Biofilms are communities of microbes embedded in a matrix of extracellular polymeric substances, largely polysaccharides. Multiple types of extracellular polymeric substances can be produced by a single bacterial strain. The distinct polymer components of biofilms are known to provide chemical protection, but little is known about how distinct extracellular polysaccharides may also protect biofilms against mechanical stresses such as shear or phagocytic engulfment. Decades-long infections of Pseudomonas. aeruginosa biofilms in the lungs of cystic fibrosis patients are natural models for studies of biofilm fitness under pressure from antibiotics and the immune system. In cystic fibrosis infections, production of the extracellular polysaccharide alginate has long been known to increase with time and to chemically protect biofilms. More recently, it is being recognized that chronic cystic fibrosis infections also evolve to increase production of another extracellular polysaccharide, Psl; much less is known about Psl’s protective benefits to biofilms. We use oscillatory bulk rheology, on biofilms grown from longitudinal clinical isolates and from genetically-manipulated lab strains, to show that increased Psl stiffens biofilms and increases biofilm toughness, which is the energy cost to cause the biofilm to yield mechanically. Further, atomic force microscopy measurements reveal greater intercellular cohesion for higher Psl expression. Of the three types of extracellular polysaccharides produced by P. aeruginosa, only Psl increases the stiffness. Stiffening by Psl requires CdrA, a protein that binds to mannose groups on Psl and is a likely cross-linker for the Psl components of the biofilm matrix. We compare the elastic moduli of biofilms to the estimated stresses exerted by neutrophils during phagocytosis, and infer that increased Psl could confer a mechanical protection against phagocytic clearance.

Summary Displacement of oil from an initially oil-filled porous rock by water consists of advancement of menisci and rupture of oil connections. In displacements controlled by capillarity, which are typical of oil reservoir floods, these pore-level events are governed by the local pore geometry, pore topology, and fluid properties, but the pressure field initiates these pore-level events and integrates them with the externally imposed Darcy flow. This paper reports the physics of the pore-level events and their integration on a computationally simple model of rock: a square network of pores. The novelty of the approach lies in keeping track of the evolution of the displacement front and in constructing an approximation of the entire pressure field that carries the information essential for predicting the evolution. The result gives insight into the state of the residual oil saturation and its dependence on pore geometry and the capillary number, Nca, of displacement. As the capillary number increases, the residual oil saturation decreases and the residual oil blobs tend to be smaller. As the pore size distribution becomes wider, the decrease of residual oil saturation with capillary number becomes smoother. Introduction Displacement of oil from water-wet porous sedimentary rock by waterflooding usually leads to entrapment of a considerable fraction of oil. The processes involved are controlled by the interplay of capillary and viscous forces1 and, in some cases, buoyancy and inertial forces. Understanding entrapment phenomena and displacement mechanisms is important for designing EOR techniques. If, as in some important processes, entrapped oil blobs are mobilized by lowering interfacial tension (IFT), then the length distribution of blobs is a key factor in determining the recovery efficiency.2,3 It is desirable to know how this distribution is created and to develop a theoretical basis for correlating the distribution with the morphology of the reservoir rock and the conditions of waterflooding. For example, knowledge of the displacement mechanism permits one to predict the integrity of the trailing edge of an oil bank. Understanding of the mechanisms of two-phase flow in porous media has progressed considerably in the last few decades. Studies in this area can be regarded as either macroscopic or microscopic. In macroscopic studies, averaged quantities, such as relative permeabilities and residual saturations, are. measured in samples that are large compared with pore scale. When correlated with average flow parameters, such as capillary number, the measurements can be interpolated and, with greater risk, extrapolated. In microscopic studies, pore-level events in model systems are visualized and measured to understand why an oil phase becomes disconnected into ganglia or blobs,4–7 how oil/water menisci move,8,9 why ganglia or blobs remain trapped,1,2,10 and what is required to mobilize them.1,2,11 The results have guided the design of macroscopic studies and the choice of correlating parameters. What has been lacking is a means of directly linking important macroscopic quantities to experiments at the microscopic level. Recently, pore-level physics has been combined with percolation theory12,13 and population balances14 to predict macroscopic quantities for tertiary oil recovery. The need for a statistical physics of flow in porous media is reflected by Fatt's15 pioneering attempts at network modeling. Recently, highly simplified pictures of pore-level physics have been combined with a stochastic approach called percolation theory13 and another known as population balance equations14 in attempts to correlate better the macroscopic quantities needed to interpret corefloods and to design EOR processes. This paper addresses the issues of more accurately modeling the pore-level events and the chaotic nature of rock and relating them to displacement on a macroscopic scale. For example, unanswered questions remain about the mechanism of oil disconnection. The disconnection process, as surmised by Reed and Healy,16 should be strongly influenced by the local pressure field near a flood front, which is unsteady even in a steady displacement process. To date, no attempt has been made to estimate that field or its effect. The increased relative permeability near the flood front estimated by Morrow and Boonraum7 for gas displacement is an important quantity but does not appear to be adequate for determination of oil-disconnection events.

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Monte Carlo simulations of a three-dimensional lattice model of an amphiphile−solvent system are presented and examined for their usefulness in predicting the thermodynamics of self-association of amphiphiles in solution. The size distribution of the aggregates obtained from the simulations is usedin combination with the mass action modelto obtain the thermodynamic properties of the solution of aggregates and to obtain expressions for the coefficients of an empirical equation for the Gibbs energy available in the literature for aqueous nonionic surfactant solutions. The results are in good agreement with the empirical model, and the essential microstructural features and thermodynamic properties of the micellar solution can be extracted reasonably well using such an empirical fitting of the simulation data. The effects of the lengths of the solvophobic and solvophilic sections of the amphiphile on critical micelle concentration extracted from the simulations are consistent with experimental results reported for several nonionic surfactants in water.

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