Smooth Silicon Wafers Research Articles

One step synthesis of durable slippery, oil/water selective, corrosion-resistant silicone coatings with grafted flexible sidechains

Slippery liquid-infused porous surfaces (SLIPSs) based on polymer matrices impregnated with liquid lubricants are multifunctional and provide a wide range of useful properties such as low adhesion of water and other liquids, selectivity, high corrosion resistance, etc. However, these properties vanish quickly due to the gradual depletion of lubricant from the matrix. This issue could be resolved by attaching lubricant molecules to the matrix as flexible, liquid-like sidechains. In the present work, thin silicone films with grafted sidechains are fabricated. The uniform coatings are obtained in “eco-friendly” pressurized (subcritical) CO2 on smooth silicon wafers as well as on rough porous substrates such as carbon aerogels. The possibility of applying films by the traditional dip-coating method is also demonstrated on titanium slides. The coatings demonstrate low sliding angles for water and water/alcohol solution droplets. Coated aerogels preserve high porosity. It is demonstrated that initially hydrophilic aerogels become highly hydrophobic after coating and selectively absorb oil from water/oil mixtures. The tests of obtained coatings on titanium substrates demonstrate a significant increase in corrosion resistance. The proposed technique has a wide range of potential applications where such slippery coatings should be applied on complex rough substrates: from filtration materials production to textile finishing.

The nucleation, growth, and adhesion of water bridges in sliding nano-contacts.

We provide experimental observations of the nucleation and growth of water capillary bridges in nanometer gaps between a laterally moving atomic force microscope probe and a smooth silicon wafer. We find rising nucleation rates with increasing lateral velocity and a smaller separation gap. The interplay between nucleation rate and lateral velocity is attributed to the entrainment of water molecules into the gap by the combination of lateral motion and collisions of the water molecules with the surfaces of the interface. The capillary volume of the full-grown water bridge increases with the distance between the two surfaces and can be limited by lateral shearing at high velocities. Our experimental results demonstrate a novel method to study in situ how water diffusion and transport impact dynamic interfaces at the nanoscale, ultimately leading to friction and adhesion forces at the macroscale.

Exploring the water capture efficiency of covalently attached liquid-like surfaces.

The capture of moisture from the atmosphere through condensation has the potential to provide a sustainable source of water. Here, we investigate the condensation of humid air at low subcooling condition (11 °C), similar to conditions for natural dew capture, and explore how water contact angle and contact angle hysteresis affect the rates of water capture. We compare water collection on three families of surfaces: (i) hydrophilic (polyethylene oxide, MPEO) and hydrophobic (polydimethylsiloxane, PDMS) molecularly thin coatings grafted on smooth silicon wafers, which produce slippery covalently attached liquid surfaces (SCALSs), with low contact angle hysteresis (CAH = 6°); (ii) the same coatings grafted on rougher glass, with high CAH (20°-25°); (iii) hydrophilic polymer surfaces [poly(N-vinylpyrrolidone), PNVP] with high CAH (30°). Upon exposure to water, the MPEO SCALS swell, which likely further increases their droplet shedding ability. MPEO and PDMS coatings collect similar volume of water (around 5 l m-2day-1), both when they are SCALS and non-slippery. Both MPEO and PDMS layers collect about 20% more water than PNVP surfaces. We present a basic model showing that, under low heat flux conditions, on all MPEO and PDMS layers, the droplets are so small (600-2000 µm) that there is no/low heat conduction resistance across the droplets, irrespective of the exact value of contact angle and CAH. As the time to first droplet departure is much faster on MPEO SCALS (28min) than on PDMS SCALS (90min), slippery hydrophilic surfaces are preferable in dew collection applications where the collection time frame is limited.

Bubble dynamics and pressure oscillation in highly subcooled water jet array impingement boiling under periodical heat flux

Jet array impingement boiling is promising to be used in the applications of large-area high-heat-flux cooling. In this study, the bubble dynamics and pressure oscillations of a one-dimensional confined jet array impingement boiling of highly subcooled water on smooth silicon wafer surface under periodical heat fluxes are investigated experimentally. It is found nucleate boiling mainly occurs nearby wall jet collision (WJC) zones and the initial bubble nucleation starts after a certain period of time in each heating cycle, which is insensitive to heating frequency under the present experimental conditions but related to the positions of WJC zones due to the crossflow effect. The initial bubbles have relatively larger size and will departure from heating surface or not, while the offspring bubbles have relatively smaller size and will not departure from heating wall but collapse on the heating wall. The collapsing of the above two kinds of normal-sized bubble will generate microbubbles (i.e., Microbubble emission boiling, MEB) and the number of microbubbles in jet chamber increases during the heating period. The periodical bubble growth and collapse causes high frequency pressure oscillation, with its frequencies being related to the bubble nucleation rates. The peak oscillation frequency shows somewhat increasing trend with the increasing of heating frequency.

Collective behavior of evaporating droplets on superhydrophobic surfaces

AbstractWe study the evaporation dynamics of multiple water droplets deposited in ordered arrays or randomly distributed (sprayed) on superhydrophobic substrates (SHP) and smooth silicon wafers (SW). The evaluation of mass of the droplets as a function of time shows a power‐law behavior with exponent 3/2, and from the prefactor of the power‐law an evaporation rate can be determined. We find that the evaporation rate on a SHP surface is slower than a normal surface for both single droplet and collection of droplets. By dividing a large droplet into more smaller ones, the evaporation rate increases and the difference between the evaporation rates on SHP and SW surfaces becomes higher. The evaporation rates depend also on the distance of the droplets which increase with increasing this distance.

Evaluation of Pre-Sliding Behavior at a Rough Interface: Modeling and Experiment

AbstractOne of the main issues in precision engineering is the lack of deep understanding of the pre-sliding behavior at the interface of mating surfaces of positioning mechanisms. In addition to the mechanical properties of the contacting bodies, their surface topography plays a key role in the pre-sliding regime and has a great impact on the frictional stiffness. This paper experimentally evaluates a boundary element method (BEM) model for the pre-sliding behavior at the interface of a smooth silicon wafer and a rough polymeric ball. The polymeric ball is either high-density polyethylene (HDPE) or polyoxymethylene (POM). The experiments are conducted at three different normal loads on five different spots on the wafer. The sliding stroke and coefficient of friction are extracted from experiments to be implemented as inputs to the numerical model. The roughness of the balls is also another input. The numerical and experimental friction hysteresis loops are compared. There is a small difference in the predicted pre-sliding distance from the experiments. The lateral stiffness, calculated at three different points on the pre-sliding regime of friction hysteresis loops, is compared with the Mindlin’s solution and experimental values for both contact interfaces and normal loads.

Freezing and melting of sessile droplet on micro- and hierarchically-structured silicon surfaces

Water droplet freezing and melting on three micro- or hierarchically-structured silicon surfaces and a smooth silicon wafer were studied experimentally. The surface temperature was kept at −13.8 °C during freezing and increased gradually to room temperature after turning off the bath circulator. The freezing behaviors including the freezing delay, solidification front movement, and the droplet profile were obtained through the photographic method. The results show that the tiny black silicon particles on hierarchical structures facilitate a longer freezing delay. The solidification stage lasted for 5–17 s depending on the surface structures, during which the freezing rate was larger in the initial seconds. Droplet on three micro-structured surfaces formed a tip with the angle of 122 ± 2°, smaller than that on the smooth silicon which was 136 ± 3°. The melting onset temperature for samples 1–3 was about 4.4 °C while melting happened on smooth silicon needed a smaller degree of superheat.

Copper nanowire arrays surface wettability control using atomic layer deposition of TiO2

Template two step electrodeposition method and atomic layer deposition were used to synthesize copper nanowires of varied length (1.2 to 26.2 μm) and copper nanowires coated with titanium dioxide. As a result of the atomic layer deposition of TiO2, coated nanowires demonstrated an up to 10-fold decrease in the wetting angle, compared with uncoated nanowires. It was found the dissipation rate is substantially higher for nanowires coated by the atomic layer deposition method (100 s) as compared with the uncoated copper nanowires (400 s), which assumes the positive properties of water propagation along the surface, necessary for improving the heat transfer. It was also found that the water contact angle for uncoated nanowires and those coated with TiO2 by the atomic layer deposition (ALD) gradually increases as the samples are kept in air. A gradual increase in wettability was also observed for smooth silicon wafers coated by ALD of TiO2, which were exposed to air. On the coated silicon substrates, the wetting angle gradually increased from 10° to approximately 56° in the course of four days. In addition, it was shown that copper nanowires coated with TiO2 by the atomic layer deposition method have an excellent corrosion resistance, compared with uncoated nanowires, when brought in contact with air and water.

Carbon-Ionomer Nanocomposite Wetting Properties: The Role of Ionomer Composition and Surface Roughness

Surface hydrophobicity changes of a series of nanocomposite films were evaluated as a function of roughness and ionomer concentration. Nanocomposite surfaces were created by coating a smooth silicon wafer and micro textured surfaces based on two types of 3M micro-replicated Brightness Enhancement Films (BEF). Multiple nanocomposite surfaces were evaluated as a function of Pt/C catalyst, a single walled carbon nanotube (SWCNT), and ionomer concentration varying between 7.5 to 27.3 wt% Nafion. An increase in hydrophobicity was observed for all nanocomposite surfaces as compared to bare substrates coated with ionomer. Bare substrates had observed water contact angles of 32.5o on silicon, 50.8o on BEF type Y, and 91.2o on BEF type P. Nanocomposites coated on BEF type P surfaces had the greatest increase in apparent contact angle starting from 101.5o on a surface coated with ionomer to 140.6o for an ionomer composite containing 100 wt% Pt/C followed by BEF type Y (78.4o - 135.4o) and Si (76.9o - 135.8o). Nanocomposite roughness increased with increasing ionomer concentration and was inversely related to the apparent contact angle of water. Nanocomposite wetting properties were strongly dependent upon ionomer concentration and micro scale roughness contributed to wetting behavior transitioning between Wenzel and Cassie modes.

Dynamics of gas micronuclei formed on a flat hydrophobic surface, the predecessors of decompression bubbles

It is a long-standing hypothesis that the bubbles which evolve as a result of decompression have their origin in stable gas micronuclei. In a previous study (Arieli and Marmur, 2011), we used hydrophilic and monolayer-covered hydrophobic smooth silicon wafers to show that nanobubbles formed on a flat hydrophobic surface may be the gas micronuclei responsible for the bubbles that evolve to cause decompression sickness. On decompression, bubbles appeared only on the hydrophobic wafers. The purpose of the present study was to examine the dynamics of bubble evolution. The numbers of bubbles after decompression were greater with increasing hydrophobicity. Bubbles appeared after decompression from 150kPa, and their density increased with elevation of the exposure pressure (and supersaturation), up to 400kPa. The normal force of attraction between the hydrophobic surface and the bubble, as determined from the volume of bubbles leaving the surface of the wafer, was 38×10−5N and the tangential force was 20×10−5N. We discuss the correlation of these results with previous reports of experimental decompression and bubble formation, and suggest to consider appropriate modification of decompression models.

On evaporation of sessile drops with moving contact lines

We consider theoretically, computationally and experimentally spontaneous evaporation of water and isopropanol drops on smooth silicon wafers. In contrast to a number of previous works, the solid surface we consider is smooth and therefore the droplets' evolution proceeds without contact line pinning. We develop a theoretical model for evaporation of pure liquid drops that includes Marangoni forces due to the thermal gradients produced by non-uniform evaporation, and heat conduction effects in both liquid and solid phases. The key ingredient in this model is the evaporative flux. We consider two commonly used models: one based on the assumption that the evaporation is limited by the processes originating in the gas (vapour diffusion-limited evaporation), and the other one which assumes that the processes in the liquid are limiting. Our theoretical model allows for implementing evaporative fluxes resulting from both approaches. The required parameters are obtained from physical experiments. We then carry out fully nonlinear time-dependent simulations and compare the results with the experimental ones. Finally, we discuss how the simulation results can be used to predict which of the two theoretical models is appropriate for a particular physical experiment.

On evaporation of sessile drops with moving contact lines

We consider theoretically, computationally and experimentally spontaneous evaporation of water and isopropanol drops on smooth silicon wafers. In contrast to a number of previous works, the solid surface we consider is smooth and therefore the droplets' evolution proceeds without contact line pinning. We develop a theoretical model for evaporation of pure liquid drops that includes Marangoni forces due to the thermal gradients produced by non-uniform evaporation, and heat conduction effects in both liquid and solid phases. The key ingredient in this model is the evaporative flux. We consider two commonly used models: one based on the assumption that the evaporation is limited by the processes originating in the gas (vapour diffusion-limited evaporation), and the other one which assumes that the processes in the liquid are limiting. Our theoretical model allows for implementing evaporative fluxes resulting from both approaches. The required parameters are obtained from physical experiments. We then carry out fully nonlinear time-dependent simulations and compare the results with the experimental ones. Finally, we discuss how the simulation results can be used to predict which of the two theoretical models is appropriate for a particular physical experiment.

Decompression sickness bubbles: Are gas micronuclei formed on a flat hydrophobic surface?

It is a long-standing hypothesis that the bubbles which evolve as a result of decompression have their origin in stable gas micronuclei lodged in hydrophobic crevices, micelles of surface-active molecules, or tribonucleation. Recent findings supported by atomic force microscopy have indicated that tiny, flat nanobubbles form spontaneously on smooth, hydrophobic surfaces submerged in water. We propose that these nanobubbles may be the gas micronuclei responsible for the bubbles that evolve to cause decompression sickness. To support our hypothesis, we used hydrophilic and monolayer-covered hydrophobic smooth silicon wafers. The experiment was conducted in three main stages. Double distilled water was degassed at the low pressure of 5.60 kPa; hydrophobic and hydrophilic silicon wafers were placed in a bowl of degassed water and left overnight at normobaric pressure. The bowl was then placed in the hyperbaric chamber for 15 h at a pressure of 1013 kPa (=90 m sea water). After decompression, bubbles were observed and photographed. The results showed that bubbles only evolved on the hydrophobic surfaces following decompression. There are numerous hydrophobic surfaces within the living body (e.g., in the large blood vessels), which may thus be the sites where nanobubbles that serve as gas micronuclei for bubble evolution following decompression are formed.

Interactions of human bone cells with diamond-like carbon polymer hybrid coatings

Diamond-like carbon (DLC) coatings produced using the plasma-accelerating filtered pulsed arc discharge (FPAD) method display excellent adherence to the substrate and improve its corrosion resistance. This article reports the interactions of human osteoblastic cells with DLC and two DLC polymer hybrid (DLC-p-h) coatings deposited on smooth, matt and rough silicon wafers by the FPAD method. The DLC-p-h materials were DLC-polytetrafluoroethylene hybrid (DLC-PTFE-h) and DLC-polydimethylsiloxane hybrid (DLC-PDMS-h) coatings. The biocompatibility of the coatings was assayed by using mesenchymal stem cells, primary osteoblasts and Saos-2 cells. Human mesenchymal stem cells proliferated when cultured on DLC and DLC-PTFE-h, but their numbers diminished on DLC-PDMS-h. In all three cell types studied, phalloidin-TRITC staining disclosed cell-type organization typical of an actin cytoskeleton on DLC and DLC-PTFE-h, but minimal and disorganized stress fibers on cells cultured on DLC-PDMS-h. The microtubular cytoskeleton was similarly disorganized on DLC-PDMS-h. Cells on DLC-PDMS-h developed a peculiar form of membrane damage, with nuclear staining by propidium iodide associated with granular calcein staining of the cytoplasm. Active caspase-3 labeling was only seen in cells cultured on DLC-PDMS-h, indicating that these cells undergo apoptosis induced by defective cell adhesion. Results suggest that DLC-PDMS-h coatings might be useful in orthopedic applications where an implant or implant-facet should be protected against bone overgrowth while DLC and DLC-PTFE-h coatings might improve osseointegration.

Indentation on very smooth silicon wafers

At present, indentation is considered as the most convenient approach for characterising materials for MEMS and even NEMS devices. Here, we demonstrate that a very slight change in the average surface roughness (Ra) of a commercial silicon wafer, that is, from 1.6 to 4.4 nm, can dramatically alter the indentation result. Another surface condition, namely, wetting, is also influential. Since it is not easy to quantitatively describe the effects of the interface between the indenter and tested material, the results of indentation test, as an approach for revealing the properties of a material, should be treated with great cautions.

Contact Electrochemical Replication of Hydrophilic−Hydrophobic Monolayer Patterns

Contact electrochemical replication (CER) is a novel pattern replication methodology advanced in this laboratory that offers the unprecedented capability of direct one-step reproduction of monolayer surface patterns consisting of hydrophilic domains surrounded by a hydrophobic monolayer background (hydrophilic @ hydrophobic monolayer patterns), regardless of how the initial "master" pattern was created. CER is based on the direct electrochemical transfer of information, through aqueous electrolyte bridges acting as an information transfer medium, between two organosilane monolayers self-assembled on smooth silicon wafer surfaces. Upon the application of an appropriate voltage bias between a patterned monolayer/silicon specimen playing the role of "stamp" and a monolayer/silicon specimen playing the role of "target", the hydrophilic features of the stamp are copied onto the hydrophobic surface of the target. It is shown that this electrochemical printing process may be implemented under a variety of experimental configurations conducive to the formation of nanometric electrolyte bridges between stamp and target; however, using plain liquid water for this purpose is, in general, not satisfactory because of the high surface tension, volatility, and incompressibility of water. High-fidelity replication of monolayer patterns with variable size of hydrophilic features was achieved by replacing water with a sponge-like hydrogel that is nonvolatile, compressible, and binds specifically to the hydrophilic features of such patterns. Since any copy resulting from the CER process can equally perform as stamp in a subsequent CER step, this methodology offers the rather unique option of multiple parallel reproduction of an initially fabricated master pattern.

Interaction forces between a poly ( N-isopropylacryamide) layer grafted onto a solid surface and a hydrophobic particle

Abstract —The interaction forces between poly ( N-isopropylacrylamide) (PNIPAAm)-grafted surfaces and colloidal particles in an aqueous solution were investigated using atomic force microscopy. The measurements were conducted between a smooth silicon wafer on which PNIPAAm was terminally grafted and silica particles hydrophobized with a silanating reagent in an aqueous electrolyte solution under controlled temperature. Large repulsive forces were observed between the surfaces below the lower critical solution temperature (LCST) of PNIPAAm, while attractive forces were observed above the LCST. The magnitude of the attractive force tended to increase with the surface hydrophobicity of the particles. The temperature-induced changes in the hydration state of the grafted PNIPAAm chains are considered to greatly alter the interaction force properties. The role of the intermolecular interaction between the PNIPAAm chains and hydrophobic particles in the interaction forces is discussed.

Pool Boiling Experiments on Multiwalled Carbon Nanotube (MWCNT) Forests

In this study, two silicon wafer substrates were coated with vertically aligned multiwalled carbon nanotubes (MWCNT) “forests” and were used for pool boiling studies. The MWCNT forests (9 and 25μm in height) were synthesized on the silicon wafer substrates using chemical vapor deposition (CVD) process. The substrates were clamped on a cylindrical copper block with embedded cartridge heaters. The heat flux was measured using sheathed K-type thermocouples, which were placed inside the cylindrical copper block. Pool boiling experiments using refrigerant PF-5060 as the working liquid were conducted to obtain the pool “boiling curve.” The experiments were conducted in nucleate and film boiling regimes to investigate the effect of MWCNT height on pool boiling performance. Reference (control) experiments were also performed with an atomically smooth bare silicon wafer (without MWCNT coating). The results show that the MWCNT forests enhanced critical heat flux (CHF) by 25-28% compared to control experiments. For the film boiling regime, Type-B MWCNT (25μm in height) yields 57% higher heat flux at Leidenfrost point (film boiling regime) compared to control experiments. However, for the Type-A MWCNT (9μm in height) the film boiling heat flux values are nearly identical to the values obtained for the control experiments performed on bare silicon.

Interaction Forces Between Thermoresponsive Surface and Colloidal Particle in Aqueous Solution Studied Using Atomic Force Microscopy

ABSTRACTThe interaction forces between poly(N-isopropylacrylamide) (PNIPAAm)-grafted surfaces and colloidal particles in an aqueous solution were investigated using an atomic force microscope (AFM). Measurements were conducted between a smooth silicon wafer on which PNIPAAm was terminally grafted and silica particles hydrophobized with a silanating reagent in an aqueous electrolyte solution under controlled temperature. Below the lower critical solution temperature (LCST) of PNIPAAm, there were large repulsive forces between the surfaces, while attractive forces were observed above LCST. When surface hydrophobicity of the particles increased, the magnitude of attractive force tended to increase. The changes of hydration state of the grafted PNIPAAm chains depending on temperature is considered to greatly alter the interaction force properties. The role of the intermolecular interaction between the PNIPAAm chains and the hydrophobic particles in the interaction forces is discussed.

Interaction forces measured between poly( N-isopropylacrylamide) grafted surface and hydrophobic particle

The interaction forces between poly( N-isopropylacrylamide) (PNIPAAm)-grafted surfaces and colloidal particles in an aqueous solution were investigated using an atomic force microscope. Measurements were conducted between smooth silicon wafers on which PNIPAAm was terminally grafted and silica particles hydrophobized with a silanating reagent in an aqueous electrolyte solution under controlled temperature. Below the lower critical solution temperature (LCST) of PNIPAAm, there were large repulsive forces between the surfaces, both on approach and separation of the surfaces. On the other hand, above LCST, attractive forces were observed both in approaching and in separating force curves. When surface hydrophobicity of the particles increased, the maximum attractive force tended to increase. The changes of hydration state of the grafted PNIPAAm chains depending on temperature are considered to greatly alter the interaction force properties. The role of the intermolecular interaction between the PNIPAAm chains and the hydrophobic particles in the interaction forces is discussed.