaces with low surface energy (e.g., Teflon, wax) repel liquids, leading to poor wetting. Liquids bead up, forming high contact angles.

Question 24

What is the influence of viscosity on surface wetting

Answer 24

High Viscosity - High-viscosity liquids (e.g., honey, thick oils) resist flow and spread more slowly. They may not wet the surface as effectively because their resistance to flow limits their ability to cover the surface uniformly.

Low Viscosity - Liquids with low viscosity (e.g., water, alcohol) flow easily and spread quickly over surfaces. They can achieve better wetting because they can penetrate into surface irregularities and spread out more uniformly.

Question 25

What is the relationship between surface energy of a surface and wettability

Answer 25

High surface energy means that the surface is more wettable

Question 26

Why is etching the tooth important for adhesive retoration

Answer 26

Etch increases the surface energy of the tooth - hence making the tooth surface more wettable meaning that it can better interact with and bond to adhesives.