Viscosity and Surface Tension Flashcards
State the three van der Waals forces
Dispersion forces: all molecules, temporary dipoles
Dipole-dipole attraction: polar molecules permanent dipoles
Hydrogen bonding: O–H, N–H, F–H molecules
Differentiate cohesive and adhesive forces
Cohesive forces are attraction between same type of molecule (bonds water molecules)
Adhesive forces are attraction between different type of molecules (droplet to surface)
Define viscosity
Resistance of fluid to flow due to internal friction
Usual unit of dynamic viscosity
mPa s or cP (centipoise)
Force required to flow is equal to?
- (dynamic viscosity)(surface area)(velocity gradient, dvy/dx)
A fluid that obeys Newton’s Law
Newtonian fluid
What is dynamic viscosity?
The resistance to shear flow. When force is applied, does it flow?
Poiseuille’s Law
describes flowing liquids under laminar (smooth) flow which allows calculation of volume flowrate V/t which depends on radius and length.
Volume flowrate is equal to?
(pie r^4)/[8(dynamic visc)] x (dP)/L
Viscosity of water at 25 deg C
0.890 mPa s
Types of flow and differentiate the two
Laminar (streamline) flow: smooth flow, one direction
Turbulent flow: chaotic flow
What is critical Re?
The transition from laminar to turbulent flow.
Reynold’s number formula
Re=[(density)(velocity)(diameter)]/(dynamic viscosity)
Define kinematic viscosity and its formula
It is the resistance to transfer of momentum; when fluid layer flows against another, does the other layer flow?
kinematic viscosity = dynamic viscosity/density
Dynamic viscosity and temperature relationship (liquid)
Inverse. Increase in thermal energy, IMF overcome
Dynamic viscosity and pressure relationship (liquid)
Direct. Due to fluid compression
Dynamic viscosity and temperature relationship (gas)
Direct. Due to increased collision leading to mentum transfer
Dynamic viscosity and pressure relationship (gas)
Direct. Due to fluid compression
Five ways to measure of viscosity
Flow rate thru capillary tube w/ known diameter
Ostwald viscometer (relative visc.)
Falling ball viscometry
Orifice viscometry
Viscosity formula using Ostwald viscometer
dynamic: nb/na = (pb tb)/ (pa ta)
kinematic: Vb = Va tb/ta
Define surface tension
Energy (work) required to extend a surface by a unit area
Force required to increase the length of a surface
Unit for Surface Area
1 N/m = 1J/m^2
IMF and surface tension relationship
Direct. Dispersion<Dipole-dipole<H-bonding<ionic
Surface tension and temperature relationship
Inverse. Increased KE, IMF overcomes
Surface tension and pressure realtionship
Surface tension of liquid decreases with pressure of gas above
At higher pressure, surface tension increases
Measurements of surface tension
Drop-weight method (stalagmometer)
Ring-pull method (Du Nouy tensiometer)
Wilhelmy plate method
Capillary rise method
Contact angle goniometry
Relationship of surface tension with weight of drop formed (drop-weight method)
Direct. st=mg/2 pie r
Surface tension depends on ___ in Du Nuoy/ ring-pull method
Force required to lift the ring. st=F/2 pie D
Surface tension depends on ___ in Wilhelmy plate method
Force required to lift the plate.
st=F-m of plate/2 pie D
Surface tension depends on ___ in Capillary rise method
height of liquid. st = rhpg/2costheta
Surface tension depends on ___ in contact angle goniometry
contact angle
T/F: Concave meniscus has higher surface tension than convex
False.
In convex meniscus: cohesive force ___ adhesive force
greater than (>)
In concave meniscus: cohesive force ___ adhesive force
less than (<)
Average mass of droplets formula
total mass of droplets/ number of drops
Surface tension formula as respect to reference liquid
st sample=st reference (m sample / m reference)
Alcohol-water concentration and surface tension relationship.
higher the concentration of alcohol in a mixture the lower the surface tension of that mixture
