RESUMO
Within the framework of a discrete Gaussian model, we present analytical results for the interaction induced by a lamellar phase between small embedded colloidal particles. We consider the two limits of particles strongly adherent to the adjacent membranes and of particles impenetrable to the membranes. Our approach takes into account the finite size of the colloidal particles, the discrete nature of the layers, and includes the Casimir-like effect of fluctuations, which is very important for dilute phases. Monte Carlo simulations of the statistical behavior of the membrane-interacting colloidal particles account semi-quantitatively, without any adjustable parameters, for the experimental data measured on silica nanospheres inserted within lyotropic smectics. We predict the existence of finite-size and densely packed particle aggregates originating from the competition between attractive interactions between colloidal particles in the same layer and repulsion between colloidal particles one layer apart.
RESUMO
The activity of cell membrane inclusions (such as ion channels) is influenced by the host lipid membrane, to which they are elastically coupled. This coupling concerns the hydrophobic thickness of the bilayer (imposed by the length of the channel, as per the hydrophobic matching principle) but also its slope at the boundary of the inclusion. However, this parameter has never been measured so far. We combine small-angle x-ray scattering data and a complete elastic model to measure the slope for the model gramicidin channel and show that it is surprisingly steep in two membrane systems with very different elastic properties. This conclusion is confirmed and generalized by the comparison with recent results in the simulation literature and with conductivity measurements.
Assuntos
Gramicidina/química , Gramicidina/metabolismo , Canais Iônicos/química , Canais Iônicos/metabolismo , Lipídeos de Membrana/química , Lipídeos de Membrana/metabolismo , Modelos Biológicos , Modelos Químicos , Membrana Celular/química , Membrana Celular/metabolismo , Interações Hidrofóbicas e HidrofílicasRESUMO
We revisit the problem of the long-range interaction between two conical proteins inserted into a lipid membrane and interacting via the induced deformation of the membrane, first considered by Goulian et al. (M. Goulian, R. Bruinsma, P. Pincus, Europhys. Lett. 22, 145 (1993); Europhys. Lett. 23, 155 (1993)). By means of a complex variables formulation and an iterative solution, we determine analytically an arbitrary high-order expansion of the interaction energy in powers of the inverse distance between two inclusions of different sizes. At leading order and for inclusions of equal sizes, we recover the result obtained by Goulian et al.. We generalize the development to inclusions of different sizes and give explicit formulas that increase the precision by ten orders in the inverse distance.
Assuntos
Elasticidade , Bicamadas Lipídicas/química , Proteínas de Membrana/química , Modelos Teóricos , Bicamadas Lipídicas/metabolismo , Proteínas de Membrana/metabolismoRESUMO
We analytically determine the lateral capillary force acting on a spherical colloid trapped at an interface of arbitrary shape. Our calculations, which are valid for colloids that are small with respect to the capillary length, take into account surface tension, pressure and gravity. We relate the force acting on the colloid to the shape of the liquid interface prior to colloid deposition. Our approach is a generalization of a previous study of ours [Ch. Blanc et al., Phys. Rev. Lett., 2013, 111, 058302] to the case in which gravity is present. Our findings are in agreement with the so-called Nicolson superposition approximation [D. Y. C. Chan, J. D. J. Henry and L. R. White, J. Colloid Interface Sci., 1981, 79, 410] and with the curvature-dependent capillary force predicted by Würger [A. Würger, Phys. Rev. E, 2006, 74, 041402], and extend these results by including higher-order terms in the ratio between the size of the colloid and the capillary length. We thoroughly validate our theoretical expressions by means of an exact nonlinear numerical calculation.
RESUMO
We report theoretical predictions and measurements of the capillary force acting on a spherical colloid smaller than the capillary length that is placed on a curved fluid interface of arbitrary shape. By coupling direct imaging and interferometry, we are able to measure the in situ colloid contact angle and to correlate its position with respect to the interface curvature. Extremely tiny capillary forces down to femtonewtons can be measured with this method. Measurements agree well with a theory relating the capillary force to the gradient of Gaussian curvature and to the mean curvature of the interface prior to colloidal deposition. Numerical calculations corroborate these results.
Assuntos
Coloides/química , Modelos Químicos , Interferometria/métodos , Tensão SuperficialRESUMO
Particles dispersed in a liquid crystal above the nematic-isotropic phase transition are wetted by a surface-induced corona of paranematic order. Such coronas give rise to pronounced two-particle interactions. In this paper, we report details on the analytical and numerical study of these interactions published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially demonstrate how for large particle separations the asymptotic form of a Yukawa potential arises. We show that the Yukawa potential is a surprisingly good description for the two-particle interactions down to distances of the order of the nematic coherence length. Based on this fact, we extend earlier studies on a temperature induced flocculation transition in electrostatically stabilized colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa potential to establish a flocculation diagram for a much larger range of the electrostatic parameters, namely, the surface charge density and the Debye screening length. As a distinguished feature, a kinetically stabilized dispersion close to the nematic-isotropic phase transition is found.
RESUMO
We show that, contrary to planar membranes under tension, tense membrane tubules exhibit important critical fluctuations originating from unidimensional Goldstone modes. The latter yield unexpected behavior, such as correlations extending over the whole tube length and the increase of the fluctuating area over the projected area with increasing tension.