Terra firma, earth, ground, soil, name it what you will, is a suspension of natural clay minerals and salt, in water and sand. The physical characteristics and ratios of clay and salt in this suspension, together with the proportion and behavioral properties of sand in everyday soil have been a matter of much research. These factors decide how soil will behave structurally irrespective of its geographical location, environmental conditions and application of external force or the lack thereof.
A series of experiments performed by Prof. Ranjini Bandyopadhyay at Raman Research Institute and Dr. Samim Ali in her group may have brought us one step closer to understanding what factors contribute to the collapse of soil that can ultimately lead to large-scale deltaic island formation in riverine systems like the Gangetic delta Sunderbans, or landslides in areas like the sub-Himalayan regions of India. The researchers have characterized the internal structure and organization of a Na-montmorillonite and salt dispersion in water using different experimental techniques to better understand how the concentration of salt in water could potentially affect soil settling. Na-montmorillonite suspension in water is a form of naturally occurring clay and is most abundantly present in soil. Na-montmorillonite are flexible nanosheets that occur in the form of charged anisotropic ‘platelets’ or simply put discs of variable sizes and shapes.
In the presence of salt, each disc acquires an anisotropic pH dependent cloud of sodium ions around itself that increases the effective volume it occupies in solution beyond what it otherwise would. The electrostatic interaction between these charged discs depends on the inter-disc distance, their relative orientation and the charge distribution on the discs. By fine tuning the concentration of Na-montmorillonite and salt and the pH of the medium, we can obtain different macroscopic phases of the suspension like a ‘soft’ glass or gel. Glass here refers to the physicist’s definition of glass transition, in which a liquid when cooled beyond a certain temperature at a very fast rate reaches a highly viscous state.
It is the ‘gel’ phase of
such a suspension
that has kept scientists
in the dark thus far
Interestingly it is the ‘gel’ phase of such a suspension that has kept scientists in the dark thus far. Dr. Ali and Dr. Bandyopadhyay have tackled this problem by keeping the clay particle concentration constant at their natural pH value, and systematically varying the salt concentration of medium to study its effect on the ‘gel’ structure.
The experiments that the group performed on their salt-added clay suspension are shear force response measurements, electroacoustic measurements and cryogenic scanning electron microscopy (cryo-SEM). The shear force response measurements are done to shed light on the viscous and elastic properties of the suspension in response to an external force. The electroacoustic measurement can directly measure if the particles are settling or shrinking under gravity. In this experiment, an ultrasound wave is passed through the sample inducing a measurable electric current, which is extremely sensitive to the physical organization of the clay particles in the suspension. The ultrasound wave creates alternating cycles of high- and low-pressure zones within the suspension giving rise to a detectable peak in the current, when the organization of suspended clay particle is loose and has tendency to settle. In the cryo-SEM experiment, the Na-montmorillonite suspension is first frozen, then broken into shards and imaged using a scanning electron microscope. This experiment yields images of Na-montmorillonite platelets in situ. The cryo-SEM experiment together with other two experiments helps in providing a better correlation and interpretation of the data.
Images show changes in the microstructure of the gel-
from an ‘overlapping coin’ to a ‘house of cards’ platelet structure
with a much decreased cavity space
in the gel network
Data from rheological measurement shows that the viscoelastic moduli and yield stress increases with an increasing salt concentration till a critical concentration is reached and thereafter decreases due to salt-induced inter-particle attractions. Overall though, the dispersion remains elastic.
The gels with varying salt concentrations are imaged using cryo-SEM. The team analyzes the gel images to find intra-network honeycomb-like cavities forming within the gel in response to variations in salt concentrations. The image analysis is used to quantify the area and number of these cavities. In addition, data from the cryo-SEM technique gives us an insight into the length of branches from platelet to platelet that hold these cavities together within the gel. These factors play a crucial role in the stability of the gel network. With gradual increase in the concentration of salt in the suspension, the images show changes in the microstructure of the gel- from an ‘overlapping coin’ to a ‘house of cards’ platelet structure with a much decreased cavity space in the gel network accounting for the increase in viscoelastic moduli and yield stress of the dispersion.
But, upon further increase in salt concentration, the suspension undergoes a drastic structural change. Under very high salt conditions, the Na- montmorillonite platelets come face-to-face making the dispersion ‘banded’ in structure. The image analysis suggests that this ‘banded’ structure indicates a possible collapse of the network with the plate to platelet branches growing thicker and longer, forming ‘bands’ within the suspension; a possibility they next explored using the electroacoustic experiment. The electroacoustic study also shows that beyond a critical salt concentration, the gel structures are prone to collapse.
Given that soil is nothing but a suspension of clay minerals and salt, in water and sand, the findings of these three experiments are useful in getting a better understanding of soil-settling that could potentially hold a key to some of the geophysical phenomena such as the massive deltas in the world’s major riverine systems or landslides in fold mountainous systems around the earth.
What our study does is to simplify a very complex problem
by completely ignoring the underlying
role of water in the physics of soil settling-Dr. Bandyopadhyay
There are caveats to the work though: “What our study does is to simplify a very complex problem by completely ignoring the underlying role of water in the physics of soil settling. Hydrogen bonding in water is known to change quite significantly when certain solutes are added to it. Since soil is messy and can contain a range of inorganic and organic additives, we plan to look more closely at changes in the structure of water and the role of additives in the formation and settling of the colloidal clay gel dispersions.” adds Dr. Bandyopadhyay as she charts the future for this project with significant geophysical implications.
About the authors and work:
Prof. Ranjini Bandyopadhyay is associated with the Soft Condensed Matter group of Raman Research Institute while Dr. Samim Ali who is a co-author in the work was associated with the same group. Ali has since successfully defended his thesis and is currently a postdoctoral fellow at National Institute of Standard and Technology, Gaithersburg, MD.
For the publication see – http://pubs.rsc.org/en/content/articlelanding/2016/sm/c5sm01700a#!divAbstract
Full paper – https://arxiv.org/pdf/1507.02844v1.pdf
Author
Debarshini is an erstwhile scientist with a deep passion for science communication, science policy and outreach – a person of many hats. Science, to her, is a way of thinking. Trained in theoretical soft condensed matter physics, through the years she has branched out in various directions away from pure academia although it’s always a scientist’s hat that goes on when she brings in her own spin to her work. Having worked in unusual roles in multiple research institutes, in an embassy science department and now in a bio innovation hub, C-CAMP Debarshini feels that strength in targeted communication requires command over both science and language, subject and matter. Debarshini is open to connecting with anyone who shares the same career goals on Facebook and LinkedIn.
Editors: Rajamani Selvam and Paurvi Shinde Ph.D.
Rajamani Selvam is currently a Neuroscience Ph.D. student at the University of Connecticut Health, Farmington, CT. Her research focuses on understanding the interactions between growth factors and endocannabinoids in modulating acute synaptic transmission in the brain. Post-graduation, she is interested in pursuing a career in medical communications. She is passionate about communicating STEM education and outreach to middle and high schoolers. She is also a mentor for 1000 girls 1000 futures program, New York Academy of Sciences. Away from science, she is an artist and enjoys leisure travel. Follow her on LinkedIn.
Paurvi Shinde did her PhD, Immunology from Uconn Health and her expertise lies in T/B cell biology and activation pathways. She currently works as a Post Doc Fellow at Bloodworks Northwest in Seattle, where she studies the mechanism of alloimmunization to certain human ‘Red Blood Cell antigens’. Apart from science, she’s loves editing scientific articles to convey the message behind it, in a clear and concise way. Follow her on Linkedin.
Illustrators
Ipsa Jain provided the cover image. She is a post-doctoral fellow at Instem, Bangalore. She tries to communicate science through visual arts as a medium. Collecting graphic books, tree trash, and reading brain pickings is few of her favourites. Follow and purchase her artwork at Ipsawonders (Facebook, Twitter, and Instagram). She will be happy to hear praises and non-praises at ipsajain.31@gmail.com.
Bhrugu Yagnik is a Postdoctoral Fellow at Emory Vaccine Centre, Yerkes National Primate Research Centre, Emory University, Atlanta, GA and works on the development of a HIV/AIDS vaccine. His doctoral research focused on development of vaccines against Shigella using food grade Lactococcus lactis as an antigen delivery vehicle. Bhrugu has many awards to his credit. He is passionate about communicating science in creative ways. In his free time, Bhrugu indulges himself into the spirituality where he attempts to bring amalgamation of science and spirituality. Follow him on Twitter or connect with him on LinkedIn or ResearchGate.
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