27 research outputs found
Coerced Mechanical Coarsening of Nanoparticle Assemblies
Coarsening is a ubiquitous phenomenon [1-3] that underpins countless processes in nature, including epitaxial growth [1,3,4], the phase separation of alloys, polymers and binary fluids [2], the growth of bubbles in foams5, and pattern formation in biomembranes6. Here we show, in the first real-time experimental study of the evolution of an adsorbed colloidal nanoparticle array, that tapping-mode atomic force microscopy (TM-AFM) can drive the coarsening of Au nanoparticle assemblies on silicon surfaces. Although the growth exponent has a strong dependence on the initial sample morphology, our observations are largely consistent with modified Ostwald ripening processes [7-9]. To date, ripening processes have been exclusively considered to be thermally activated, but we show that nanoparticle assemblies can be mechanically coerced towards equilibrium, representing a new approach to directed coarsening. This strategy enables precise control over the evolution of micro- and nanostructures
Controlling Pattern Formation in Nanoparticle Assemblies via Directed Solvent Dewetting
We have achieved highly localised control of pattern formation in two dimensional nanoparticle assemblies by direct modification of solvent dewetting dynamics. A striking dependence of nanoparticle organisation on the size of atomic force microscope-generated surface heterogeneities is observed and reproduced in numerical simulations. Nanoscale features induce rupture of the solvent-nanoparticle film, causing the local flow of solvent to carry nanoparticles into confinement. Microscale heterogeneities instead slow the evaporation of the solvent, producing a remarkably abrupt interface
between different nanoparticle patterns
Controlling pattern formation in nanoparticle assemblies via directed solvent dewetting.
We have achieved highly localized control of pattern formation in two-dimensional nanoparticle assemblies by direct modification of solvent dewetting dynamics. A striking dependence of nanoparticle organization on the size of atomic force microscope-generated surface heterogeneities is observed and reproduced in numerical simulations. Nanoscale features induce a rupture of the solvent-nanoparticle film, causing the local flow of solvent to carry nanoparticles into confinement. Microscale heterogeneities instead slow the evaporation of the solvent, producing a remarkably abrupt interface between different nanoparticle patterns
Fingering Instabilities in Dewetting Nanofluids
The growth of fingering patterns in dewetting nanofluids (colloidal solutions of thiol-passivated gold nanoparticles) has been followed in real time using contrast-enhanced video microscopy. The fingering instability on which we focus here arises from evaporatively-driven nucleation and growth
a nanoscopically thin "precursor" solvent film behind the macroscopic contact line. We find that well-developed isotropic fingering structures only form for a narrow range of experimental parameters. Numerical simulations, based on a modification of the Monte Carlo approach introduced by Rabani et al. [Nature 426, 271 (2003)], reproduce the patterns we observe experimentally
Front instabilities in evaporatively dewetting nanofluids
Various experimental settings that involve drying solutions or suspensions of
nanoparticles -- often called nanofluids -- have recently been used to produce
structured nanoparticle layers. In addition to the formation of polygonal
networks and spinodal-like patterns, the occurrence of branched structures has
been reported. After reviewing the experimental results we use a modified
version of the Monte Carlo model first introduced by Rabani et al. [Nature 426,
271 (2003)] to study structure formation in evaporating films of nanoparticle
solutions for the case that all structuring is driven by the interplay of
evaporating solvent and diffusing nanoparticles.
After introducing the model and its general behavior we focus on receding
dewetting fronts which are initially straight but develop a transverse
fingering instability. We analyze the dependence of the characteristics of the
resulting branching patterns on the driving chemical potential, the mobility
and concentration of the nanoparticles, and the interaction strength between
liquid and nanoparticles. This allows us to understand the underlying
instability mechanism.Comment: 35 pages, 28 figure
Modelling approaches to the dewetting of evaporating thin films of nanoparticle suspensions
We review recent experiments on dewetting thin films of evaporating colloidal nanoparticle
suspensions (nanofluids) and discuss several theoretical approaches to describe the ongoing
processes including coupled transport and phase changes. These approaches range from
microscopic discrete stochastic theories to mesoscopic continuous deterministic descriptions. In
particular, we describe (i) a microscopic kinetic Monte Carlo model, (ii) a dynamical density
functional theory and (iii) a hydrodynamic thin film model.
Models (i) and (ii) are employed to discuss the formation of polygonal networks, spinodal
and branched structures resulting from the dewetting of an ultrathin ‘postcursor film’ that
remains behind a mesoscopic dewetting front. We highlight, in particular, the presence of a
transverse instability in the evaporative dewetting front, which results in highly branched
fingering structures. The subtle interplay of decomposition in the film and contact line motion is
discussed.
Finally, we discuss a simple thin film model (iii) of the hydrodynamics on the mesoscale.
We employ coupled evolution equations for the film thickness profile and mean particle
concentration. The model is used to discuss the self-pinning and depinning of a contact line
related to the ‘coffee-stain’ effect.
In the course of the review we discuss the advantages and limitations of the different
theories, as well as possible future developments and extensions
Fingering Instabilities in Dewetting Nanofluids
The growth of fingering patterns in dewetting nanofluids (colloidal solutions of thiol-passivated gold nanoparticles) has been followed in real time using contrast-enhanced video microscopy. The fingering instability on which we focus here arises from evaporatively-driven nucleation and growth
a nanoscopically thin "precursor" solvent film behind the macroscopic contact line. We find that well-developed isotropic fingering structures only form for a narrow range of experimental parameters. Numerical simulations, based on a modification of the Monte Carlo approach introduced by Rabani et al. [Nature 426, 271 (2003)], reproduce the patterns we observe experimentally