Description

Clay flocs are abundant in natural soils (a particulate material) and water-borne sediments. As the basic microscale, loading-bearing fundamental units, their mechanical properties control the macroscopic response of bulk soils and sediment transport. Owing to their tiny size and extremely soft consistency (especially for suspended water-borne flocs), significant difficulties and challenges exist for mechanical characterization of clay micro flocs. A novel nanomechanical characterization technique was developed to probe the elasticity and yield shear strength of individual micron-sized clay and clay-biopolymer flocs that were prepared in laboratory and sampled in natural waters. Artificial clay flocs were prepared using four types of pure clay minerals (i.e., kaolinite, Na-smectite, Ca-smectite, and illite) and two types of polysaccharides of dissimilar polarities (i.e., cationic vs. anionic) to investigate the effects of how polarity affects clay–biopolymer interactions and the mechanical behavior of resulting biopolymer-bearing clay flocs. Natural marine flocs were sampled offshore near the Northern Gulf of Mexico. A nano universal testing system was used to compress individual spherical flocs submerged in water between two rigid platens, and the load-deformation curves were analyzed using Hertz elastic contact theory and Tresca yield criterion. The reduced modulus of these flocs ranges from 0.45 to 4.82 kPa, and the yield strength from 0.07 to 0.51 kPa. Moreover, the distribution of both elasticity and strength of the flocs can be fit by Weibull distribution, allowing the estimation of the mean value for the mechanical properties of flocs with highly variable properties. The developed technique was also employed to study the thixotropism of pure kaolinite flocs prepared in salt water at a salinity of 5 psu. Results indicate that thixotropic hardening leads to an increase in both elasticity and yield strength, and such an increase obeys a kinetics law of chemical reactions. Through the statistical analysis of the thixotropic increase, it was found that the mechanisms of thixotropy originates not only through interparticle bonding development, but also the redistribution of the nonuniform structural and mechanical flaws into a more uniform distribution. Finally, the developed techniques are readily applicable to the study of the clay aggregates in natural soils.

Share

COinS
 

Nanomechanical characterization of clay micro flocs

Clay flocs are abundant in natural soils (a particulate material) and water-borne sediments. As the basic microscale, loading-bearing fundamental units, their mechanical properties control the macroscopic response of bulk soils and sediment transport. Owing to their tiny size and extremely soft consistency (especially for suspended water-borne flocs), significant difficulties and challenges exist for mechanical characterization of clay micro flocs. A novel nanomechanical characterization technique was developed to probe the elasticity and yield shear strength of individual micron-sized clay and clay-biopolymer flocs that were prepared in laboratory and sampled in natural waters. Artificial clay flocs were prepared using four types of pure clay minerals (i.e., kaolinite, Na-smectite, Ca-smectite, and illite) and two types of polysaccharides of dissimilar polarities (i.e., cationic vs. anionic) to investigate the effects of how polarity affects clay–biopolymer interactions and the mechanical behavior of resulting biopolymer-bearing clay flocs. Natural marine flocs were sampled offshore near the Northern Gulf of Mexico. A nano universal testing system was used to compress individual spherical flocs submerged in water between two rigid platens, and the load-deformation curves were analyzed using Hertz elastic contact theory and Tresca yield criterion. The reduced modulus of these flocs ranges from 0.45 to 4.82 kPa, and the yield strength from 0.07 to 0.51 kPa. Moreover, the distribution of both elasticity and strength of the flocs can be fit by Weibull distribution, allowing the estimation of the mean value for the mechanical properties of flocs with highly variable properties. The developed technique was also employed to study the thixotropism of pure kaolinite flocs prepared in salt water at a salinity of 5 psu. Results indicate that thixotropic hardening leads to an increase in both elasticity and yield strength, and such an increase obeys a kinetics law of chemical reactions. Through the statistical analysis of the thixotropic increase, it was found that the mechanisms of thixotropy originates not only through interparticle bonding development, but also the redistribution of the nonuniform structural and mechanical flaws into a more uniform distribution. Finally, the developed techniques are readily applicable to the study of the clay aggregates in natural soils.