Spatiotemporal Evolution Of Biomineralized Cementation Based On Microfluidics

Geotechnical engineering is a typical practical discipline,which has ever relied on the principles of mechanics,physics and chemistry to solve related geotechnical problems.Cement,polymer and other synthetic materials have been widely used in the traditional geotechnical solutions and are considered to have a great impact on the environment.The using of those materials is also inconsistent with the green concept of development.Microbially-geotechnical engineering has been emerged as a cross discipline between microbial technology and geotechnical engineering in recent years.It uses microorganisms that harbored in the soil to alternate the engineering properties of geomaterials.This technology shows the advantages of low disturbance during construction and environmental compatibility,which is promising in the mitigation of liquefaction,seepage control,reinforcement,pollution control etc.Microbially-induced calcium carbonate precipitation(MICP)is a typical microbial geotechnical engineering technique,which uses the biomineralization process to cement particles and clog the pores to improve strength,permeability and other engineering properties.This technique has become a hot research topic in recent years.However,previous studies are mainly focuses on macroscale tests such as element scale and model scale tests.Microscopic research is still insufficient and the understanding of mechanism of MICP is still lacking.Inspired by the element scale test,this paper fabricates a reinforced mold within micrometer,establishes corresponding specimen preparation method and analytical procedures,and carries out a systematic study on the regimes of the MICP processes from the point of microscale view.The main research work and results in this paper can be summerized as follows:(1)The preparation process of microchannel and porous medium is elaborated,the method of filling sand particles into the microchannel is also proposed,and double-particle cemented specimens and single-layer sand particles biocemented specimens could be tailored using the microchannel.On the basis of the traditional microfluidics observation platform,the MICP micromodel,data acquisition and monitoring control equipment are integrated to establish a visualization platform for biocementation with high temporal and spatial resolutions.The injection process is controlled by a high-precision syringe pump,the bacteria are fluorescence stained and marked,and the growth of calcium carbonate is captured by a high-speed camera.The whole process of MICP from the precipitation of minerals in the solution to the biocementation of sand particles could be continuously monitored and imaged.(2)The raw images during the MICP processes are deep processed based on digital image processing technology,including the use of depth-of-field extended technique to improve image clarity,image recognition technology to distinguish microorganisms and calcium carbonate crystals from background or sand particles,and layer stacking technology to show the crystal growth process.On the basis of the image with micron resolutions,the quantification of the microscopic properties such as the number,area and size of calcium carbonate and the identification of the position of the bacteria and the morphology of calcium carbonate are realized.The interactions between bacteria,calcium carbonate precipitation and sand particles are analized,the growth and evolution of calcium carbonate single crystals,and the precipitation patterns and distributions of calcium carbonate precipitates between particles are identified;the influence of various environmental factors on the MICP process are investigated.Consequently,a complete analysis process of the microscale system for the MICP visualization platform is established,and the novel methodology of micro-scale research for MICP is raised.(3)Homogeneous porous medium tests were conducted to investigate the microcosmic process of MCP reinforcement in ideal pore space.The effects of flow rate and calcium ion concentration on the growth of calcium carbonate were discussed.The microscopic growth mechanisms,such as crystallization rule,growth pattern,and evolution pattern of calcium carbonate during MICP were analyzed.The transverse diffusion of bacteria and the kinetic behavior of calcium carbonate growth were studied,and the variation of Damk?hler number with the same injection amount and different flow rates is analyzed.Then,the growth mechanism of calcium carbonate reaction and migration competition in the process of MICP reinforcement is proposed by comparing the distribution characteristics of calcium carbonate at different concentrations and flow rates.(4)The MCP tests of the single-row sand-filled pipeline in the static and flowing conditions were carried out to investigate the adsorption of sand particles to bacteria and calcium carbonate.Then,the effect of calcium ion concentration on the free diffusion of bacteria was discussed under static condition,and it is found that the sand particles cannot accumulate bacteria,a high concentration of calcium ions can inhibit the spread of bacteria,calcium carbonate is preferentially deposited on the surface of the particles,the concentration can affect the calcium carbonate distribution.Besides,the growth pattern of calcium carbonate induced by the MICP in the pore structures with different sizes is different from that precipitated at flow conditions.The influence of pore fluid flow on the growth of calcium carbonate single crystals and the cementation between particles was analyzed,and the growth rate of calcium carbonate over time was further discussed.It is verified that the single-row sand-filled pipeline has the ability to prepare large size MICP single crystal and particle scale cement.(5)Microchip tests with channels full filled with sand particles were carried out to study the microscopic process of MCP reinforcing heterogeneous sand particles.The effects of sand particle size,bacterial concentration,and calcium ion concentration on the growth of calcium carbonate on the micro-scale were investigated and the heterogeneity in the growth of Ca CO₃ were analyzed.The deposition pattern of calcium carbonate and the growth law of calcium carbonate in the sand-filled pipeline were studied,and the evolution pattern of calcium carbonate precipitation among heterogeneous sand particles was analyzed.The micro-mechanism which may affect the distribution of Ca CO₃precipitation and its related physical and chemical effects were studied.It is found that the higher the concentration,the faster the growth rate of calcium carbonate,and the smaller the effect of particle size on the micro-scale distribution of calcium carbonate.Besides,to investigate the distribution and growth of Ca CO₃ precipitation formed under abiotic condition,chemical method was used to precipitate Ca CO₃ in the microchip with channels full filled with sand particles.The difference in the precipitation between the chemical method and MICP was analyzed,which demonstrated the key factor of bacteria in the MICP process.