Using dissipative particle dynamics for modeling surfactants

Oil recovery is an industrial process that injects aqueous solutions into an oil reservoir to pump out crude oil and promote the oil production. The aqueous solution contains surfactants for reducing the interfacial tension (IFT) between aqueous phase and oil. The critical micelle concentration (CMC) is the concentration of surfactant above which micelles form and the interfacial tension reaches a plateau. Our research seeks to measure IFT and CMC for surfactants using dissipative particle dynamics (DPD) technique, which is a coarse-grained method based on the molecular dynamics. We first study how IFT is influenced by the surfactant concentration. Furthermore, another simulation is performed, in which an oil drop passes through porous media in the presence of surfactants under various capillary numbers. Using the simulation of emulsion flow through porous media, we get qualitatively similar velocity profile and average velocity as the results from computational fluid dynamics. In addition, the porous media simulation indicates that the presence of surfactants would slightly reduce the average velocity of the oil drop

Analytical Solution Of Microbes Interacting With Surfaces

Nowadays, there is a rising interest in studying the behavior of microbes and their interactions with flow and surfaces. In order to explore the velocity field, pressure and forces around the microbes, the solution of Stokes equations, which is called a Stokeslet, is used. This solution represents a singular velocity field due to a concentrated external force acting on fluid at a single point. This singularity could cause the expression of velocity not integrable. We use the Regularized Stokeslet Method and Method of Images to deal with this problem. The expression of force is replaced by a radially symmetric function, which distributes the force on a certain area rather than at a single point. We perform different numerical simulations to validate the code against analytical solution for flow around a sphere and a swimmer. The numerical results match well with the exact solutions. It can be concluded that the analytical solutions of microbes interacting with surfaces can be well simulated using the method of Regularized Stokeslet

Near-wall motion of a squirmer in viscoelastic fluids

Microbial biofilms ubiquitously occur on natural and man-made surfaces and are closely related to various health and environmental issues. During the biofilm formation, the hydrodynamic interaction between microorganisms, surfaces and biofilm itself, which shows viscoelastic properties, are important. Despite this, the hydrodynamic interaction of swimming microorganisms and solid surfaces in viscoelastic fluids is poorly understood. We perform a three-dimensional direct numerical simulation of a microorganism swimming in viscoelastic fluids near a wall. The background viscoelastic fluid is modeled using a Giesekus constitutive equation and the microorganism is modeled using a squirmer model, which is composed of a spherical cell body with a tangential surface motion. We found that the viscoelasticity of the fluid affects the near-wall motion of a squirmer depending on the swimming mode. For a neutral squirmer, the wall-contact time increases with viscoelasticity and reaches a maximum at Wi ~ 1 due to a negative polymeric torque acting on the squirmer and impeding its rotation away from the wall. The neutral squirmer eventually escapes from the wall. On the other hand, the pusher is found to be trapped near the wall in viscoelastic fluids due to the highly stretched polymers behind its body. The near-wall motion of a puller swimmer is less affected in viscoelastic fluids

Bacteria Movement Near Surfaces

Understanding the behaviors of bacteria near surfaces is crucial in many biological and ecological applications. This knowledge can be used to hinder undesired biofilm formation on medical instruments and wounds. On top of that, it could also provide further insights in biodegradation of dispersed oil. In this work, the behavior of Escherichia Coli near a surface was experimentally studied. We utilized an inverted microscope in the phase filed illumination mode and processed acquired images to track the motions of bacteria near surfaces with high accuracy and repeatability. Distribution of the cells when they reached a steady state shows that the number of bacteria near solid surfaces increases, which is consistent with previous studies

Pore Scale Transport of Miscible and Immiscible Fluids in Porous Media

The separation of harmful or valuable substances entrapped in porous media has applications in processes such as enhanced oil recovery, diffusion in tissue, and aquifer remediation. In this study the motion and removal rate of immiscible and miscible solutions have been analyzed to gain understanding of solvent effectiveness as it is diluted due to diffusion or mixing within porous materials. The extraction of oil using water, a surfactant solution of 4% CTAB in water, and a foam produced form the surfactant solution is observed using two dimensional flows between parallel slides containing cylindrical obstacles. The fluid motion is visualized. The foam proved to be the most effective solution at removing oil. The formation of large air bubble during foam propagation indicated that foam is not capable of holding its structure. The dissolution of two miscible fluids (glycerol and water) is visualized in square and round capillary tubes of various diameters. The capillaries are filled with solute before being immersed in a bath in which the solute concentration within the solvent is increased. The observation of the miscible liquid-liquid interfaces in a tube help us quantify the effective diffusion process

Movement and Distribution of Bacteria near Surfaces

Bacteria are found everywhere in nature, including within human/animal bodies, biomedical devices, industrial equipment, oceans and lakes. They can be found in planktonic state within a bulk liquid phase or attached to surfaces with the potential to form biofilms. In this study we are interested in the movement and distribution of bacteria near surfaces. The concentrations and fluid interactions of bacteria were characterized at various distances from a surface. Psuedomonas putida F1 was observed in a suspension near a surface. Bacteria movements were visualized with an inverted microscope at 40x magnification. P. putida F1 exhibited greater density in close proximity to the surface when compared to the bulk. Additionally, the ability to move in a direction normal to the surface was significantly reduced