Ivan U. Vakarelski

Stabilization of Leidenfrost vapour layer by textured superhydrophobic surfaces

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.

Dynamic interactions between microbubbles in water

The interaction between moving bubbles, vapor voids in liquid, can arguably represent the simplest dynamical system in continuum mechanics as only a liquid and its vapor phase are involved. Surprisingly, and perhaps because of the ephemeral nature of bubbles, there has been no direct measurement of the time-dependent force between colliding bubbles which probes the effects of surface deformations and hydrodynamic flow on length scales down to nanometers. Using ultrasonically generated microbubbles (∼100 μm size) that have been accurately positioned in an atomic force microscope, we have made direct measurements of the force between two bubbles in water under controlled collision conditions that are similar to Brownian particles in solution. The experimental results together with detailed modeling reveal the nature of hydrodynamic boundary conditions at the air/water interface, the importance of the coupling of hydrodynamic flow, attractive van der Waals–Lifshitz forces, and bubble deformation in determining the conditions and mechanisms that lead to bubble coalescence. The observed behavior differs from intuitions gained from previous studies conducted using rigid particles. These direct force measurements reveal no specific ion effects at high ionic strengths or any special role of thermal fluctuations in film thickness in triggering the onset of bubble coalescence.

Dynamic forces between bubbles and surfaces and hydrodynamic boundary conditions.

A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.

Role of interaction forces in controlling the stability and polishing performance of CMP slurries.

Chemical mechanical polishing (CMP) is an essential step in metal and dielectric planarization in multilayer microelectronic device fabrication. In the CMP process it is necessary to minimize the extent of surface defect formation while maintaining good planarity and optimal material removal rates. These requirements are met through the control of chemical and mechanical interactions during the polishing process by engineering the slurry chemistry, particulate properties, and stability. In this study, the performance of surfactant-stabilized silica CMP slurries at high pH and high ionic strengths are investigated with particular emphasis on the particle-particle and particle-substrate interactions. It is shown that for the design of consistently high performing slurries, stability of abrasive particles must be achieved under the dynamic processing conditions of CMP while maintaining sufficient pad-particle-wafer interactions.

Bubble colloidal AFM probes formed from ultrasonically generated bubbles.

Here we introduce a simple and effective experimental approach to measuring the interaction forces between two small bubbles (approximately 80-140 microm) in aqueous solution during controlled collisions on the scale of micrometers to nanometers. The colloidal probe technique using atomic force microscopy (AFM) was extended to measure interaction forces between a cantilever-attached bubble and surface-attached bubbles of various sizes. By using an ultrasonic source, we generated numerous small bubbles on a mildly hydrophobic surface of a glass slide. A single bubble picked up with a strongly hydrophobized V-shaped cantilever was used as the colloidal probe. Sample force measurements were used to evaluate the pure water bubble cleanliness and the general consistency of the measurements.

Dynamic air layer on textured superhydrophobic surfaces.

We provide an experimental demonstration that a novel macroscopic, dynamic continuous air layer or plastron can be sustained indefinitely on textured superhydrophobic surfaces in air-supersaturated water by a natural gas influx mechanism. This type of plastron is an intermediate state between Leidenfrost vapor layers on superheated surfaces and the equilibrium Cassie-Baxter wetting state on textured superhydrophobic surfaces. We show that such a plastron can be sustained on the surface of a centimeter-sized superhydrophobic sphere immersed in heated water and variations of its dynamic behavior with air saturation of the water can be regulated by rapid changes of the water temperature. The simple experimental setup allows for quantification of the air flux into the plastron and identification of the air transport model of the plastron growth. Both the observed growth dynamics of such plastrons and millimeter-sized air bubbles seeded on the hydrophilic surface under identical air-supersaturated solution conditions are consistent with the predictions of a well-mixed gas transport model.

Particulate Templates and Ordered Liquid Bridge Networks in Evaporative Lithography

We investigate the properties of latex particle templates required to optimize the development of ordered liquid bridge networks in evaporative lithography. These networks are key precursors in the assembly of solutions of conducting nanoparticles into large, optically transparent, and conducting microwire networks on substrates (Vakarelski, I. U.; Chan, D. Y. C.; Nonoguchi, T.; Shinto, H.; Higashitani, K. Phys. Rev. Lett., 2009, 102, 058303). An appropriate combination of heat treatment and oxygen plasma etching of a close-packed latex particle monolayer is shown to create open-spaced particle templates which facilitates the formation of ordered fully connected liquid bridge networks that are critical to the formation of ordered microwire networks. Similar results can also be achieved if non-close-packed latex particle templates with square or honeycomb geometries are used. The present results have important implications for the development of the particulate templates to control the morphology of functional microwire networks by evaporative lithography.

Strategies for Optimal Chemical Mechanical Polishing (CMP) Slurry Design

Abstract Chemical mechanical polishing (CMP) has become the preferred route for achieving wafer‐level global planarization in microelectronics device manufacturing. However, the micro‐ to molecular‐level mechanisms that control its performance and optimization are not well understood. In CMP, complex slurry chemistries react with the first few atomic layers on the wafer surfaces forming a chemically modified film. This film is subsequently mechanically abraded by nanosized slurry particles to achieve local and global planarity for multi‐level metalization. For optimal CMP performance, high material removal rates with minimal surface defectivity are required. This can be achieved by controlling the extent of interparticle and particle–substrate interactions, which are facilitated through the manipulation of the slurry composition, solution chemistry, as well as operational parameters. Interparticle interactions must be engineered to maintain slurry stability to minimize the number and extent of surface defects during polishing while maintaining adequate removal rates. The fundamental considerations, which are necessary for the development of high performance CMP slurries, are discussed in this article through model silica CMP systems.

Foam-Film-Stabilized Liquid Bridge Networks in Evaporative Lithography and Wet Granular Matter

Evaporative lithography using latex particle templates is a novel approach for the self-assembly of suspension-dispersed nanoparticles into ordered microwire networks. The phenomenon that drives the self-assembly process is the propagation of a network of interconnected liquid bridges between the template particles and the underlying substrate. With the aid of video microscopy, we demonstrate that these liquid bridges are in fact the border zone between the underlying substrate and foam films vertical to the substrate, which are formed during the evaporation of the liquid from the suspension. The stability of the foam films and thus the liquid bridge network stability are due to the presence of a small amount of surfactant in the evaporating solution. We show that the same type of foam-film-stabilized liquid bridge network can also propagate in 3D clusters of spherical particles, which has important implications for the understanding of wet granular matter.

Photoresist Templates for Wafer‐Scale Defect‐Free Evaporative Lithography

DOI: 10.1002/adma.201002644 Template guided evaporative self-assembly of nanoparticulates that are initially dispersed in suspension could provide a fl exible and energy effi cient method for large scale patterning, including that of ordered microwire networks. [ 1–5 ] A possible application for such networks is to create transparent-conducting coatings of metallic microwires that could be used as an alternative to the industry standard indium tin oxide (ITO). [ 1 , 6–10 ] In this communication we introduce an evaporative lithography method that extends previous approaches by using a photolithography defi ned photoresist as the template, thus enabling the production of wafer-scale, defect-free functional microwire networks in a variety of well controlled topologies ( Figure 1 ). The present approach, using photoresist templates, is based upon the phenomena of stable liquid bridge formation which was recently observed in the evaporative lithography method initiated by Vakarelski et al. [ 1 , 2 ] This method utilized as a template two dimensional arrays of latex particles (50 to 100 μ m diameter) deposited on a fl at glass substrate. The template is then covered with a gold nanoparticle (Au NP) suspension. During the evaporation of the solvent from the suspension, a connected liquid bridge network is fi rst formed between the latex particles and the substrate. Further evaporation of the solvent then leaves a network of microwires (composed of the Au NP) on the substrate. The presence of small amounts of surfactant in the suspension is crucial for the stability and formation of the initial liquid bridges. The use of latex particles is a fl exible and easy-to-apply approach to create a template for research purposes, but could prove tedious and diffi cult to control on a manufacturing scale. [ 1 , 2 , 11 ] Even after optimization of the template preparation it is diffi cult to produce templates with no defects over a large area. [ 2 ] As demonstrated below, we can overcome this drawback using photolithography to produce defect-free photoresist templates over wafer-scale areas. Moreover, unlike latex particle templates, we are not restricted to a hexagonal network and can be fl exible in the topology of the template pattern. The basic unit of the latex particle template is two contacting latex particles and after the evaporation lithography process is complete, there remains a microwire connecting the particles over the substrate ( Figure 2 a ). [ 1 , 2 ] It is diffi cult to produce similar spherical shape structures using photolithography. The simplest alternative for photolithography is to create cylindrical pillars arranged in a similar geometry as the latex particle spheres. However, tests conducted using such pillar templates indicated that stable liquid bridge formation (and hence microwire creation) does not occur between two pillars. In all pillar geometries investigated (10 μ m diameter and 30 μ m high pillars spaced by 10 to 30 μ m) the receding meniscus around two pillars begins breaking before the formation of a stable liquid bridge. It was found the pillars must be connected with a “roof” or “arch” shape structure, to achieve a workable confi guration as shown in Figure 2b . A second requirement is that the pillarroof structures must be interconnected in a symmetric openspaced pattern, [ 2 ] as shown for example in Figure 2g or by the network patterns of Figure 1 . Given the above conditions, stable liquid bridge/microwire formation can occur, similar to the case using latex particles, for pillar-roof type templates. To produce the pillar-roof “arch” structures we use a double exposure UV-photolithography process schematized in Figure 2c to 2f . The photoresist used is SU-8, a negative, epoxy-type, photoresist appropriate for the production of thick, high aspect ratio structures as required for our templates. [ 12 ] A 20 to 30 μ m thick SU-8 layer is spin-coated on a glass wafer substrate. A fi rst UV exposure is done through the pillar pattern mask using a long exposure time to allow SU-8 to crosslink through the entire layer thickness (Figure 2c ). A second UV exposure is subsequently done through the roof pattern mask using a short exposure time allowing SU-8 crosslinking only on the top part of the layer (Figure 2d ). This combination of two exposures creates a pillar-roof or arch structure following the baking (to crosslink) and the development of the SU-8 layer (Figure 2e,f , see Experimental section for details). Figure 2g shows a top view microscopic image of a SU-8 rectangular roof structure with pillars beneath, Figure 2h shows a side view image of the arch structure, and Figure 2i shows a scanning electron microscopy (SEM) image of the template array. At present, exposure is done over 4 inches glass wafers on which 4 rows by 5 columns of 1cm 2 square SU-8 patterns are created. The SU-8 squares are connected along each edge with pre-deposited gold busbars to facilitate the measurement of the conductivity of the microwire coating. We used three different pattern topologies: rectangular (Figure 1a , 2g ); honeycomb (Figure 1b ); fully interconnected hexagonal (Figure 1c ). To demonstrate the SU-8 template lithography process we use a Au NP suspension of 1 to 3 wt% Au NP content. The Au NP (10 to 20 nm diameter) are stabilized with poly (vinyl pyrrolidone) (PVP) polymer and a small amount of 0.1 mM sodium dodecyl sulfate (SDS) surfactant added to the suspension to ensure stable liquid bridge formation (see Experimental). [ 1 , 2 ]