Low Withdrawal Speed Research Articles

The normal force evolution of the substrate during the withdrawal process

AbstractThe evolution of normal force on the smooth plate immersed in Newtonian and yield stress fluids was investigated during the dip coating. The influences of withdrawal velocity, liquid rheological property, and plate roughness were elaborated. For all fluids, the variation of force versus the withdrawing height was divided into four stages. Importantly, the control forces for coating in each stage were determined, which are in turn viscous force‐capillary, viscous force‐gravity, capillary‐viscous force, and gravity. The critical value of normal force controlled from capillary to viscous effect during drainage was adopted to calculate the surface tension. The possibility of measuring the surface tension was verified under the negligible interference of viscous effect and film instability through a detailed analysis on the evolution of force in a series of tests. Accordingly, the applicable conditions with the low withdrawal speed, high viscosity, and rough surface were finally achieved. This work can provide a theoretical basis for the film‐formation mechanism and surface tension measurement in dip coating.

Influence of PLGA nanoparticles on the deposition of model water-soluble biocompatible polymers by dip coating

This work relies on the use of dip-coating at various withdrawal speeds to form nanocomposite films, with a detailed analysis of the influence of the mode of deposition on the nanoparticle (NP) concentration in the dried film. While the deposition of polymer solutions on the one hand and colloidal suspensions of NPs on the other hand have been separately studied by dip coating, their combination is far being a simple superposition of the two separate behaviors. The formation of nanocomposite thin films composed of model water-soluble biocompatible polymers (polyvinyl alcohol - PVA and polyvinylpyrrolidone - PVP) loaded with poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) was studied through a dip coating process. Flat silicon substrates were removed at a controlled withdrawal speed from an aqueous colloidal suspension. Thin films of PLGA NPs with concentrations ranging from 1 to 10 % wt./wt. in PVA or PVP matrices were prepared. The presence of nanoparticles on the well-established process of thin film deposition was examined, as well as the influence of the deposition regime on the nanoparticle concentration in the deposited coating. We demonstrate that the presence of colloidal dispersion of PLGA nanoparticles in water solution of PVA and PVP does not modify the process of film deposition by dip coating. A typical “V” shaped curve was observed, with two well-known deposition regimes: capillary and draining modes respectively obtained at low and high withdrawal speeds. Due to crystallization at low withdrawal speed (favored by slow evaporation of the solvent) it was not possible to identify individual PLGA nanoparticles by AFM in the case of the PVA matrice. Amorphous PVP nanocomposite films were successfully prepared by dip coating, and allowed us to identify individual PLGA nanoparticles with AFM. Because of the prevalence of an evaporation-driven phenomenon at low withdrawal speed, incorporation of NPs was observed over the whole range of withdrawal speeds, showing original behavior compared to recent studies relying on a pure Landau-Levich regime (i.e. non-evaporative systems). Our results indicate that the nanoparticles were not equally retrieved from the solution in the capillary and the draining regimes. This suggests that the balance between the viscous drag and the interfacial effects depends on the deposition mode and calls for a more detailed analysis of the physical processes involved in both regimes.

The effect of the withdrawal speed on properties of nickel oxide thin films

AbstractNickel oxide thin films have been prepared by sol-gel dip-coating technique on glass substrate. It is shown that nickel oxide thin films have poly crystalline nature. Nickel oxide thin films exhibit high transmission (39–85%) in the wavelength range of 400–900 nm, strong absorption between 300 and 400 nm wavelengths and decrease of band gap values are in the range 3.69–3.27 eV with increase of withdrawal speed. High band gap at low withdrawal speed is because of the small average crystallite size, which decreases with increase in withdrawal speed. The SEM micrograph shows cubic crystallites and surface of thin films become dense, smooth and homogeneous with an increase in withdrawal speed. Assessment of nickel oxide deposition conditions provides gateway for effective and cheap solar cells.

Fabrication and Characterisation of Titania Nanoporous thin Film for Photoelectrochemical (PEC) Conversion of Water

In this paper, nanoporous titania thin films for PEC conversion of water were dip coated by using acetic acid as a stabiliser and poly(ethylene glycol) (PEG) as a pore former. The evolution of microstructure and morphology of the thin films at different conditions was investigated by XRD, SEM and AFM. Anatase was the primary crystalline phase of the as-obtained titania nanoporous thin film after heat treatment at 500°C/2h. A small amount of PEG, a low ratio of acetic acid/Titania butoxide (HoAC/TBOT) and a relatively low withdrawal speed facilitate the formation of TiO2 nanoporous thin films with a homogeneous distributed nanostructure.

EFFECTS OF SUBSTRATE AND EXPERIMENTAL CONDITIONS ON THE FABRICATION OF WELL-ORDERED SILICA PARTICULATE LANGMUIR–BLODGETT FILMS

The dependence of particulate Langmuir–Blodgett (LB) monolayer quality on the surface nature of the substrate was investigated by pretreating the substrates with different methods, and the optimal withdrawal speed was determined by detecting the quality of particulate monolayers deposited at different withdrawal speeds. Furthermore, the effects of stearic acid (SA) concentration on the quality of particulate monolayers were discussed. Results indicated that surface amination would favor the transfer of the floating particulate monolayer from the interface to the substrate, which would be attributed to the formation of chemical bonds between the – NH 2 groups on the substrate surface and the functional groups of – COOH and Si – OH groups on surfaces of particles floating at the interface. And the withdrawal speed of 1 mm ⋅ min-1 was appropriate for the fabrication of high-quality particulate LB monolayer because low withdrawal speed can offer sufficient time to form chemical bonds and expel the subphase from the substrate. Additionally, SEM images illustrated that the SA concentration of 0.40 mg ⋅ mL-1 was suitable for the fabrication of well-ordered particulate LB monolayers composed by the particles of 380 nm.

Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells

Nanostructured porous zinc oxide electrodes for use in dye-sensitized solar cells (DSSCs) were coated with thin niobium oxide layers by using sol–gel transformation of niobium pentaethoxide in air. Coating solutions were prepared by mixing niobium pentaethoxide and ethanol. A dip-coating technique was adopted at a low withdrawal speed of 100μms−1. The coated electrodes were then heat-treated at temperatures between 400 and 600°C. The presence of niobium in the coated electrodes was confirmed by X-ray photoelectron spectroscopy. As expected, the niobium oxide layers worked as an energy barrier between the ZnO electrode and electrolyte. Open-circuit voltage (VOC) of the cells using the coated electrodes was then enhanced up to 0.768V, which was attributable to the suppression of the recombination of photogenerated electrons with oxidized species in electrolytes. An additional benefit of the coating was that grain growth of ZnO particles in the electrodes was hindered and short-circuit photocurrent density (JSC) was kept relatively high due to large amounts of adsorbed dye. An overall light-to-electricity conversion efficiency was increased to a maximum of 5.19%, indicating that the proper coating technique was the key for improving the performance of ZnO-based DSSCs.

Formation of Silica Nanolayers on ZnO Electrodes in Dye‐Sensitized Solar Cells

AbstractZinc oxide electrodes for use in dye‐sensitized solar cells were coated with thin silica layers by using a sol–gel transformation of tetraethylorthosilicate (TEOS). Coating solutions were prepared by mixing TEOS and diluted ammonia while stirring and ultrasonificating simultaneously. Two kinds of coating methods (immersing and dip‐coating) were adopted and compared to control the thickness of the silica layers on a nanometer scale. With the immersing method a relatively thick SiO2 coating was made on zinc oxide particles on the electrodes, which resulted in an increase in the open‐circuit photovoltage (Voc) and a large decrease in the short‐circuit photocurrent density (Jsc). In contrast, by using the dip‐coating method, with a relatively low withdrawal speed of 100 μm s–1, thinner SiO2 layers could be formed on the zinc oxide particles and a high Jsc as well as a high Voc was obtained. These results demonstrate that the Voc characteristics of solar cells using silica‐coated zinc oxide electrodes can be enhanced by controlling the structure of the silica‐coating layers.

Fabrication of Fe3O4 nanoparticle arrays via patterned template assisted self-assembly

Magnetic nanoparticle arrays have been fabricated by combining chemically synthesizedFe3O4 nanoparticles with a diblock copolymer template substrate consisting of self-assembled polystyrene(PS) dots in a polymethylmethacrylate (PMMA) matrix. The influence of the volume fraction of theFe3O4 suspending solution and the withdrawal speed of the template on the formation of arraystructures was investigated. A small volume fraction of the nanoparticles and lowwithdrawal speed play an important role in the fabrication of the patterned arrays ofnanoparticles via template assisted self-assembly. Below a withdrawal speed of0.5 mm s−1 and a nanoparticle volume fraction below 0.05 vol% (in particular, at extremely highdilutions of less than 0.01 vol%), the selective deposition of one to several nanoparticles onevery single PS dot becomes possible.

Concentration Polarization at the Langmuir Monolayer Deposition: Theoretical Considerations

A mathematical model describing the concentration polarization in the three-phase contact region during the Langmuir-Blodgett deposition process is developed. It is shown that the stationary deposition is only possible when, in additional to convective fluxes, electrodiffusion ionic fluxes and corresponding concentration gradients are developed in the system. At a sufficiently low withdrawal speed, the occurring diffusion and migration ionic fluxes restore the steady-state ionic balance. As well, electric charge is accumulated in close vicinity to the three-phase contact line to produce a stationary electric field. The concentration polarization affects the parameters of the deposition process (dynamic contact angle, work of adhesion, maximum deposition rate) as well as morphology, composition, and structure of the deposited monolayer. When the withdrawal speed exceeds a critical value, the transport of counterions becomes insufficient to compensate interfacial charge in close vicinity to the three-phase contact line. Consequently, the electrostatic repulsion between the monolayers becomes sufficiently strong to disrupt the deposition process. The latter can result in meniscus instability. The proposed mechanism correlates with some experimental observations.

Silica Films Containing Ordered Pores Prepared by Dip Coating of Silica Nanoparticles and Polystyrene Beads Colloidal Mixture

Silica films containing three dimensionally (3D) ordered pores were prepared by a simple dip coating method. A colloidal sol containing silica particles in the nanometer size range and a polystyrene latex (PSL) colloidal sol containing particles of tens of nanometers to one micrometer in size were used as precursors. The pore periodicity, which in turn produces the dielectric periodicity, can be easily altered by changing the size of the PSL beads. Films having a high surface smoothness were obtained by using small silica particles, large PSL particles, and a low withdrawal speed in the dip-coating. When the films were irradiated with a white light source, the reflective spectrum was changed by varying the incident angle, indicating its possible use as a monochromator. The change in the reflective spectrum was explained using effective medium approximation combined with a simple Bragg reflection equation.