The factors affecting the nucleation and growth of gold nanorods, (Jana et al., Adv. Mater. 2001, 13, 1389) have been investigated. It is shown that the size and aspect ratio can be controlled through the use of different sized seed particles. The length of the rods can be tuned from 25–170 nm, while the width remains almost constant at 22–25 nm. The formation of rods requires the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Lower temperature favors rod formation, although this reduces CTAB solubility. The addition of chloride ions or the use of dodecyltrimethylammonium bromide (DTAB) leads to shorter‐aspect rods. AuIII and AuI are shown to be quantitatively bound to the CTAB micelles. We propose an electrochemical mechanism for rod formation, whereby the flux of AuI bound to cationic micelles to the seed surface is maximized at points of highest curvature, where the electrical double layer gradient is highest. Initial numerical solutions to the electric potential and field around an ellipsoid in a 1:1 electrolyte are provided, which indicate that the field at the particle tip scales linearly with the aspect ratio. Mean free passage times for ions are found to be shortest at the tips. The results provide a general explanation for the formation of non‐equilibrium crystal habits and a mechanism for controlling crystal growth.

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In 1756, Leidenfrost observed that water drops skittered on a sufficiently hot skillet, owing to levitation by an evaporative vapour film. Such films are stable only when the hot surface is above a critical temperature, and are a central phenomenon in boiling. In this so-called Leidenfrost regime, the low thermal conductivity of the vapour layer inhibits heat transfer between the hot surface and the liquid. When the temperature of the cooling surface drops below the critical temperature, the vapour film collapses and the system enters a nucleate-boiling regime, which can result in vapour explosions that are particularly detrimental in certain contexts, such as in nuclear power plants. The presence of these vapour films can also reduce liquid-solid drag. Here we show how vapour film collapse can be completely suppressed at textured superhydrophobic surfaces. At a smooth hydrophobic surface, the vapour film still collapses on cooling, albeit at a reduced critical temperature, and the system switches explosively to nucleate boiling. In contrast, at textured, superhydrophobic surfaces, the vapour layer gradually relaxes until the surface is completely cooled, without exhibiting a nucleate-boiling phase. This result demonstrates that topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling--by heat transfer--the liquid-gas phase transition at hot surfaces. This concept can potentially be applied to control other phase transitions, such as ice or frost formation, and to the design of low-drag surfaces at which the vapour phase is stabilized in the grooves of textures without heating.

The understanding of static interactions in colloidal suspensions is well established, whereas dynamic interactions more relevant to biological and other suspended soft-matter systems are less well understood. We present the direct force measurement and quantitative theoretical description for dynamic forces for liquid droplets in another immiscible fluid. Analysis of this system demonstrates the strong link between interfacial deformation, static surface forces, and hydrodynamic drainage, which govern dynamic droplet-droplet interactions over the length scale of nanometers and over the time scales of Brownian collisions. The results and analysis have direct bearing on the control and manipulation of suspended droplets in soft-matter systems ranging from the emulsions in shampoo to cellular interactions.

Interfacial nanobubbles (INBs) on a solid surface in contact with water have drawn widespread research interest. Although several theoretical models have been proposed to explain their apparent long lifetimes, the underlying mechanism still remains in dispute. In this work, the morphological evolution of INBs was examined in air-equilibrated and partially degassed water with the use of atomic force microscopy (AFM). Our results show that (1) INBs shrank in the partially degassed water while they grew slightly in the air-equilibrated water, (2) the three-phase boundary of the INBs was pinned during the morphological evolution of the INBs. Our analyses show that (1) the lifetime of INBs was sensitive to the saturation level of dissolved gases in the surrounding water, especially when the concentration of dissolved gases was close to saturation, and (2) the pinning of the three-phase boundary could significantly slow down the kinetics of both the growth and the shrinkage of the INBs. We developed a one-dimensional version of the Epstein-Plesset model of gas diffusion to account for the effect of pinning.

A combination of atomic force microscopy (AFM) and reflection interference contrast microscopy (RICM) was used to measure simultaneously the interaction force and the spatiotemporal evolution of the thin water film between a bubble in water and mica surfaces with varying degrees of hydrophobicity. Stable films, supported by the repulsive van der WaalsÀCasimirÀLifshitz force were always observed between air bubble and hydrophilic mica surfaces (water contact angle, θ w < 5°) whereas bubble attachment occurred on hydrophobized mica surfaces. A theoretical model, based on the Reynolds lubrication theory and the augmented YoungÀLaplace equation including the effects of disjoining pressure, provided excellent agreement with experiment results, indicating the essential physics involved in the interaction between air bubble and solid surfaces can be elucidated. A hydrophobic interaction free energy per unit area of the form: W H (h) = Àγ(1 À cos θ w )exp(Àh/D H ) can be used to quantify the attraction between bubble and hydrophobized solid substrate at separation, h, with γ being the surface tension of water. For surfaces with water contact angle in the range 45°< θ w < 90°, the decay length D H varied between 0.8 and 1.0 nm. This study quantified the hydrophobic interaction in asymmetric system between air bubble and hydrophobic surfaces, and provided a feasible method for synchronous measurements of the interaction forces with sub-nN resolution and the drainage dynamics of thin films down to nm thickness. KEYWORDS: bubble . hydrodynamic force . hydrophobic interaction . AFM . mica . thin film drainage ARTICLE SHI ET AL.