M. Elwenspoek

Osmotic shrinkage of giant egg-lecithin vesicles

Osmotic shrinkage of giant egg-lecithin vesicles was observed by phase-contrast microscopy. The vesicles remained or became spherical when shrinking. Small and thick-walled vesicles formed visible fingers attached to the sphere. The water permeability of the single bilayer was found to be 41 micrometers/s. A variety of observations indicate that osmosis induces a parallel lipid flow between the monolayers of the bilayer, leading to a strong positive spontaneous curvature. They also suggest the formation of mostly submicroscopic daughter vesicles. The estimated coupling constant, 2 . 10(-6) mol/mol, is large enough to be biologically significant.

Mie scattering from thin spherical bubbles

The electromagnetic scattering problem for a thin optically isotropic spherical shell of arbitrary size and refractive index has been solved exactly in closed form. This is the only closed form solution in Mie scattering known to the authors. The standard series solution to the layered sphere problem is shown to be summable and simple exact expressions are obtained for the scattering amplitudes. The Rayleigh–Debye approximation results are also obtained by direct summation of the infinite series. The horizontal depolarization ratio, given a zero value for isotropic shells in the Rayleigh–Debye theory, is given considerable attention and is compared to the Rayleigh–Debye calculation for anisotropic radially oriented segments in the shell. Some comparisons with the Mie scattering from solid spheres are also included. In addition, the deviations of Rayleigh–Debye theory from the exact Mie theory of thin shell scattering are examined in detail as a function of various parameters.

Molecular motion in supercooled glycerol

Quasi-elastic Rayleigh scattering of 14·4 keV photons has been measured on supercooled liquid glycerol at -30°C and 0°C by employing the Mossbauer effect. Total scattered intensity, quasi-elastically scattered intensity I q and energy width of I q(k, ω) have been determined for k=0·6 to 4·2 A-1. The molecular motion is modelled as: random-walk diffusional motions for the centre-of-mass translation and for the orientation of independent rigid molecules, plus fast-bounded translational jitter (vibration). The model parameters are evaluated. The temperature dependence of the translational diffusion constant corresponds to an activation energy of 12 kcal/mol. Comparison is made especially with N.M.R. results for rotational motion. The effect of orientational jitter (libration) is considered and its possible influence on nuclear magnetic relaxation is pointed out.

Molecular motion in supercooled glycerol a Mössbauer scattering experiment

Rayleigh scattering of photons with λ=0·86 A was applied to liquid glycerol at T=-16·5°C. Using the Mossbauer effect, the width and intensity of the quasi-elastic line as well as the inelastic scattering intensity were measured for momentum transfers between 0·6 A-1 and 4·2 A-1. A model of the molecular motions is developed in order to analyse the results quantitatively. It comprises the molecular structure factor, a radial distribution function for the centres of mass, random-walk diffusion of the centres of mass and of the molecular orientation, and a high frequency motion; translation-rotation coupling is assumed to produce a common jump time. The results are compared with other experiments, especially N.M.R. relaxation.

Electric field fluctuations and bond character in liquid alloys

Abstract Electric field fluctuations within a liquid alloy can be measured by nuclear quadrupolar spin relaxation rate RQ of noble gas atoms imbedded in the liquid. The method is demonstrated using Xe as a prove in liquid Te alloys, viz. Teue5f8Ag, Ga, Ge, In, Sn, Tl. It is proposed to use RQ in order to derive a measure of ionic contribution to bonding. Combining the RQ data with conductivity data, and applying a simple model approach, we estimate the fraction of metallic-covalent-ionic bonds for each of the alloys.

High frequency motion and nuclear magnetic spin relaxation in highly viscous liquids

The influence of high frequency translational jitter in highly viscous liquids on the dipolar relaxation rate is calculated on the basis of an Einstein model. The frequency integral over the diffusional peak of the dipolar relaxation rate decreases with increasing mean squared vibrational amplitude. The results are applied to N.M.R. experiments on glycerol.

Nuclear quadrupolar relaxation and association in liquid InSb

Abstract A thermodynamic model of associates is applied to discuss the large increase of nuclear quadrupolar relaxation rate R Q found in many liquid alloys with compound-forming tendency, by considering the concentration and lifetime of associates. Data for concentration and temperature dependence of R Q of Sb in In 1- c Sb c are presented and explained by known thermodynami properties of the alloy; a characteristic time of the order of 10 −11 s is estimated.

Association in strongly interacting liquid binary alloys and nuclear spin relaxation

An experimental quantity sensitive to the time development of the distance between two atoms in a liquid alloy, is the quadrupolar nuclear spin relaxation rate RQ. The existing material for s-p-alloys shows a systematic occurrence of string enhancements of RQ if there is an attractive interaction among the partner elements in alloys. Using a model of associates for the alloy, it is possible to connect RQ rather directly with the lifetime of the associate. For In---Hg at 260 K as an example, 10?11 sec is obtained.

Microscopic charge fluctuations in the liquid semiconducting alloys Ga-Te and In-Te

Measurements are presented of the nuclear quadrupolar spin relaxation rate on an inert probe atom, 123Xe (using the TDPAD technique), in GacTe1-c and IncTe1-c alloys. Despite the apparent similarities of these systems, the relaxation rate in In-Te alloys is larger than in Ga-Te alloys. A possible connection of this difference with a difference in bond type is discussed.

Nuclear quadrupolar relaxation and the dynamics of pairs of atoms in liquids

We have developed a model calculation for the electrical field gradient correlation function on a probe atom in the liquid, cEFG(t)=. In this model, symmetry of the liquid is introduced explicitly and the distribution function for therelative coordinate ri(t) between the probe atom and particle i is calculated using Smoluchowskis diffusion equation with a mean force potential Φ(r)=kBT In g(r). The results for cEFG(t) can be characterized by two correlation times,, the shorter one being responsible for the small values of RQ in pure liquid metals, the longer one producing the increase of RQ in alloys. Also good agreement is found with recent results for cefg(t) from molecular dynamics studies.