Coagulation - definitions

1. Coagulation - definitions • when relative motion of particles is Brownian, process = thermal coagulation • when relative motion arises from external forces (eg gravity, electrical forces, aerodynamic effects) = kinematic coagulation • coagulation of solid particles = agglomeration

2. Collision frequency function collision frequency - # collisions/time between particles of size i and size j = vi, vj are volumes of particles of size i and j b depends on the size of the colliding particles, and properties of system such as temperature and pressure Consider the change in number concentration of particles of size k, where vk = vi + vj

3. Coagulation - discrete distributions For a discrete size distribution, the rate of formation of particles of size k by collision of particles of size i and j, is given by: where the factor 1/2 is introduced because each collision is counted twice in summation Rate of loss of particles of size k by collision with all other particles is given by: Change in number concentration of particles of size k given by: theory of coagulation for discrete spectrum developed by Smoluchowski (1917) change = formation - loss

4. Simple example

5. Collision frequency functions for particles in continuum regime: (Stokes-Einstein relationship valid) for particles in free molecular regime: (derived from kinetic theory of collisions between hard spheres) interpolation formulas between regimes given by Fuchs (1964) The Mechanics of Aerosols

6. Collision frequency values: where particle diameters, d1 and d2 are in microns

7. Coagulation: simple test case Assume we have a monodisperse population initially, and particle diameter is greater than gas mean free path (continuum regime), so vi = vj coagulation equation is then: let coagulation equation simplifies to:

8. Simple test case continued solving for total number concentration as f(t) using the boundary condition: solving for time

9. Example problem: Estimate the time for the concentration of a monodisperse aerosol to fall to 10% of its original value for an initial size of dp = 1 micron and for the following initial concentrations: 103 and 108 # / cm3

10. Coagulation, continuous distributions continuous nomenclature n(v) = number of particles per unit volume of size v, a continuous distribution collision rate: where b is the collision frequency function described earlier The rate of formation of particles of size v by collision of smaller particles of size and is given by: Here, loss is from collisions with all other particles, so must integrate over entire size range

11. Continuous distributions continued How to solve?

12. Recall: similarity transformation The similarity transformation for the particle size distribution is based on the assumption that the fraction of particles in a given size range (ndv) is a function only of particle volume normalized by average particle volume: defining a new variable, and rearranging,

13. Self-preserving size distribution For simplest case: no material added or lost from the system, V is constant, but is decreasing as coagulation takes place. If the form of is known, and if the size distribution corresponding to any value of V and is known for any one time, t, then the size distribution at any other time can be determined. In other words, the shapes of the distributions at different times are similar, and can be related using a scaling factor. These distributions are said to be ‘self-preserving’. t1 t2 t3

14. Special cases - free molecular regime, coagulation of spheres • Brownian coagulation in free molecular regime:Lai, Friedlander, Pich, Hidy, J. Colloid Int. Sci. 39, 395 (1972). • allows estimation of change in total number concentration resulting from coagulation, if a self-preserving distribution is assumed • a is an integral function of y, and is found to be about 6.67 Note V =volume fraction, vol aerosol / total volume gas plus aerosol

15. Coagulation of hard spheres • When primary particles collide and stick, but do not coalesce, irregular structures are formed agglomerate spherical equivalent • Recall - we can use a fractal dimension to characterize these structures Above equation gives: the relationship between radius r (rgyration usually) of aerosol agglomerates, and the volume of primary particles in the agglomerate

16. Special cases - free molecular growth of fractal agglomerates • The change of agglomerate size distribution with time also can be described using a self-preserving size distribution scaling law. • Using a similarity transform, and extrapolating out to long times, average agglomerate radius grows as a power law function of time: (Matsoukas and Friedlander, J. Colloid Int. Sci.146, 495 (1991) and

17. Interesting result! • For Df = 3, growth rate is independent of primary particle size. • But, for Df < 3, then the exponent q is negative • So smaller primary particles result in larger agglomerates!

18. Summary of processes affecting suspended particles • Transport by diffusion and external fields • Formation by nucleation • Condensation and evaporation • Coagulation Balances: