Foam formation during drainage of a surfactant solution in a microfluidic porous medium model.

Foam has been shown to have great potential to significantly improve sweep efficiency during gas injection in oil recovery, remediation of contaminated sites, gas storage, and acidification processes. The gas mobility reduction largely depends on the generation and stability of lamellae in the pore space that traps the gas phase. Most available analyses focus on foam formation during the co-injection of gas and liquid phases at different fractional flow (foam quality) or flow of foam formed before being injected in the porous media. During surfactant-alternating-gas (SAG) injection, foam is formed as the aqueous phase is displaced by the gas slug that follows. The dynamics of lamellae formation and their stability are different from that of a co-injection process, since the amount of surfactant available to stabilize the gas-liquid interfaces is fixed as fresh surfactant solution is not injected together with the gas phase. This work studies foam formation during the drainage of a surfactant solution by gas injection at a fixed flow rate. A transparent microfluidic model of a porous medium is used in order to enable the correlation of pore-scale phenomena and macroscopic flow behavior. The results show that the maximum number of lamellae increases with surfactant concentration, even much above the critical micelle concentration (CMC). The availability of surfactant molecules needed to stabilize newly formed gas-liquid interfaces rises with concentration. The higher number of lamellae formed at higher surfactant concentration leads to stronger mobility reduction of the gas phase and longer time needed for the gas to percolate through the porous medium.

Use of a Geometric Parameter for Characterizing Rigid Films at Oil-Water Interfaces.

Interfacial tension and dilatational rheology are often used to characterize the mechanical response of a liquid interface using axisymmetric drop shape analysis (ADSA). It is important to note that for systems dominated by adsorption/desorption of surfactants, the contributions of extra mechanical stresses are negligible; thus, the Young-Laplace equation remains valid. However, for interfaces dominated by extra stresses, as in the case of particle monolayers or asphaltenes that clearly exhibit a skin (a rigid film), the nature of the elastic response is fundamentally different and the validity of the equation is questionable. Calculation of the interfacial tension and dilatational elasticity using drop shape analysis depends critically on the drop shape following the Young-Laplace equation. If the interface becomes more like a solid, the drop shape will deviate from being purely Laplacian. Indeed, the drop will exhibit a wrinkled surface as collapse continues. The geometric parameter RV/A, defined as the ratio (dV/V)/(dA/A) with V is the volume of the drop and A is the area of the interface), allows one to measure the deviation of the drop shape from purely Laplacian. For a simple interface (pure liquids or surfactant solutions), RV/A is quite close to the theoretical value of 1.5 of a perfect sphere. Nevertheless, if the molecules adsorbed at the interface begin to interact strongly, the ratio can vary. In the limit of long-time-scale experiments, RV/A of some drops approaches 2. We studied the evolution of the parameter RV/A for different systems, from simple to complex, as a function of oscillation frequencies and amplitudes of drop volume. The results obtained were compared to the values of the interfacial moduli and drop shape behavior to better characterize the regime change.

Freeze-Dried Microfluidic Monodisperse Microbubbles as a New Generation of Ultrasound Contrast Agents.

We succeeded in freeze-drying monodisperse microbubbles without degrading their performance, that is, their monodispersity in size and echogenicity. We used microfluidic technology to generate cryoprotected highly monodisperse microbubbles (coefficient of variation [CV] <5%). By using a novel retrieval technique, we were able to freeze-dry the microbubbles and resuspend them without degradation, that is, keeping their size distribution narrow (CV <6%). Acoustic characterization performed in two geometries (a centimetric cell and a millichannel) revealed that the resuspended bubbles conserved the sharpness of the backscattered resonance peak, leading to CVs ranging between 5% and 10%, depending on the geometry. As currently observed with monodisperse bubbles, the peak amplitudes are one order of magnitude higher than those of commercial ultrasound contrast agents. Our work thus solves the question of storage and transportation of highly monodisperse bubbles. This work might open pathways toward novel clinical non-invasive measurements, such as local pressure, impossible to carry out with the existing commercial ultrasound contrast agents.

Effect of interfacial rheology on drop coalescence in water-oil emulsion.

Over the last years several studies have been conducted to understand emulsion formation and its behavior. In some applications, the aim is the phase separation of the emulsions through the coalescence of the drops, as in the oil industry. In this study, the relationship between rheological properties of oil-water interfaces and the drainage time of a thin oil film between two aqueous drops was investigated. Interfacial tension and dilatational rheology were measured using the axisymmetric drop shape analysis. We evaluated different concentrations of a nonionic surfactant (Span 80) dissolved in mineral oil (Primol 352) phase. The results indicate a direct relationship between the properties of the structure formed at the oil-water interface and the absence of droplet coalescence. For low surfactant concentrations, below the critical micelle concentration (CMC), the interface is weakly elastic (fluid-like) and the coalescence process always occurs; the draining time is not to related to the aging time of the interface. For surfactant concentrations above CMC, the elastic and viscous moduli showed significant changes with aging leading to the formation of a solid-like film at the interface preventing further coalescence. We used the characteristic times of change in interfacial rheological behavior to better explain the non-coalescence process.

Deformation and rupture of microcapsules flowing through constricted capillary.

The dynamics of deformable microcapsules flowing through constricted channels is relevant in target delivery of chemicals in physiological systems, porous media, microfluidic medical diagnostic devices and many other applications. In some situations, the microcapsules need to sustain the stress they are subjected to as they flow through constricted channels and in others, the stress may be the rupture trigger used to release the internal content. We experimentally investigate the flow of monodispersed gellan gum microcapsules through a constricted capillary tube by measuring the evolution of the pressure difference and flow visualization. The maximum pressure difference and capsule deformation is obtained for capsules with different diameter and shell thickness. We map the conditions, e.g. diameter and shell thickness, at which the capsule membrane ruptures during the flow, releasing its internal phase.

Pore Scale Visualization of Drainage in 3D Porous Media by Confocal Microscopy.

We visualize the dynamics of immiscible displacement of a high viscosity wetting phase by a low viscosity non-wetting phase in a three-dimensional (3D) glass bead packing using confocal microscopy. Both phases were doped with two different fluorescent dyes, which enabled visualization of both phases simultaneously and quantification of the phase volumes without the need of image subtraction operations. The transient results show details of the displacement process and how pores are invaded by the non-wetting displacing phase. The static images at the end of the displacement process reveal how the trapped ganglia volume and morphology change with capillary number. The wetting phase is trapped as pendular rings spanning one to multiple pore necks. Details of the pore scale flow of oil wet media revealed with the experimental methods presented here can lead to better fundamental understanding of the physical processes and optimized enhanced oil recovery methods, CO2 sequestration and aquifer remediation.

Flow of Tunable Elastic Microcapsules through Constrictions.

We design and fabricate elastically tunable monodisperse microcapsules using microfluidics and cross-linkable polydimethylsiloxane (PDMS). The overall stiffness of the microcapsules is governed by both the thickness and cross-link ratio of the polymer shell. Flowing suspensions of microcapsules through constricted spaces leads to transient blockage of fluid flow, thus altering the flow behavior. The ability to tune microcapsule mechanical properties enables the design of elastic microcapsules that can be tailored for desired flow behavior in a broad range of applications such as oil recovery, reactor feeding, red blood cell flow and chemical targeted delivery.

Stretching liquid bridges with moving contact lines: comparison of liquid-transfer predictions and experiments.

Transfer of liquid from one surface to another plays a key role in printing processes. During liquid transfer, a liquid bridge is formed and then undergoes significant extensional motion while its contact lines are free to move on the bounding solid surfaces. In this work, we develop one-dimensional (1D) slender-jet and two-dimensional (2D) axisymmetric models of this phenomenon and compare the resulting predictions with previously published experimental data. For very low capillary numbers (Ca) (quasi-static stretching), predictions from both models of the amount of liquid transferred agree well with the experimental data, provided that the difference in receding contact angles (|Δθr|) between the two surfaces is sufficiently large. Notably, the amount of liquid transferred is primarily governed by the overall bridge shape and is not significantly influenced by contact-line motion toward the end of bridge stretching. For O(1) values of Ca, the models predict that each surface receives half the liquid, in agreement with experimental observations. For intermediate values of Ca (and very low values of Ca when |Δθr| is small enough), predictions from each model can deviate substantially from the experimental data, which we speculate is due to the influence of surface heterogeneities that are not included in the models. In this regime, there can be significant differences between the predictions of the 1D and 2D models, which is due to the tendency of the contact line to slip more in the 1D model. The models are also used to understand the influence of initial bridge shape on liquid transfer and to rationalize related experimental observations. The results from these fundamental studies will aid the optimization of gravure and other printing processes for manufacturing of printed electronic devices.

Minimization of viscous fluid fingering: a variational scheme for optimal flow rates.

Conventional viscous fingering flow in radial Hele-Shaw cells employs a constant injection rate, resulting in the emergence of branched interfacial shapes. The search for mechanisms to prevent the development of these bifurcated morphologies is relevant to a number of areas in science and technology. A challenging problem is how best to choose the pumping rate in order to restrain the growth of interfacial amplitudes. We use an analytical variational scheme to look for the precise functional form of such an optimal flow rate. We find it increases linearly with time in a specific manner so that interface disturbances are minimized. Experiments and nonlinear numerical simulations support the effectiveness of this particularly simple, but nontrivial, pattern controlling process.

Experiments and network model of flow of oil-water emulsion in porous media.

Transport of emulsions in porous media is relevant to several subsurface applications. Many enhanced oil recovery (EOR) processes lead to emulsion formation and as a result conformance originating in the flow of a dispersed phase may arise. In some EOR processes, emulsion is injected directly as a mobility control agent. Modeling the flow of emulsion in porous media is extremely challenging due to the complex nature of the associated flows and numerous interfaces. The descriptions based on effective viscosity are not valid when the drop size is of the same order of magnitude as the pore-throat characteristic length scale. An accurate model of emulsion flow through porous media should describe this local change in mobility. The available filtration models do not take into account the variation of the straining and capturing rates with the local capillary number. In this work, we present experiments of emulsion flow through sandstone cores of different permeability and a first step on a capillary network model that uses experimentally determined pore-level constitutive relationships between flow rate and pressure drop in constricted capillaries to obtain representative macroscopic flow behavior emerging from microscopic emulsion flow at the pore level. A parametric analysis is conducted to study the effect of the permeability and dispersed phase droplet size on the flow response to emulsion flooding in porous media. The network model predictions qualitatively describe the oil-water emulsion flow behavior observed in the experiments.

Stretching liquid bridges with bubbles: the effect of air bubbles on liquid transfer.

Liquid bridges containing bubbles are relevant to industrial printing and are also a topic of fundamental scientific interest. We use flow visualization to study the stretching of liquid bridges, both with and without bubbles, at low capillary numbers. We find that whereas the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force air bubbles selectively toward the least hydrophilic plate confining the liquid bridge.

Leveling of thin films of colloidal suspensions.

We present an analysis of leveling in thin films of colloidal suspensions. The colloidal particles are assumed to be much smaller than the film thickness and influence the film rheology through a concentration-dependent viscosity and bulk diffusivity. A system of coupled nonlinear partial differential equations based on lubrication theory is used to describe the film height and the particle concentrations in the bulk and at the film surface. Linear stability analysis is applied to develop expressions for leveling rates in a number of limiting cases. It is found that for soluble particles, there exist regimes where increasing the Marangoni number slows down leveling at both short and long times, in contrast to the case of insoluble particles. Nonlinear simulations show that the linear theory accurately predicts leveling times even for large amplitude disturbances, and that the presence of a concentration-dependent viscosity and bulk diffusivity speed up leveling. The results of this work should be useful for estimating leveling rates in coatings laden with colloidal particles, and also in coatings containing soluble surfactant.

Estudo histopatológico da estrutura óssea da bolha etmoidal em rinossinusite crônica / Histopathological study of the bone structure of etmoidal bulla in chronic rhinosinusitis

Apesar dos avanços no campo da pesquisa e da clínica, a exata fisiopatologia da rinossinusite crônica ainda permanece desconhecida.Vários estudos têm demonstrado as mais variadas alteraçöes histopatológicas que ocorrem em rinossinusite crônica. A mucosa nasal e dos seios paranasais têm sido o local primário dessas pesquisas, porém muito pouco se conhece sobre as alteraçöes ósseas encontradas nesta doença. OBJETIVO: Descrever as características histopatológicas da estrutura óssea da bolha etmoidal em pacientes com rinossinusite crônica. FORMA DE ESTUDO: Clínico prospectivo. MATERIAL E MÉTODO: Foram avaliados 19 indivíduos com rinossinusite crônica. Por meio da microscopia de luz, foram analisadas as trabéculas ósseas da bolha etmoidal quanto à extensäo do depósito de osteóide, a presença de osteoblastos e osteoclasto na superfície das trabéculas, reabsorçäo, linhas de aposiçäo e fibrose entre as:trabéculas ósseas. Foi analisada também a lâmina própria quanto ao infiltrado inflamatório e aos elementos celulares. A microscopia eletrônica de varredura analisou-se a morfologia óssea. CONCLUSÄO: A microscopia eletrônica de varredura foi possível observar com mais nitidez as margens das trabéculas ósseas reabsorvidas. Nenhum caso foi considerado normal, mas necrose óssea näo foi encontrada. Por meio da microscopia de luz, observou-se: depósitos de osteóide, agrupamentos de osteoblastos, fibrose e remodelaçäo óssea em cerca de 90 por cento dos casos. Apesar da ausência de osteoclastos, a reabsorçäo óssea esteve presente em cerca de 50 por cento dos casos. Estudos futuros acerca da açäo dos mediadores inflamatórios presentes na lâmina própria sobre o osso subjacente poderäo esclarecer a fisiopatologia da rinossinusite crônica em nível celular

Infecçäo no trajeto dos fios e pinos do fixador externo de Ilizarov: estudo bacteriológico / Pin tract infection using the Ilizarov external fixator: bacteriological analysis

Infecçäo no trajeto dos fios e dos pinos é complicaçäo comum e muitas vezes significante durante o uso de fixadores externos. A utilizaçäo de drogas antimicrobianas na interface implante-pele pode criar condições desfavoráveis ao crescimento desses germes, reduzindo o número de infecções. O objetivo com esta pesquisa foi realizar um estudo bacteriológico da secreçäo colhida do trajeto de fios e pinos infectados, em pacientes utilizando o fixador externo de Ilizarov. Foram coletadas 28 amostras (27 pacientes), sendo 20 no trajeto de fios de Kirschner e oito no trajeto de pinos de Schanz. Das 28 culturas, 25 apresentaram crescimento de um microorganismo e três, de dois microorganismos. O germe prevalente foi o Staphylococcus aureus em 20 das: amostras (prevalência de 0,645). Aplicando-se o teste do qui-quadrado para amostras independentes (Χc2), näo foi possível encontrar associaçäo estatisticamente significante entre o tipo de microorganismo isolado e o sexo do paciente, faixa etária, tipo de implante, período entre a cirurgia e o início da infecçäo, e regiäo acometida (p > 0,05). Diante dos resultados observados, os autores sugerem a adoçäo profilática de uma droga antimicrobiana tópica, com espectro para bactérias gram-positivas, na interface fio/pino-pele durante o uso de dispositivos de fixaçäo externa