Mihail N. Popescu

Topographical pathways guide chemical microswimmers

Achieving control over the directionality of active colloids is essential for their use in practical applications such as cargo carriers in microfluidic devices. So far, guidance of spherical Janus colloids was mainly realized using specially engineered magnetic multilayer coatings combined with external magnetic fields. Here we demonstrate that step-like submicrometre topographical features can be used as reliable docking and guiding platforms for chemically active spherical Janus colloids. For various topographic features (stripes, squares or circular posts), docking of the colloid at the feature edge is robust and reliable. Furthermore, the colloids move along the edges for significantly long times, which systematically increase with fuel concentration. The observed phenomenology is qualitatively captured by a simple continuum model of self-diffusiophoresis near confining boundaries, indicating that the chemical activity and associated hydrodynamic interactions with the nearby topography are the main physical ingredients behind the observed behaviour.

Reversed Janus Micro/Nanomotors with Internal Chemical Engine

Self-motile Janus colloids are important for enabling a wide variety of microtechnology applications as well as for improving our understanding of the mechanisms of motion of artificial micro- and nanoswimmers. We present here micro/nanomotors which possess a reversed Janus structure of an internal catalytic “chemical engine”. The catalytic material (here platinum (Pt)) is embedded within the interior of the mesoporous silica (mSiO2)-based hollow particles and triggers the decomposition of H2O2 when suspended in an aqueous peroxide (H2O2) solution. The pores/gaps at the noncatalytic (Pt) hemisphere allow the exchange of chemical species in solution between the exterior and the interior of the particle. By varying the diameter of the particles, we observed size-dependent motile behavior in the form of enhanced diffusion for 500 nm particles, and self-phoretic motion, toward the nonmetallic part, for 1.5 and 3 μm ones. The direction of motion was rationalized by a theoretical model based on self-phoresis. For the 3 μm particles, a change in the morphology of the porous part is observed, which is accompanied by a change in the mechanism of propulsion via bubble nucleation and ejection as well as a change in the direction of motion.

Effective Interaction between Active Colloids and Fluid Interfaces Induced by Marangoni Flows

We show theoretically that near a fluid-fluid interface a single active colloidal particle generating, e.g., chemicals or a temperature gradient experiences an effective force of hydrodynamic origin. This force is due to the fluid flow driven by Marangoni stresses induced by the activity of the particle; it decays very slowly with the distance from the interface, and can be attractive or repulsive depending on how the activity modifies the surface tension. We show that, for typical systems, this interaction can dominate the dynamics of the particle as compared to Brownian motion, dispersion forces, or self-phoretic effects. In the attractive case, the interaction promotes the self-assembly of particles into a crystal-like monolayer at the interface.

Kinetics of submonolayer epitaxial growth

Abstract Molecular beam epitaxy is an important method for growing thin-films and nanostructures. One of the scientific challenges is to understand the fundamental processes that control the evolution of thin film structure and morphology. The results of kinetic Monte Carlo simulations carried out to study the dependence of the submonolayer scaled island-size distribution on the critical island-size are presented and compared with experiments. A recently developed method which involves a self-consistent coupling of evolution equations for the capture-zone distributions with rate-equations for the island-size distribution is also described. Our method explicitly takes into account the existence of a denuded (“capture”) zone around every island and the correlations between the size of an island and the corresponding average capture zone, and has been used to develop a quantitative rate-equation approach to irreversible submonolayer growth on a two-dimensional substrate. The resulting predictions for the capture-zones, capture numbers, and island-size distributions are in excellent agreement with experimental results and kinetic Monte Carlo simulations.

Self-consistent rate-equation approach to irreversible submonolayer growth in one dimension

Abstract A self-consistent rate-equation (RE) approach to irreversible submonolayer growth in one dimension is presented. Our approach is based on a set of dynamical equations for the evolution of gaps between islands which is coupled to the island-density REs via local capture numbers and explicitly takes into account correlations between the size of an island and the corresponding capture zone. In the most simple formulation, fragmentation of capture zones is not directly included, but accounted for through a uniform rescaling, while nucleation is assumed to generate only gaps with average length. Using this approach, we have been able to accurately predict the scaled island-size, capture-number, and average-gap-size distributions in the pre-coalescence regime. Our approach also leads to a novel analytical expression for the monomer capture number σ 1 =(4/ RN 1 γ ) 1/2 where N 1 is the monomer density, γ is the fraction of the substrate covered by islands, and R is the ratio D / F of the diffusion rate to deposition flux which agrees with simulations over the entire pre-coalescence regime, and implies a novel scaling behavior for the island density at low coverage, in contrast to earlier predictions. Comparisons between our RE results and kinetic Monte Carlo simulations are presented for both point islands and extended islands.

Self-consistent rate equation theory of cluster size distribution in aggregation phenomena

Cluster nucleation and growth by aggregation is the central feature of many physical processes, from polymerization and gelation in polymer science, flocculation and coagulation in aerosol and colloidal chemistry, percolation and coarsening in phase transitions and critical phenomena, agglutination and cell adhesion in biology, to island nucleation and thin-film growth in materials science. Detailed information about the kinetics of aggregation is provided by the time dependent cluster size-distribution, a quantity which can be measured experimentally. While the standard Smoluchowski rate-equation approach has been in general successful in predicting average quantities like the total cluster density, it fails to account for spatial fluctuations and correlations and thus predicts size distributions that are in significant disagreement with both experiments and kinetic Monte Carlo simulations. In this work we outline a new method which takes into account such correlations. We show that by coupling a set of evolution equations for the capture-zone distributions with a set of rate-equations for the island densities one may obtain accurate predictions for the time- and size-dependent rates of monomer capture. In particular, by using this method we obtain excellent results for the capture numbers and island-size distributions in irreversible growth on both one- and two-dimensional substrates.

Self-Consistent Rate-Equation Approach to Nucleation and Growth in Point/Extended Island Models of 1-D Homoepitaxy

A self-consistent rate equation (RE) approach to submonolayer growth on a one-dimensional surface is presented. This approach explicitly takes into account the existence of gaps between clusters and can successfully predict the coverage dependence of the average densities of monomers N 1 , and clusters, N . It also implies an unusual dependence for the monomer-monomer capture number σ 1 as a function of monomer density. To obtain the island size-distribution, a second set of mean-field equations is used describing the evolution of the size-dependent capture zones and leading to explicit size- and coverage-dependent capture numbers. The solution of this fully self-consistent RE approach is then compared with kinetic Monte Carlo results

Capture-Numbers and Island Size-Distributions in Irreversible Homoepitaxial Growth: A Rate Equation Approach

A fully self-consistent rate-equation approach to irreversible submonolayer growth is presented. This approach explicitly takes into account the correlation between the size of an island and the corresponding average capture zone. It is shown that this leads to capture numbers which depend explicitly on the island-size, and excellent agreement with experimental and Monte Carlo results is found for this size-dependence. Consequently, the predictions for the island-size distributions are in very good agreement with Monte Carlo simulation results over the whole range of coverages in the pre-coalescence regime.