NORAINI BINTI ALUWISITI AUNI NABILAH KAMARUDDINNOOR HIDAYAH BINTI HASBAH
HAZWANI BINTI BASRI
COLLOIDS AND FINE PARTICLES
BROWNIAN MOTION• A phenomenon experienced by colloids when
dispersed in a liquid.• Vibration of molecules of the liquid caused by
thermal energy from environment.• The molecules collide with each other and with
particles surface causing the random walk of particles.
Figure 1: Random walk of a Brownian particle
Average velocity of particles in suspension:
where,k is Boltzmann’s constant ()T is temperature (K)m is mass of the particle
• The simple application of a kinetic model:•Show either increasing or decreasing temperature the particles mass increases Brownian motion
•Cannot be used in determining the actual distance of the particle from its original position because it does not move in a straight line
• Thermodynamic principles: the lowest free energy state (greatest entropy) of a suspension is a uniform distribution of particles throughout the volume of the fluid
• Thus, the random walk of particle provides a mechanism to arrange the particles uniformly throughout the volume of the fluid
• The diffusion of particles from regions of high concentration to lower concentration occur
• Increasing temperature increases distance travelled over a period of time
• Increasing particles size and fluid viscosity redyced the distance travelled
Schematic representations of interparticle potential energy and force versus particle surface to surface separation distance
(a)Energy versus separation distance curve for an attractive interaction. The particles will reside at the separation distance where the minimum in energy occurs
(b)Force versus separation distance for the attractive potential shown in (a). The particles feel no force if they are at the equilibrium separation distance. An applied force greater than a maximum is required to pull the particles apart
(c)Energy versus separation distance curve for a repulsive interaction. When the potential energy barrier is greater than the available thermal and kinetic energy the particles cannot come in contact and move away from each other to reduce their energy
(d)Force versus separation distance for the repulsive potential shown in (c). There is no force on the particles when they are very far apart. There is a maximum force that must be exceeded to push the particles into contact
SURFACE
FORCES
1) VAN DER WAALS FORCES
2) ELECTRICAL DOUBLE LAYER FORCES
3) ADSORBING POLYMERS, BRIDGING, STERIC FORCES
4) OTHER FORCES
5) NET INTERACTION FORCE
1) VAN DER WAALS FORCES– Van der Waals forces is a group of electrodynamic interactions that occur between the atoms in two different particles
– The + represents the nucleus of the atom– The – represents the centre of the electron density– A dipole moment exist between the two opposite charges in each atom as the centre of electron density is typically not coincident with the nucleus
– The lowest free energy configuration and the resulting position of positive and negative charges leads to an attraction between two atoms is due to Coulumb’s law
– Dominant contribution to the van der Waals interaction between two particles is dispersion force
• The dispersion force is a result of columbic interactions between correlated fluctuating instantaneous dipole moment within the atoms of particles.
• The combine attraction between all the dipoles in the two particles results in an overall attraction between the particles
• Van der Waals interaction can be attractive or repulsive depending on the dielectric properties of the two particles and the medium between particles
For two spherical particles of the same size:
where, is the van der Waals interaction energy is the van der Waals force is distance between particles is diameter of the particles is Hamaker constant
𝑉 𝑉𝐷𝑊=− 𝐴𝑥24𝐷
𝐹𝑉𝐷𝑊=− 𝐴𝑥24 𝐷2
A > 0, attractive interaction
A < 0, repulsive
interaction
Notation used to indicate the type of material for each particle and the intervening medium
Material 1
Material 3
Material 2
When materials 1 and 3 are the same
• Van der Waals interaction is always attractive
• Van der Waals interaction is reduced when the particles are in water compared with air
• Easier to separate (disperse) fine particles in liquids than in air
When materials 1 and 3 are different
• Van der Waals interaction is repulsive
• The dielectric properties of the intervening medium are between the two particles are increased
• Difficult to separate
Force, easy to separate
Dielectric, difficult to
separate
2) ELECTRICAL DOUBLE LAYER FORCES
• EDL repulsion - same charges• EDL attraction – different charges
• Determine by osmotic pressure – counterion concentration
• Debye length (equation 5.10, pg 126)
• Repulsive, k increase, small counterion conc.
• Attractive, k decrease, high counterion conc.
4) OTHER FORCES
• Columbic interaction• Short ranged repulsions
known as hydration / structural force
• Strong attractionhydrophobic surface immerse
in water
5) NET INTERACTION FORCES
DLVO Theory – Derjaguin,
Landau, Verway and Overbeek
Many other forces may be combined in the same way
determined overall interparticle interaction
FACTORS INFLUENCE THE EFFICIENCY OF SOLID/LIQUID SEPARATION BY
GRAVITY
MOISTURE CONTENT OF SEDIMENT
RATE OF SEDIMENTATIO
NMaximized
Minimized
SEDIMENTATION RATE
Time frame for stability of a colloidal suspension against
gravity depends uponSedimentation Flux Brownian Flux
:Tends to move
particles denser than fluid downward &
particles less denser than fluid
upward
Tends to randomize position of particles
SEDIMENT CONCENTRATION & CONSOLIDATION
The moisture content of sediment and how sediment consolidates in
response to an applied consolidation pressure depends upon the
interparticle forcesApplied to particle network: Direct application (filter press/centrifuge) By weight of particles sitting above a particular level in a sediment
Concentration of sediment will vary from top to bottom due to
local solids pressure
Because fine particles and colloids typically produce compressible
sediments
Why ???
Sendiment rate is slow when repulsion and Brownian dominate
Sediment bed eventually forms is quite
concentrated & approaches a value near random dense packing of monodisperse
sphereØmax = (1-ε)max = 0.64
Because repulsive particles joining sediment bed are able to rearrange into a lower energy (lower
height) position
Why ???
Attractive and aggregates form
sediments that are quite open & contain
high levels of residual moisture
Strong attraction between particles creates a strong bond between
individual particlesPrevent rearrangement into a
compact sediment structure
Why ???
pH 4 Strong repulsion between particles
results in consolidation to high densities over a wide range of pressures
pH 5Weak attraction, with added salts results in intermediate behaviour
pH 9IEP of powder,strong attraction produces
difficult to consolidate and
pressure-dependent filtration behaviour
Figure of equilibrium volume fraction as a function of
consolidation pressure in a filter press (data Franks and Lange,1996) for 200nm diameter
alumina
Kreiger-Dougherty model :
Either low/high shear rate Newton plateau viscosity
Liquid viscosity
Intrinsic viscosity
2.5 Spherical particles
Volume fraction of solid
Real powder:(Ømax ) x [η](2.5
x 0.64) ≈ 2
Quemada model : with Ømax = 0.631
Batchelor’s model
Einstein’s model
Quemada’s model
Relative viscosity at
low shear rate of sphere silica
suspensions
REPULSIVE FORCE
• Particles that interact with long range repulsive force behave much like hard spheres when distance between particles larger than range of repulsive force
Volume fraction is low
Particles are
relatively large
Particles behave like
hard spheres
AND / OR
REPULSIVE FORCE
Volume fraction is high
• Repulsive force fields of particles overlap
• Viscosity of the suspension is increased compared with hard spheres
Particles are very small
• Average distance between particles is on order of the range of repulsion
• Repulsive force fields overlap
• Viscosity is increased
If……
• The influence of repulsive forces on suspension viscosity is handled considering the effective volume fraction of the particles
• Effective volume fraction:Repulsive region
REPULSIVE FORCE
ATTRACTIVE FORCE
• The bonds between particles must be broken in order to allow flow to occur
• An attractive particle network is formed when the suspension is at rest
• Stronger attraction between particles result in higher viscosities
• Shear thinning of attractive particle network is more pronounced than for hard sphere suspensions caused by a different mechanism
Particle network broken up into large clusters and liquid is trapped
Particle clusters broken down into smaller flow units releasing more liquid
Completely broken down, flow as individuals
• Attractive particle networks exhibit a yield stress (minimum stress required for flow) must be exceeded in order to pull two particles apart
• The yield stress depends upon the magnitude of the attraction :
The stronger the attraction, the higher the yield stress
ATTRACTIVE FORCE
• When the particles in a suspension are attractive, smaller particle size results in increase rheological properties
• Shear yield stress:
• The strength of the bond increases linearly with particle size
ATTRACTIVE FORCE
• The number of bonds that need to be broken per unit volume depends on the structure and the size of particles
• The number per unit volume simply varies with the inverse cube of particle size:
• Therefore, rheological properties vary inversely with the square of the particle size:
ATTRACTIVE FORCE
• A nanoparticle (nanopowder, nanocluster, nanocrystal) is a microscopic particle with at least one dimension less than 100 nm.
• Nanoparticle are finding application due to their unique properties high ratio of surface atoms to bulk atoms
• Because of the unusual properties of nanoparticles, there are numerous emerging applications where nanoparticles will be used
NANOPARTICLES
Applications of
nanoparticles
Optical
used for anti-
reflection product coatings
Magnetic
potential to increase the density of various storage media
Thermal
improve the transfer of heat from collectors of solar energy to their storage
tanks
Mechanical
improved wear and tear
resistance for almost any mechanical
device
Electronic
nanoparticle electronics can create digital displays that
are more electricity-efficient
Biomedical
produce “quantum
dots,” which can detect diseases
Energy
Nanoparticle batteries would
be longer-lasting