 Motion of heavy particles settle down in response to an external force such as gravity, centrifugal force or electric forcegravitycentrifugal forceelectric force  Rate of sedimentation depends on the strength of the field, masses and shapes of the particles Sedimentation Theory (a) Gravitational sedimentation (b) Centrifugal sedimentation -Sedimentation velocity -Sedimentation Equilibrium 2.2 Kinetic property Sedimentation

(a) Gravitational SedimentationGravitational Sedimentation m F1F1 F2F2 F b (buoyancy) F g (gravity)

(a) Gravitational Sedimentation m F 1 = fV g F 2 = mg(1 -  2 /  1 ) At equilibrium : fV g = mg(1 -  2 /  1 ) m = fV g g(1 -  2 /  1 ) M = fN A V g g(1 -  2 /  1 )  1 = density of particle  2 = density of medium F1F1 F2F2

(b) Centrifugal Sedimentation sedimentation velocity sedimentation equilibrium  = angular velocity (rpm)

b.1 Sedimentation velocity an analytical ultracentrifugation method that measures the rate at which molecules move in response to centrifugal force generated in a centrifuge.  This sedimentation rate provides information about both the molecular mass and the shape of molecules.  In some cases this technique can also measure diffusion coefficients (D).

b.1 Sedimentation velocity F1F1 F 1 = fV = f(dr/dt) F 2 = m  2 r (1 -  2 /  1 ) At equilibrium : f(dr/dt) = m  2 r (1 -  2 /  1 ) M = sfN A = sRT (1 -  2 /  1 ) D (1 -  2 /  1 ) S = (dr/dt) = m (1 -  2 /  1 )  2 r f r F2F2 s = sedimentation coefficient (Svedberg, 1Sv= s)  ~ 80,000 rpm

b.2 Sedimentation equlibrium M = 2RT ln (C 2 /C 1 ) (1 -  2 /  1 )  2 (r 2 2 -r 1 2 )  ~ 10,000 rpm =

Assignment-3 Group Presentation Comparing to sedimentation G1 : precipitation, aggregation G2 : coagulation, flocculation (10 students per group) (5 min)

3. Optical property A colloidal system contains particles that affect a light beam by scattering and absorption.  If the particles are of a size comparable to the wavelength ( ) of light or larger, they scatter or absorb light independently.  The same thing happens if they are separated by distances comparable to or greater than the wavelength ( ) of light.

3. Common Optical phenomena the interaction of light from the sun or moon with the atmosphere, clouds, water, or dust and other particulates

Assignment-4 Group Presentation G3 : Rayleigh Scattering G4 : Raman Scattering G5 : Mie Scattering G6 : Bragg Scattering (10 students per group) (5 min)