Lipid membrane instability and poration driven by capacitive charging

A new model for the interaction of an electric pulse with a lipid membrane is proposed. Using this model we show that when a DC electric pulse is applied to an insulating lipid membrane separating fluids with different conductivities, the capacitive charging current through the membrane drives electrohydrodynamic flow that destabilizes the membrane. The instability is transient and decays as the membrane charges. The bulk conductivity mismatch plays an essential role in this instability because it results in a different rate of charge accumulation on the membrane's physical surfaces. Shearing stresses created by the electric field acting on its own induced free charge are non-zero as long as the charge imbalance exists. Accordingly, the most unstable mode is related to the ratio of membrane charging time and the electrohydrodynamic time.Comment: 4 pages, 4 figure

Interfacial effects on droplet dynamics in poiseuille flow

Many properties of emulsions arise from interfacial rheology, but a theoretical understanding of the effect of interfacial viscosities on droplet dynamics is lacking. Here we report such a theory, relating to isolated spherical drops in a Poiseuille flow. Stokes flow is assumed in the bulk phases, and a jump in hydrodynamic stress at the interface is balanced by Marangoni and surface viscous forces according to the Boussinesq–Scriven constitutive law. Our model employs a linear equation of state for the surfactant. Our analysis predicts slip, cross-stream migration and droplet-circulation velocities. These results and the corresponding interfacial parameters are separable: e.g., cross-stream migration occurs only if gradients in surfactant concentration are present; slip velocity depends on viscosity contrast and dilatational properties, but not on shear Boussinesq number. This separability allows a new and advantageous means to measure surface viscous and elastic forces directly from the drop interface

Shear and Dilational Interfacial Rheology of Surfactant-Stabilized Droplets

A new measurement method is suggested that is capable of probing the shear and dilational interfacial rheological responses of small droplets, those of size comparable to real emulsion applications. Freely suspended aqueous droplets containing surfactant and non-surface-active tracer particles are transported through a rectangular microchannel by the plane Poiseuille flow of the continuous oil phase. Optical microscopy and high-speed imaging record the shape and internal circulation dynamics of the droplets. Measured circulation velocities are coupled with theoretical descriptions of the droplet dynamics in order to determine the viscous (Boussinesq) and elastic (Marangoni) interfacial effects. A new Marangoni-induced stagnation point is identified theoretically and observed experimentally. Particle velocimetry at only two points (including gradients) in the droplet is sufficient to determine the amplitudes of the dilational and shear responses. We investigate the sensitivity for measuring interfacial properties and compare results from droplets stabilized by a small-molecule surfactant (butanol) and those stabilized by relatively large block copolymer molecules. Future increased availability of shear and dilational interfacial rheological properties is anticipated to lead to improved rules of thumb for emulsion preparation, stabilization, and general practice

Os Contornos e o Entorno da Nova Sociologia da Moral

Neste ensaio vou expor brevemente como vejo o desenvolvimento do campo da Sociologia da Moral, com foco em seu potencial fundamentalmente interdisciplinar, destacando os estudos e tradições que merecem ser incorporados à sociologia. A moral, como tema de investigação da ciência social, perpassa os campos da psicologia (social e do desenvolvimento), sociologia, antropologia, neurociências e economia. Aqueles entre nós implicados no seu desenvolvimento afirmam que ela serve de fundamento para toda a organização e interação social. Assumo, implicitamente, a posição do filósofo Charles Taylor e do sociólogo Christian Smith de que os seres humanos vivem envolvidos em teias de significados, pelas quais são moldados, conforme versões de "certo" e de "bem". Os seres humanos são fundamentalmente morais, não no sentido de serem convencionalmente altruístas ou de se preocuparem com os outros, mas de que as pessoas humanas, por serem seres sociais habitando um espaço social, devem assumir posições sobre temas relevantes nessas sociedades e grupos. As pessoas, de um modo geral, nesse meu paradigma, ancoram seus sentidos de si em posicionamentos morais, padrões que oferecem um solo a partir do qual dão sentido ao mundo através de lentes morais. Uma sociologia da moral compreende a formação dessas crenças, sua relativa imutabilidade ou as circunstâncias pelas quais elas mudam, sua influência sobre a ação e sua reconstrução retrospectiva diante de efeitos desajustados ou de pressões sociais