Clear as Mud

University of Illinois researchers Leonardo Chamorro and Jim Best used clear mud in order to study the structure of turbulence in water flows. You might be thinking, "mud isn’t clear!" If so, you would be wrong.

Most work done by researchers in the Earth surface community has looked at the dynamics of water flows in the absence of suspended sediments -- essentially clear water flows. However, natural water flows, including rivers, estuaries, and oceans, contain very fine suspended particles (clays). Turbulence in these environmental flows is affected by clay concentration, and understanding how their dynamics change can be applied to understanding how these processes affect natural and industrial environments. 

In a recent paper published in Physical Review Fluids, the team wanted to use modern fluid mechanical tools (including laser light) to characterize turbulence in muddy flows, rather than traditional acoustic techniques, which are often limited to being only one- or two- dimensional.

The Problem: Mud is opaque.  The Solution: Make mud clear.

Laponite is a synthetic clay used to produce paints, inks, and household cleaning materials. When mixed in water, the unique synthetic clay behaves in a similar way to natural clays. He said Laponite produces a clear or semi-clear suspension that is essentially a clear mud that “we can then shine laser light into and we can use different techniques to look at the dynamics," explained Best, professor in the departments of Geology and Geography and Geographic Information Science. 

The researchers aimed to understand the role of clay concentration on flow dynamics. To accomplish this goal, they performed a simplified experiment that used an idealized box to look at fluid mixing and quantify how increased clay concentrations changed the dynamics of the flow.

One major challenge the team faced was building such an idealized box that generates turbulence almost equally in each direction (isotropically). Mechanical engineering graduate students Jeffrey Cheng and Vaibhav Tipnis and geology student Shaelynn Kaufman stepped up to the challenge. After going through numerous iterations of the design, they were able to create a high-quality box with nearly isotropic flow characteristics. The box was a closed cube made of acrylic sheets with eight mixers situated symmetrically in each corner, creating a region of near-isotropic turbulence at the box center.

The concentration of Laponite used in this research ranged from 0-2%, which mimicked rheological (deformation and flow of matter) behaviors found in a range of natural clays. Interestingly, Cheng said that when clay concentration increases, even at relatively low concentrations, the clay particles begin to interact and aggregate with each other in what’s called two- and four-way interactions, therefore changing the structure of the turbulence dynamics. When clay particles are added to a flow, there are first separated particles; at higher concentration, they begin to form larger clumps, called flocs. As more clay is added, it begins to form longer chains of clay particles, affecting the rheology. As the clay concentration increased, they observed lowered turbulent kinetic energy. In other words, the flows became less turbulent with more clay.

Their findings show that even very low concentrations change the structure of turbulence, which in turn will have commensurate implications for sediment transport, for example, the way that particles are transported along the bed of a river and moved out into the deep sea.

“When we want to try and model and predict where these flows are going, how they transport their sediment, we need to parameterize the flow and the turbulence correctly. We can’t just assume a muddy flow, even at low concentrations, behaves the same as a clay-free flow, it actually is different. And those changes happen at really low concentrations, much lower than we thought previously. Some of the assumptions that we have made about turbulence before will have to be changed,” Best said.

This work is only the first step towards achieving the goal of understanding natural flows with clay. Chamorro and Best have plans to increase the complexity of their system. “We will increase the complexity of the flows. The logical next step is to uncover the role of mean shear; we aim to explore that effect soon to approach environmental flow settings,” said Chamorro, professor in the Department of Mechanical Science and Engineering. 

On the collaboration as a whole, Chamorro stresses, “This is a good example of how engineering, mechanical engineering, and geology meet together to try to solve environmentally relevant problems. It is a cool synergy.”

The publication, "Effects of low clay concentrations on nearly isotropic turbulence," can be found here.