Ceco D. Dushkin

Nucleation and growth of two-dimensional colloidal crystals

Abstract We first grow two-dimensional crystals of nanometer latex particles from a thin film of water suspension placed on a flat solid substrate. The crystallization is performed by controlling the evaporation rate of water and the meniscus profile. The kinetic observations of the crystallization demonstrate two distinct processes: nucleation, preceded by thinning of the thin liquid film, and crystal growth. The crystal grows linearly in size, and its area increases as a quadratic function of time, due to the convective influx of particles from the suspension. A simple model of crystal growth interprets the kinetic data.

The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix

The first comprehensive study on the kinetics of nanocrystal growth in a hot amphiphile medium is presented. An example is given with CdSe semiconductor nanocrystals grown after the injection of precursor (a mixture of Cd- and Se-reagents) in concentrated tri-octylphosphine oxide matrix (heated to more than 300 degrees C). The particle size distribution is reconstructed as a function of time from the absorption and photoluminescence spectra collected during the synthesis process. For this purpose a new expression is used relating the exciton energy due to quantum confinement with the nanocrystal radius. The growth kinetics is considered as a two-stage process in order to describe the time variation of nanoparticle size. During the first stage, called reaction-limited growth, the size of initial nucleus rapidly increases due to a sort of surface reaction exhausting the precursor in the nanoparticle vicinity. The growth in such conditions favors also a remarkable narrowing of the size distribution. The nanocrystal develops further on account of a slow precursor transfer from a distant space driven by the concentration gradient--classical diffusion-limited growth. The width of size distribution also increases proportional to the average particle size. Any growth will stop after the precursor concentration reaches a minimum value defining the limit for the final nanocrystal size in a batch. Solving the kinetic equations for the growth rate in each case of kinetics derives analytical expressions for the mean radius and variance of size distribution. Then the respective expressions are matched in a uniform solution valid during the entire synthesis. The theoretical model is in a good quantitative agreement with the experimental data for independent syntheses. Important characteristic scales of the processes (time-constant and length) and microscopic parameters of the reacting system (interfacial energy and reaction rate constant) are estimated from the data. It turns out that the fast reaction-limited growth is important to obtain well-defined nanocrystals of high optical quality by using less energy, time and consumable. However, to make them reproducibly uniform one should control also the ultra-fast nucleation process preceding the nanocrystal growth, which is still unknown. Nevertheless, our current findings allow the conceptual design of a new continuos-flow reactor for the manufacturing of a large amount of uniform nanocrystals.

The kinetics of the surface tension of micellar surfactant solutions

Abstract The effect of diffusion of polydisperse micelles on the kinetics of the surface tension is studied theoretically. It is shown that the surface properties of micellar surfactant solutions depend on which of the relaxation processes of micellization (fast or slow), has a time constant comparable with the characteristic time of diffusion. General equations, describing the diffusion of the free monomers and the micelles, are derived. They include new expressions for the source terms, accounting for the kinetics of micellization of the polydisperse micelles. Analytical solutions of these equations for the surface tension as a function of time are obtained. Since the micelles are additional sources of monomers, the relaxation of the surface tension is faster than the relaxation below the CMC. The theory allows computation of the relaxation time constants of micellization from data obtained by surface stress experiments.

Dynamic surface tension of micellar solutions studied by the maximum bubble pressure method. 1. Experiment

The effect of the micelles on the dynamic surface tension of micellar surfactant solutions is studied experimentally by means of the maximum bubble pressure method. Different frequencies of bubbling ranging approximately between 1 and 30 s−1 are applied. The time dependence of the surface tension is calculated using a dead time correction. Water solutions of two types of surfactants with different concentrations are investigated: sodium dodecyl sulfate and nonylphenol polyglycol ether. The surface tension relaxes more quickly in the presence of micelles. The characteristic times of relaxation of the surface tension seem to be in the millisecond range. The time constants observed experimentally are explained in terms of the theory of surfactant diffusion affected by micellization kinetics.

Effect of growth conditions on the structure of two-dimensional latex crystals: experiment

Abstract Essential experimental features of the nucleation and growth of a 2D colloidal crystal on a solid substrate are modeled. The crystal, composed of sub-micron-sized latex spheres, is grown by the evaporation of water from the particle suspension in a circular cell. The calculation of the meniscus profile in the cell allows the prediction of the particle volume fraction in the suspension surrounding the crystal as a function of time. This quantity enters into a convective-diffusion model for the crystal growth which calculates the crystal radius as a function of time. Comparison with experimental data for 2D latex particle crystals shows predominant convective growth over a wide range of evaporation rates set by varying the humidity of the air. Microscopic parameters of the particle assembly can also be estimated such as the particle velocity, diffusivity, characteristic time constants, Peclet number, etc. The nucleation is simulated by simultaneously solving the equations of motion for the ensemble of particles trapped in a thin liquid film using the discrete-element method. These equations account for the forces which are physically important in the system: contact particle–particle friction, increased viscous resistance during the particle motion in a wetting film, long-range capillary attraction between two particles screened by the rest of particles. The final result of the simulation is a particle cluster of hexagonal packing, whose structure resembles very much the monolayer nucleus of latex particles observed experimentally. The models proposed by us could also be implemented for the aggregation of species in a variety of practical processes such as coating, texturing, crystal growth from a melt or liquid solution, or a biological array.

Optical Memory Media Based on Excitation-Time Dependent Luminescence from a Thin Film of Semiconductor Nanocrystals

We describe the increase of photoluminescence intensity from densely packed semiconductor nanocrystals (quantum dots) with excitation time, which is clearly observed by the naked eye under ambient conditions. This enabled the invention of the first luminescence-based optical memory media feasible for practical applications. Bright luminescent images are stored and then read out by exciting the medium, a thin film of cadmium selenide nanocrystals, with blue or UV light. The increase in emission intensity is attributed to the trapping and accumulation of photo-generated electrons in the matrix of organic molecules capping the nanocrystals.

Nanotribology of thin liquid-crystal films studied by the shear force resonance method

Abstract We present the first resonance study on the tribology of molecularly thin liquid films. For this purpose we utilize a shear force apparatus, constructed as an attachment to the surface force apparatus, and measure the response of a film pressed between two solid surfaces and subjected to lateral oscillations with differing frequencies. There is a dramatic change in the amplitude and the phase of the output signal in the vicinity of the resonance frequency of the mechanical system. The resonance curves obtained at various normal loads are highly sensitive to the dissipation processes in the sample. The tribological parameters are extracted from the experimental data using an original theoretical model. In this way one can measure not only the friction coefficient, but also the damping coefficient, which provides information on the energy loss in the film. Attributing this loss to the work done against friction, one can calculate the energy of binding of a liquid molecule to the substrate. The samples are thin liquid crystals, 4-cyano-4- n -alkylbiphenyls ( n = 5, 6, 8) in various states, which can also serve as model lubricants. It is found that the films of these compounds exposed to friction are of different mechanics and energetics depending on their structure and on the intermolecular interactions.

A resonance shear force rheometer modeled as simple oscillating circuit

A novel resonance method for studying the viscoelasticity of very thin liquid films and elastic materials is developed using a shear force apparatus. The shear stress created by an oscillating piezo unit attached to leaf springs is recorded as the lateral displacement by capacitance probe. The oscillation frequency is varied around the resonance frequency of the mechanical system in order to trace the amplitude and the phase of the resonance peak. Two reference states are obtained: the resonance of free oscillations in air and one under constrain introduced by the cantilever spring in contact with the shear mechanical unit. The presence of a liquid film changes these resonance states depending on the film thickness and the cantilever load. A simple mechanical model is proposed entrapping the contribution of different parts in effective spring, mass, and damping constants. The model separates the effect of the liquid film from the background oscillation of the mechanical parts. The method is applied here t...

Model of the quasi-monodisperse micelles with application to the kinetics of micellization, adsorption and diffusion in surfactant solutions and thin liquid films

Abstract The micelles in a water surfactant solution are considered as quasi-monodisperse aggregates of two kinds: large and small. They give rise to two micellization processes, fast and slow, in response to any external perturbation. During the fast process the bigger micelles release a certain number of monomers to transfer into the smaller ones, while the total micelle concentration remains constant. During the slow process both types of micelle decrease in number until the effect of perturbation is compensated. Using an asymptotic mathematical procedure, we derive analytical expressions for the concentrations of species and the characteristic time constants of relaxation. The time constants obtained are consistent with the experimental data for the micellization kinetics and also with their counterparts known from the polydisperse micelle model. The advantage of the quasi-monodisperse model dealing with only two micelle fractions is further exploited to solve a more complicated diffusion problem for the kinetics of adsorption. It turns out that the micellization process is important for surfactant mass transfer; because the time constant of this process is commensurable with the characteristic time of diffusion. The set of coupled equations derived for the diffusion of free monomers and micelles contains source terms accounting for the exchange of material among the species. The equations are solved for a semi-infinite micellar solution and the analytical expression for the dynamic surface tension is compared with experimental data for different surfactants, measured by various methods. In all cases the slow micellization process predetermines the adsorption kinetics, i.e. the micelles have to be destroyed in order to fill the adsorption layer with monomers. In contrast to this observation, the fast relaxation process affects the adsorption on the two opposite surfaces of a thin liquid film, because the diffusion in the film is much faster than in a semi-infinite medium. For this reason the micelles can compensate the surface tension gradients created in the course of film thinning by simply releasing monomers. The calculated velocity of thinning is effectively increasing above the critical micelle concentration, which is in accord with existing experimental data. The thin film hydrodynamics turns out to be affected by the bulk diffusion of monomers enhanced by the micellization kinetics. This is opposite to the mechanism accepted for films without micelles, whose hydrodynamics is governed by the surface diffusion in the adsorbed layer.

Effect of the surface expansion and wettability of the capillary on the dynamic surface tension measured by the maximum bubble pressure method

The dynamic surface tension (DST) of sodium dodecyl sulfate solutions in the presence of sodium chloride is studied by the maximum bubble pressure method. The pressure oscillations are measured with a pressure transducer, while the change of the bubble area with time is determined by means of a video system. The role of the wettability of the capillary is studied by means of measurements with hydrophilic and hydrophobic capillaries. A strong effect of wettability of the capillary on the bubble growth and the DST is observed. The DST data are interpreted with a model for diffusion-controlled adsorption assuming different laws of bubble expansion. The real law of expansion is found to be important for correct interpretation in the case of the hydrophobic capillary. However, the surface expansion is not of primary importance for interpretation of the DST data obtained with the hydrophilic capillary. It is proved that the maximum pressure does not correspond to the hemispherical shape of the bubble in the presence of surfactant. Neglecting this effect does not lead to a significant error in the DST for bubbling periods smaller than several seconds.