Viscosity and Surface Tension

Question 1

State the three van der Waals forces

Answer 1

Dispersion forces: all molecules, temporary dipoles
Dipole-dipole attraction: polar molecules permanent dipoles
Hydrogen bonding: O–H, N–H, F–H molecules

Question 2

Differentiate cohesive and adhesive forces

Answer 2

Cohesive forces are attraction between same type of molecule (bonds water molecules)
Adhesive forces are attraction between different type of molecules (droplet to surface)

Question 3

Define viscosity

Answer 3

Resistance of fluid to flow due to internal friction

Question 4

Usual unit of dynamic viscosity

Answer 4

mPa s or cP (centipoise)

Question 5

Force required to flow is equal to?

Answer 5

• (dynamic viscosity)(surface area)(velocity gradient, dvy/dx)

Question 6

A fluid that obeys Newton’s Law

Answer 6

Newtonian fluid

Question 7

What is dynamic viscosity?

Answer 7

The resistance to shear flow. When force is applied, does it flow?

Question 8

Poiseuille’s Law

Answer 8

describes flowing liquids under laminar (smooth) flow which allows calculation of volume flowrate V/t which depends on radius and length.

Question 9

Volume flowrate is equal to?

Answer 9

(pie r^4)/[8(dynamic visc)] x (dP)/L

Question 10

Viscosity of water at 25 deg C

Answer 10

0.890 mPa s

Question 11

Types of flow and differentiate the two

Answer 11

Laminar (streamline) flow: smooth flow, one direction
Turbulent flow: chaotic flow

Question 12

What is critical Re?

Answer 12

The transition from laminar to turbulent flow.

Question 13

Reynold’s number formula

Answer 13

Re=[(density)(velocity)(diameter)]/(dynamic viscosity)

Question 14

Define kinematic viscosity and its formula

Answer 14

It is the resistance to transfer of momentum; when fluid layer flows against another, does the other layer flow?

kinematic viscosity = dynamic viscosity/density

Question 15

Dynamic viscosity and temperature relationship (liquid)

Answer 15

Inverse. Increase in thermal energy, IMF overcome

Question 16

Dynamic viscosity and pressure relationship (liquid)

Answer 16

Direct. Due to fluid compression

Question 17

Dynamic viscosity and temperature relationship (gas)

Answer 17

Direct. Due to increased collision leading to mentum transfer

Question 18

Dynamic viscosity and pressure relationship (gas)

Answer 18

Direct. Due to fluid compression

Question 19

Five ways to measure of viscosity

Answer 19

Flow rate thru capillary tube w/ known diameter
Ostwald viscometer (relative visc.)
Falling ball viscometry
Orifice viscometry

Question 20

Viscosity formula using Ostwald viscometer

Answer 20

dynamic: nb/na = (pb tb)/ (pa ta)
kinematic: Vb = Va tb/ta

Question 21

Define surface tension

Answer 21

Energy (work) required to extend a surface by a unit area
Force required to increase the length of a surface

Question 22

Unit for Surface Area

Answer 22

1 N/m = 1J/m^2

Question 23

IMF and surface tension relationship

Answer 23

Direct. Dispersion<Dipole-dipole<H-bonding<ionic

Question 24

Surface tension and temperature relationship

Answer 24

Inverse. Increased KE, IMF overcomes

Question 25

Surface tension and pressure realtionship

Answer 25

Surface tension of liquid decreases with pressure of gas above At higher pressure, surface tension increases

Question 26

Measurements of surface tension

Answer 26

Drop-weight method (stalagmometer) Ring-pull method (Du Nouy tensiometer) Wilhelmy plate method Capillary rise method Contact angle goniometry

Question 27

Relationship of surface tension with weight of drop formed (drop-weight method)

Answer 27

Direct. st=mg/2 pie r

Question 28

Surface tension depends on ___ in Du Nuoy/ ring-pull method

Answer 28

Force required to lift the ring. st=F/2 pie D

Question 29

Surface tension depends on ___ in Wilhelmy plate method

Answer 29

Force required to lift the plate. st=F-m of plate/2 pie D

Question 30

Surface tension depends on ___ in Capillary rise method

Answer 30

height of liquid. st = rhpg/2costheta

Question 31

Surface tension depends on ___ in contact angle goniometry

Answer 31

contact angle

Question 32

T/F: Concave meniscus has higher surface tension than convex

Answer 32

False.

Question 33

In convex meniscus: cohesive force ___ adhesive force

Answer 33

greater than (>)

Question 34

In concave meniscus: cohesive force ___ adhesive force

Answer 34

less than (<)

Question 35

Average mass of droplets formula

Answer 35

total mass of droplets/ number of drops

Question 36

Surface tension formula as respect to reference liquid

Answer 36

st sample=st reference (m sample / m reference)

Question 37

Alcohol-water concentration and surface tension relationship.

Answer 37

higher the concentration of alcohol in a mixture the lower the surface tension of that mixture