Advances in Colloid and Interface Science (v.170, #1-2)

Historical perspective by Reinhard Miller; Alidad Amirfazli (1).

Advances in top–down and bottom–up surface nanofabrication: Techniques, applications & future prospects by Abhijit Biswas; Ilker S. Bayer; Alexandru S. Biris; Tao Wang; Enkeleda Dervishi; Franz Faupel (2-27).
This review highlights the most significant advances of the nanofabrication techniques reported over the past decade with a particular focus on the approaches tailored towards the fabrication of functional nano-devices. The review is divided into two sections: top–down and bottom–up nanofabrication. Under the classification of top–down, special attention is given to technical reports that demonstrate multi-directional patterning capabilities less than or equal to 100 nm. These include recent advances in lithographic techniques, such as optical, electron beam, soft, nanoimprint, scanning probe, and block copolymer lithography. Bottom–up nanofabrication techniques—such as, atomic layer deposition, sol–gel nanofabrication, molecular self-assembly, vapor-phase deposition and DNA-scaffolding for nanoelectronics—are also discussed. Specifically, we describe advances in the fabrication of functional nanocomposites and graphene using chemical and physical vapor deposition. Our aim is to provide a comprehensive platform for prominent nanofabrication tools and techniques in order to facilitate the development of new or hybrid nanofabrication techniques leading to novel and efficient functional nanostructured devices.This review describes prominent top–down and bottom–up nanofabrication techniques for the fabrication of functional nano-devices.Display Omitted► The review presents the major top-down/bottom-up nanofabrication techniques. ► It contains the most up to date fabrication technologies. ► The fabrication of advanced functional nano-devices is described. ► Fabrication of complex nanocomposites and graphene structures is presented. ► This review facilitates the understanding of hybrid nanofabrication techniques.
Keywords: Nanofabrication; Lithography; Self assembly; Vapor phase deposition; Nanocomposites;

Silica–metal core–shell nanostructures by B.J. Jankiewicz; D. Jamiola; J. Choma; M. Jaroniec (28-47).
Silica–metal nanostructures consisting of silica cores and metal nanoshells attract a lot of attention because of their unique properties and potential applications ranging from catalysis and biosensing to optical devices and medicine. The important feature of these nanostructures is the possibility of controlling their properties by the variation of their geometry, shell morphology and shell material. This review is devoted to silica–noble metal core–shell nanostructures; specifically, it outlines the main methods used for the preparation and surface modification of silica particles and presents the major strategies for the formation of metal nanoshells on the modified silica particles. A special emphasis is given to the Stöber method, which is relatively simple, effective and well verified for the synthesis of large and highly uniform silica particles (with diameters from 100 nm to a few microns). Next, the surface chemistry of these particles is discussed with a special focus on the attachment of specific organic groups such as aminopropyl or mercaptopropyl groups, which interact strongly with metal species. Finally, the synthesis, characterization and application of various silica–metal core–shell nanostructures are reviewed, especially in relation to the siliceous cores with gold or silver nanoshells. Nowadays, gold is most often used metal for the formation of nanoshells due to its beneficial properties for many applications. However, other metals such as silver, platinum, palladium, nickel and copper were also used for fabrication of core–shell nanostructures. Silica–metal nanostructures can be prepared using various methods, for instance, (i) growth of metal nanoshells on the siliceous cores with deposited metal nanoparticles, (ii) reduction of metal species accompanied by precipitation of metal nanoparticles on the modified silica cores, and (iii) formation of metal nanoshells under ultrasonic conditions. A special emphasis is given to the seed-mediated growth, where metal nanoshells are formed on the modified silica cores with deposited metal nanoparticles. This strategy assures a good control of the nanoshell thickness as well as its surface properties.Display Omitted► A brief overview of silica–metal nanostructures is presented. ► Main methods for synthesis of silica particles are reviewed. ► Surface modification of silica particles is discussed. ► Synthesis and applications of silica–metal core–shell particles are reviewed. ► Special emphasis is placed on SiO2–Au and SiO2–Ag core–shell particles.
Keywords: Silica–metal core–shell particles; Synthesis of silica particles; Surface modification of silica; Silica–gold core––shell particles; Silica–silver core–shell particles; Stöber method; Applications of core–shell particles;

The Cassie equation: How it is meant to be used by A.J.B. Milne; A. Amirfazli (48-55).
A review of literature shows that the majority of papers cite a potentially incorrect form of the Cassie and Cassie–Baxter equations to interpret or predict contact angle data. We show that for surfaces wet with a composite interface, the commonly used form of the Cassie–Baxter equation, cos  θ c  =  f 1  cos  θ  − (1 −  f), is only correct for the case of flat topped pillar geometry without any penetration of the liquid. In general, the original form of the Cassie–Baxter equation, cos  θ c  =  f 1  cos  θ 1  −  f 2, with f 1  +  f 2  ≥ 1, should be used. The differences between the two equations are discussed and the errors involved in using the incorrect equation are estimated to be between ~ 3° and 13° for superhydrophobic surfaces. The discrepancies between the two equations are also discussed for the case of a liquid undergoing partial, but increasing, levels of penetration. Finally, a general equation is presented for the transition/stability criterion between the Cassie–Baxter and Wenzel modes of wetting.Display Omitted cos  θ c  =  f  cos  θ  − (1 −  f) is a simplified form of the Cassie–Baxter equation. cos  θ c  =  f 1  cos  θ 1  −  f 2 (with f 1  +  f 2  ≥ 1) is the correct, full, Cassie–Baxter equation. ► A majority of papers incorrectly cite the simplified Cassie–Baxter equation. ► Neglecting solid–liquid and liquid–vapor roughness under-predicts contact angle by 3–13°. ► New and universal formulation to predict Cassie mode stability is provided.
Keywords: Cassie; Cassie–Baxter; Contact angle; Contact area; Contact line; Rough surface;

The history of colloid science, from its modern foundations in the mid-nineteenth century, has been strongly concerned with studies of the aggregation of colloidal particles. It was Thomas Graham (1861) who defined the word “colloid” (from the Greek word for glue) for those materials which could not pass through membranes, unlike smaller, truly-dissolved materials. Subsequently, Graham (1864), following earlier studies, principally by Selmi and Faraday, described “the power possessed by salts for destroying colloidal solutions”. Although numerous, quantitative studies of particle aggregation were made in the years that followed, in particular, the determination of minimum electrolyte concentrations for the onset of particle aggregation and aggregation rates, no general theoretical framework emerged to account for these quantitative findings until the middle of the twentieth century. It was during and immediately following the Second World War that two sets of authors, Derjaguin and Landau, in Russia, and Verwey and Overbeek, in the Netherlands, independently came up with the theory that is now universally referred to as the DLVO theory of particle interactions and aggregation. All modern developments of the theory of particle aggregation use the DLVO theory as the keystone. However, the DLVO theory itself was based on a large body of experimental data in regard to particle aggregation obtained over the previous hundred years or so. This article is an attempt to review that body of experimental data and to show how this guided the DLVO authors in their thinking.
Keywords: Particle aggregation; Critical electrolyte concentration; Aggregation rate; Colloid stability; DLVO theory;

A sessile drop is an isolated drop which has been deposited on a solid substrate where the wetted area is limited by a contact line and characterized by contact angle, contact radius and drop height. Diffusion-controlled evaporation of a sessile drop in an ambient gas is an important topic of interest because it plays a crucial role in many scientific applications such as controlling the deposition of particles on solid surfaces, in ink-jet printing, spraying of pesticides, micro/nano material fabrication, thin film coatings, biochemical assays, drop wise cooling, deposition of DNA/RNA micro-arrays, and manufacture of novel optical and electronic materials in the last decades. This paper presents a review of the published articles for a period of approximately 120 years related to the evaporation of both sessile drops and nearly spherical droplets suspended from thin fibers. After presenting a brief history of the subject, we discuss the basic theory comprising evaporation of micrometer and millimeter sized spherical drops, self cooling on the drop surface and evaporation rate of sessile drops on solids. The effects of drop cooling, resultant lateral evaporative flux and Marangoni flows on evaporation rate are also discussed. This review also has some special topics such as drop evaporation on superhydrophobic surfaces, determination of the receding contact angle from drop evaporation, substrate thermal conductivity effect on drop evaporation and the rate evaporation of water in liquid marbles.Display Omitted► A review of the articles related to the evaporation of sessile drops is presented. ► Basic theory on the evaporation of sessile drops and self cooling on the drop surface. ► Determination of the receding contact angle from drop evaporation. ► Drop evaporation on superhydrophobic surfaces. ► Evaporation of water in liquid marbles.
Keywords: Drop evaporation; Contact angle; Diffusion; Superhydrophobic; Liquid marbles;