Nanoscale Surface Heterogeneities Research Articles

Controlling the Collision Type and Frequency of Single Pt Nanoparticles at Chemically Modified Gold Electrodes.

Studying the electrochemical response of single nanoparticles at an electrode surface gives insight into the dynamic and stochastic processes that occur at the electrode interface. Herein, we investigated single platinum nanoparticle collision dynamics and type (elastic vs inelastic) at gold electrode surfaces modified with self-assembled monolayers (SAMs) of varying terminal chemistries. Collision events are measured via the faradaic current from catalytic reactions at the Pt surface. By changing the terminal, solution-facing group of a thiolate monolayer, we observed the effect of hydrophobicity at the solution-electrode interface on single-particle collisions by employing either a hydrophobic -CH3 terminal group (1-hexanethiol), a hydrophilic -OH terminal group (6-mercaptohexanol), or an equimolar mixture of the two. Changes in the terminal group lead to alterations in collision-induced current magnitude, collisional frequency, and the distinct shape of the collision event current transient. The effects of the terminal group of the SAM were probed by measuring quantitative differences in the events monitored through both the hydrogen evolution reaction (HER) and hydrazine oxidation. In both cases, a platinum nanoparticle (PtNP) favors adsorption to bare and hydrophilic surfaces but demonstrates elastic collision behavior when it collides with a hydrophobic surface. In the case of a mixed monolayer, distinct characteristics of hydrophobic and hydrophilic surfaces are observed. We report how single nanoparticle collisions can reveal nanoscale surface heterogeneity and can be used to manipulate the nature of single-particle interactions on an electrode surface by functionalized self-assembled monolayers.

Nonlinear Phase Imaging of Gold Nanoparticles Embedded in Organic Thin Films.

The phase detection in the dynamic mode of the atomic force microscopes is a known technique for mapping nanoscale surface heterogeneities. We present here an additional functionality of this technique, which allows high-resolution imaging of embedded inorganic nanoparticles with diameter and interparticle distances of a few nanometers. The method is based on a highly nonlinear tip-sample interaction occurring markedly above the nanoparticles, giving thus a high phase contrast between zones with and without nanoparticles. A relationship between the tip-sample interaction strength and the phase signal is established in experiments and from calculations conducted with the model developed by Haviland et al. [ Soft Matter 2016 , 12 , 619 ], which is based on solving a combined equation of motion for both the cantilever and surface while taking into account the time-varying interaction forces. The nonlinear phase behavior at the origin of the subnanometer spatial resolution is found by numerical analyses to be the result of a local mechanical stiffening of the zone containing nanoparticles, which is enhanced by 2 orders of magnitude or more.

New insights into the impact of nanoscale surface heterogeneity on the wettability of polymeric membranes

The impact of surface nanoscale physical heterogeneity on the wettability of polymeric membranes is still elusive. Conventional wettability analysis includes quantifying the membrane surface roughness using AFM followed by measuring the apparent equilibrium contact angle of DI water over the membrane surface. Here, we present a novel experimental approach, solely based on contact angle analysis, to elucidate the impact of surface heterogeneity on the wettability of dense polymeric membranes. The proposed approach involves evaluation of equilibrium and advancing contact angles of at least three non-polar liquids over the membrane surface. Using this information, the wettability parameters including the surface roughness-ratio, frictional pinning force, and the dispersive surface tension component of the polymeric membranes were successfully quantified. The comparison with the conventional approach showed that there are 10%–20% discrepancies between the AFM-based and contact angle-based wettability parameters. The results revealed that the AFM measurements strongly depend on the size of the scanned area, particularly for samples with large surface heterogeneity. Furthermore, the water flux decline due to colloidal fouling was found to be in good agreement with the results of our proposed model. This study can provide new insights into developing advanced membrane materials with desired surface wettability and antifouling property.

How nanoscale surface heterogeneity impacts transport of nano- to micro-particles on surfaces under unfavorable attachment conditions

The impact of nanoscale surface heterogeneity on retention of nano-to-micro-scale particles (colloids) on surfaces governs colloid transport in the environment where unfavorable conditions (repulsive barrier present) are prevalent.

Spatial Heterogeneities and Onset of Passivation Breakdown at Lithium Anode Interfaces

Effective passivation of lithium metal surfaces, and prevention of battery-shorting lithium dendrite growth, are critical for implementing lithium-metal-anodes for batteries with increased power densities. Nanoscale surface heterogeneities can be "hot spots" where anode passivation breaks down. Motivated by the observation of lithium dendrites in pores and grain boundaries in all-solid batteries, we examine lithium metal surfaces covered with Li(2)O and/or LiF thin films with grain boundaries in them. Electronic structure calculations show that, at >0.25 V computed equilibrium overpotential, L(2)O grain boundaries with sufficiently large pores can accommodate Li(0) atoms which aid electron leakage and passivation breakdown. Strain often accompanies Li-insertion, applying a ~1.7% strain already lowers the computed overpotential to 0.1 V. Lithium metal nanostructures as thin as 12 Angstroms are thermodynamically favored inside cracks in Li(2)O films, becoming "incipient lithium filaments." LiF films are more resistant to lithium metal growth. The models used herein should in turn inform passivating strategies in all-solid-state batteries.

Frictional behavior of micro-patterned silicon surface

A micro-patterned silicon surface, consisting of depressions with walls having a tilt angle of 30°, was created by photolithography followed by etching. The friction forces in single asperity contact acting between such a surface and an AFM tip was measured in air. This allowed elucidation of the validity of some common friction rules for this particular situation where a small tip traces a surface having roughness features that are significantly larger than the tip itself. The rules that was compared with our data were Amontons’ first rule of friction stating that the friction force should be proportional to the load; Amontons’ third rule stating that the friction force should be independent of sliding speed, and Euler’s rule providing a relation between slope angle and friction coefficient. We found that both nanoscale surface heterogeneities and the μm-sized depressions affect friction forces, and considerable reproducible variations were found along a particular scan line. Nevertheless Amontons’ first rule described average friction forces well. Amontons’ third rule and Euler’s rule were found to be less applicable to our system.

Impact of Nanoscale Surface Heterogeneity on Precursor Film Growth and Macroscopic Spreading of [Rmim][NTf2] Ionic Liquids on Mica

The connection between the interfacial properties of ionic liquids and their wetting behavior has been studied very little to date and not at all on heterogeneous surfaces. Therefore, we have investigated the static and dynamic wetting for a family of ionic liquids, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], on mica, where R represents an ethyl, butyl, or hexyl alkyl chain on the imidazolium ring. Spreading is impacted greatly by a precursor film that forms on both homogeneous and heterogeneous mica surfaces. Macroscopically, the initial viscous spreading of the ionic liquid droplet on bare mica occurs within seconds but is then followed by a very slow relaxation that can be closely correlated with the typical time-scales of the precursor film growth. The contact angle for [emim][NTf2] and [bmim][NTf2] relaxes from about 40° to 23° over 30 and 90 min, respectively. For [hmim][NTf2], the process takes approximately 24 h and approaches complete wetting. The thickness of the precursor films for [emim][NTf2], [bmim][NTf2], and [hmim][NTf2] were 0.53, 0.65, and 1.0 nm, respectively, according to atomic force microscopy (AFM). These values are consistent with a monolayer of ionic liquid cations on mica, rather than ion pairs. A monolayer of octadecylphosphonic acid (OPA) on mica prevents both the formation of a precursor film and the relaxation of the contact angle. However, only a partial surface coverage of ~60% OPA is required to have the same effect. Quenching of precursor film formation (and associated contact angle relaxation) is due to an increasingly connected network of OPA regions that closes the nanoscale paths of bare mica on which the precursor film can develop via surface diffusion.

Physical ageing of the contact line on colloidal particles at liquid interfaces

Young's law predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle. This has been used to explain why colloids often bind to liquid interfaces, and has been exploited in emulsification, water purification, mineral recovery, encapsulation and the making of nanostructured materials. However, little is known about the dynamics of binding. Here we show that the adsorption of polystyrene microspheres to a water/oil interface is characterized by a sudden breach and an unexpectedly slow relaxation. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role. Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface.

The impact of nanoscale chemical features on micron-scale adhesion: Crossover from heterogeneity-dominated to mean-field behavior

This work explores the impact of nanoscale surface heterogeneity, small relative to the effective contact area between two surfaces, on pairwise colloid-scale interactions. Polycation-based positive patches, of order 10 nm in diameter, arranged randomly and lying flat on otherwise negative substrates, were used to create surfaces whose competing attractive and repulsive features determined the net interactions with opposing surfaces. Lab experiments and simulations of the adhesion of gently flowing dilute negative microparticles varied particle size (0.5–2 μm), ionic strength ( κ −1 = 1–12 nm) and the density of heterogeneity on the collectors. Limiting behaviors from heterogeneity-controlled at high ionic strength to mean-field-like interactions at low ionic strength are reported. When heterogeneities are important, pairwise interactions are more attractive than predicted by average surface properties (e.g. per DLVO), and an adhesion threshold, describing the minimum average density of cationic features needed for single particle capture (adhesion), depends strongly on Debye length. In the opposite limit, the threshold becomes insensitive to the Debye length, and the average surface character approximates the interactions. An analytical treatment, reduced to a simple scaling argument predicts a −1/2 power-law dependence of the adhesion threshold on Debye length and particle size. A slightly stronger particle size dependence in experiments and simulations results from hydrodynamic contributions along with slight scaling differences in electrostatic, van der Waals, and hydrodynamic forces. An analogy to biological ligands is made for the heterogeneity-dominated limit: it is discovered, for this particular system, that engagement of as few as 20–100 cationic patches dictates particle adhesion (with details depending on flow, particle size, and ionic strength), similar to reports for selectin-mediated rolling of white blood cells during the inflammatory pathway. Also discovered is a heterogeneity-dependent crossover in the effect of ionic strength on particle capture, where added salt promotes particle adhesion in most cases but stabilizes the particles when the heterogeneity becomes relatively dense.

Evaporation and Sliding of Water Droplets on Fluoroalkylsilane Coatings with Nanoscale Roughness

Evaporation and sliding behaviors of water droplets were investigated on smooth and rough fluoroalkylsilane coatings. The rough coating possesses nanoscale roughness and chemical heterogeneity on its surface. Evaporation behaviors for these two coatings differed when nanoliter-scale droplets were used, although they were nearly identical for microliter-scale droplets. The droplets on the smooth coating exhibit greater sliding acceleration and a larger slipping velocity ratio than those on the rough coating. Both the evaporation behavior of nanoliter-scale droplets and sliding velocity of microliter-scale droplets were affected by nanoscale surface heterogeneity. They are indicators for highly homogeneous smooth silane coatings.

The Effect of Surface Ions on Water Adsorption to Mica

We have measured the adsorption isotherms of water on a single surface of freshly cleaved mica with K+ on the surface, and on mica where the K+ has been exchanged for H+. Using a very sensitive interferometric technique, we have found a significant difference between the two isotherms at submonolayer coverage, for relative vapor pressures p/p0 < 0.5. The K+-mica isotherm shows a pronounced convexity, suggesting distinct adsorption sites, whereas the H+-mica isotherm is flatter. The two isotherms converge above monolayer coverage. The results give a graphic demonstration of the importance of nanoscale surface heterogeneities for vapor adsorption at submonolayer coverage.

Hyperbranched Fluoropolymers and their Hybridization into Complex Amphiphilic Crosslinked Copolymer Networks

AbstractThis feature article highlights three types of hyperbranched fluoropolymers (HBFPs) with different structural features, which were synthesized by either polycondensation of fluorinated ABx monomers or self‐condensing vinyl (co)polymerization of fluorinated inimers and/or fluorinated comonomers. Amphiphilic crosslinked networks with hybridization of these hydrophobic HBFPs and linear hydrophilic poly(ethylene glycol)s are also discussed. As microphase‐segregated materials with nanoscale surface heterogeneities, these networks possessed unusual anti‐biofouling abilities, atypical sequestration and release behaviors for guest molecules, and special mechanical properties.magnified image

Observed and Simulated Fluid Drag Effects on Colloid Deposition in the Presence of an Energy Barrier in an Impinging Jet System

The deposition and re-entrainment behaviors of carboxylate-modified latex microspheres (0.2-, 0.5-, 1.0-, and 2.0-microm diameter) were examined in an impinging jet system on both glass and quartz substrata under a variety of environmentally relevant fluid velocities (2.12 x 10(-4) to 1.06 x 10(-3) m sec(-1)) in both the absence and the presence of an energy barrier to deposition. In the absence of an energy barrierto deposition, deposition fluxes onto glass and quartz substrata increased with increasing fluid velocity for all four microsphere sizes, in accordance with expectations from theory. In contrast, in the presence of an energy barrier to deposition, deposition efficiencies onto glass and quartz substrata decreased with increasing fluid velocity for all four microsphere sizes. Lack of re-entrainment and observed strong attachment were consistent with the expectation that deposition occurs via the primary energy minima where nanoscale surface heterogeneity locally reduces or eliminates the energy barrier to deposition. Colloid deposition onto an overall opposite-charged surface was simulated using a particle transport model with randomly distributed hetero domains that were like-charged relative to the colloid. Varying the size and number of the hetero domains showed that simulated colloid deposition efficiencies decreased with increasing fluid velocity when the hetero domains were small relative to the colloid. The simulations thereby demonstrate that observed decreases in colloid deposition efficiencies with increasing fluid velocity are consistent with the hypothesis that colloid deposition onto overall like-charged surfaces occurs at nanoscale hetero domains where repulsion is locally reduced or eliminated.

Nanoscale surface heterogeneities and glass durability

The surface of a high quality soda-lime silicate glass is found to be inhomogeneous, consisting of nanometer-high ridges that extend for tens of microns. Exposure to mild and aggressive environments results in an increase in the ridge heights as they dissolve much more slowly than the rest of the glass. The ridges also exhibit enrichment of sodium, magnesium, aluminum and oxygen. The parallel nature of the ridges suggests that they originate from compositional heterogeneity during the glass processing.