Settling Rates of Clay in Salt Water

experiment image
Experiments/Research performed by Kai Jaelle Barrett,
May - August 2011,
Department of Physics, University of Alberta


Very little quantitative study has been done into the empirical settling rates of clay. The problem of settling rates in clay has primarily been approached from a geological standpoint wherein the difference between flocculation and settling rate has been catalogued in fresh and sea water as it applies to real world examples, such as flocculation in estuaries. These studies only observe that flocculation occurs more in salt water with little emphasis on laboratory experiments or quantitative analysis.

Solving the problem of settling rate of clay is more complicated than with other particles, because the structure of clay particles causes the clay to interact in a physical and electrodynamic way. Clay particles have a flat sheet-like shape, where their face is negatively charged and their edges are positively charged. Clay groups together into clumps of particles, called 'flocs', this process is called 'flocculation'. Flocs settle faster than clay particles alone.

In fresh-water the structure of the clay particles forces them to flocculate by stacking like a house of cards, face to edge. When salt is added the positive sodium ions in the saline solution attach to the clay particles and, the clay flocculates more easily.

We seek to examine settling clay from a quantitative perspective, looking at data produced from laboratory experiments to create a viable empirical theory to predict the dynamics of clay settling.

Experimental Setup and Analysis Methods

The experiments are performed in a rectangular tank measuring 30cm in height, 5cm in width, and 20cm in length. This equates to a one litre volume. The tank is filled to a 10cm height with a uniform density fluid.

Once the tank is filled with the correct salinity of water, the density is verified, and a predetermined amount of powdered clay is added to the tank and is mechanically mixed to suspend it in the ambient fluid.

A video camera records the descending front as the suspended clay particles settle.

Every experiment explores 2 parameters: salinity, and clay density. The salinity is varied between fresh water (0.998g/cm3) and very salty water (1.05g/cm^3) and the clay density was varied between 15g/L and 50g/L.

Once the experiment is completed the video is converted into a usable format and analysed. The analysis is done by taking a vertical time series. From the vertical time series the slope the settling rate is measured from the descending front.


The results presented here include 2 experiments with the following fixed parameters: H=10cm, mass=30g/clay. From left-to-right, the density of the ambient fluid (i.e. the salinity) is at 1.01229g/cm3, and 0.99839g/cm3. Below are 2 videos of the settling rates and corresponding and vertical time series. Since the actual experiments take place on the order of an hour these videos are time lapsed to 60 times real time.

video of salt-water case with 30g/clay video of fresh-water case with 30g/clay
image from E110627b image from E110624c

VTS averaged across the tank (below)
VTS averaged across the tank (below)
salt.png fresh.png
NOTE: h = height of the tank in cm

Analysis of our results reveals that settling rate increases as more salt is added to the tank, and decreases as more mass of clay is added. The qualitative difference between the fresh-water and salt-water cases can clearly be seen in the above videos, as they settle on completely different timescales and at different rates. Analysis of these results is ongoing.


Department of Physics, University of Alberta

Dr. Bruce Sutherland, for an excellent and informative summer.

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