**Particle Dynamics & Fractal Coagulation Processes**
Particles are extremely important in the natural
environment and in engineered systems. Particles contribute to water
turbidity and many engineered systems are designed to treat water
through particle removal. Particles can enhance the transport
of relatively insoluble chemicals sorbed on their surfaces, and
through coagulation can facilitate carbon export to sediments.
My
research over the past 20+ years has examined particle transport
dynamics, including the transport of chemicals to particles, and the
formation of larger, fractal particles through aggregation
processes, and how these particles and processes can be
characterized.
Large aggregates that form in the ocean, called
**marine snow,** can rapidly form and sink, resulting potentially
in the export of carbon to deep ocean sediments. The formation of
these aggregates can thus be important in global ocean carbon
balances and can affect the rate of carbon loss to deep sediments,
thus potentially sequestering this carbon. Working we researchers at
the University of California, Santa Barbara, we discovered a new
type of particle that can form in the ocean (transparent exopolymer
particles; **TEP**) that are responsible for the rapid formation
of marine snow.
(More information: See the paper by Alldrege et al (1993), one of my
most highly cited papers)
Particles produced through aggregation are
highly amorphous, non-spherical and fractal, and have
three-dimensional fractal dimensions that span a range of ~1.1 to
the maximum of 3. Previous methods for calculating fractal
dimensions of inorganic aggregates have been based on the analysis
of a single particle, but most systems of interest consist of a
spectrum of particle sizes. In order to analyze average properties
of aggregates in real coagulating suspensions with broad size
distributions, we developed
new spectral techniques to estimate
fractal dimensions. Using these techniques, we demonstrated that
there was no universality of the fractal dimension for aggregates
formed from fluid shear and differential sedimentation processes.
Fractal dimensions of microspheres formed in three
different mixing
environments, for example, were: 1.9, in a paddle mixer; 1.59 in a
rolling cylinder; and 1.43 to 1.74 in a laminar shear device
(depending on shear rate and salt concentration).
The fractal nature of these particles has been
found to have important implications for calculations of particle
properties such as their settling velocities and collision
efficiencies. Fractal aggregates of inorganic microspheres, for
example, settle an average of eight times faster than spheres of
identical size and mass. Collision frequencies between large
bacterial aggregates (100 um) and small microspheres (~ 1 um) were
five orders-of-magnitude higher than predicted using sphere-based
(curvilinear) coagulation models. Similarly high but slightly larger
collision frequencies were obtained for aggregates made from
microspheres. These results demonstrate that fractal aggregates of
particles can collide much more frequently than expected based on
spherical-particle coagulation models. They suggest that coagulation
rates in natural and engineered systems are much more rapid than
predicted by coagulation models based on impermeable spheres.
Use the links provided through this web page to
see examples of fractals and to see a list of publications
associated with this topic. |