NORAINI BINTI ALUWISITI AUNI NABILAH KAMARUDDINNOOR HIDAYAH BINTI HASBAH

HAZWANI BINTI BASRI

COLLOIDS AND FINE PARTICLES

BROWNIAN MOTION

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

SURFACE FORCES

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.

3) ADSORBING POLYMERS,BRIDGING AND STERIC FORCE

Bridging Flocculati

on

Steric Repulsion

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

RESULT OF SURFACE FORCES

ON BEHAVIOUR IN AIR AND WATER

INFLUENCES OF PARTICLE SIZE AND SURFACE FORCES ON SOLID/LIQUID SEPARATION BY

SEDIMENTATION

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

Rearranged Stokes’ Law

Solving for time

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 ???

SUSPENSION RHEOLOGY

Study of flow & deformation of matter

What is RHEOLOGY ???

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

Relative viscocity

(μS/μl) of hard silica

particle suspension

Einstain’s relationship

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

INFLUENCES OF SURFACE FORCES ON SUSPENSION FLOW

Influences of surface forces on suspension

flow

Repulsive

forces

Attractive forces

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

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

The yield stress increases as salt is increase at pH away from the IEP

• 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

NANOPARTICLES

• 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