Particle Size Evolution Research Articles

On anomalous nanoparticle size evolution in glowing wire generator

This paper deals with nanoparticle generation by a glowing wire generator (GWG). The glowing chamber has been customized enabling using either single- or double-wire attached to the electrodes. The double-wire configuration was expected to significantly extend the operation window of the GWG. The undertaken measurements of the particle size distribution revealed, however, an anomalous behaviour of the particle size evolution depending on the applied voltage. At low voltages (<1.1V), the double-wire configuration allows for a substantial particle enlargement (up to two times) as compared to the single-wire configuration, which looks promising for the development of industrial aerosol generators. However, when the voltage exceeded the above value, the opposite behaviour was detected, i.e., some decrease in the particle size for the double-wire configuration was observed. This anomalous behaviour was attributed to interplay between nucleation and coagulation in the system. The corresponding mechanisms are discussed in this paper.

Structural effect of ZrO2 on supported Ni-based catalysts for supercritical water gasification of oil-containing wastewater

Supercritical water gasification (SCWG) is a promising technology to treat oil-containing wastewater. This study reports a Ni/ZrO2 catalyst with mixed phase of tetragonal ZrO2 and monoclinic ZrO2 synthesized by a one-step sol-gel method, which shows good catalytic activity and stability in SCWG of oil mixture. The evolution of particle size, crystal structure, and pore size caused by increasing calcination temperature is demonstrated by X-ray diffraction and transmission electron microscopy. When calcined at 700 °C, Ni/ZrO2 shows superior catalytic activity and stability in SCWG reaction, with a high carbon gasification efficiency of 91.3% at 500 °C. The characterizations of fresh and spent catalysts suggest that Ni/ZrO2 catalyst with the fraction of monoclinic ZrO2 around 75% would be highly active and stable in SCWG reaction. In addition, two sets of optimal gasification conditions of SCWG of oil are obtained by varying operating parameters, which provides guidance for different concentrations of oil-containing wastewater treatment.

Experimental study on calcination and fragmentation characteristics of limestone in fluidized bed

The fragmentation degree of limestone affects seriously the particle size distribution of sorbent in the desulfurization process in fluidized bed combustion. In this paper, the variation of particle size and differences in microstructure of calcination and fragmentation were measured under different fluidizing medium in the same equipment. The effects of temperature, reaction rate, SO2 and CO2 atmosphere, firing exhaust gas atmosphere on the calcination and fragmentation of limestone with different properties in the fluidized bed were studied. It was found that the size evolution of larger limestone was more obvious than that of finer ones. Under the condition of fragmentation, the particle size evolution of limestone was more intense than that in calcination. The particle size was smaller under the condition of a higher reaction rate. Higher temperature can promote the calcination reaction and reduce the particle size to a certain extent. The coarser fragmentation products at high temperature may be related to the enhancement of agglomeration between particles. SO2 and CO2 can inhibit the fragmentation of limestone and lead to coarsening of products, which further confirms the main contribution of calcination reaction to limestone size evolution. The main chemical reaction in a given atmosphere is direct sulphation reaction and the primary fragmentation is restrained by the high concentration of CO2 in firing exhaust gas, which is more obvious in the oxygen-enriched exhaust gas.

Particle Size Evolution during the Synthesis of Gold Nanoparticles Using In Situ Time-Resolved UV–Vis Spectroscopy: An Experimental and Theoretical Study Unravelling the Effect of Adsorbed Gold Precursor Species

The quantitative analysis of nanomaterials synthesis kinetics is valuable both for understanding the synthesis and for development of manufacturing processes, and it usually requires the use of synchrotron-based instrumentation, making it challenging to perform experiments in the large parametric space needed to develop quantitative kinetic models. UV–vis spectroscopy represents a convenient technique to circumvent such difficulties, as it is available in most chemistry laboratories and allows fast data acquisition. This technique can in theory be used for the characterization of plasmonic nanomaterials synthesis kinetics. However, linking UV–vis spectra with characteristic features of the produced nanomaterials, such as size and shape, is a challenging task. This work presents a detailed spectroscopic analysis of gold nanoparticles syntheses via in situ time-resolved UV–vis spectroscopy, with emphasis on the role of gold precursor adsorption on the particle surface during nanoparticle growth. We show that the classic Turkevich synthesis and the growth of preformed gold nanoparticles with two different methods exhibit significant commonalities in the spectra evolution, explained in terms of the interaction between gold precursor species and the nanoparticle surface. Such interaction was accounted for in a model based on the Mie theory, describing the growing nanoparticles as core–shell spheres with an outer shell of few Ångströms characterized by reduced conductivity and increased electron damping rate. The proposed model led to the determination of the nanoparticle size and concentration evolution throughout the synthesis with good quantitative agreement against literature SAXS data. Furthermore, the core–shell model enabled the reproduction of the progressive blue-shift in the SPR peak position observed during the synthesis. Thus, this work reconciles the “seed-mediated growth” mechanism for the Turkevich synthesis with the temporal evolution of the spectra within the framework of the Mie theory, showing that the distinctive purple-greyish hue observed during the synthesis can be related to the interaction of gold precursor with the growing gold nanoparticles.

The Effect of Temperature and SEI Formation on the Nucleation and Growth of Electrochemically Plated Lithium

Understanding the nucleation and growth process during electroplating of lithium metal is essential for enabling long cycle life in lithium metal batteries. The morphology of electrochemically plated lithium is strongly correlated with Coulombic efficiency and cycle life, and the nucleation and growth of lithium is dictated by the nature of the substrate, the characteristics of the solid electrolyte interphase (SEI), and temperature. However, there is a lack of knowledge about how these factors affect fundamental nucleation and growth processes, especially at temperatures other than room temperature. In this work, we investigate the nucleation, growth, and morphology evolution of lithium metal under different temperature conditions. Stainless steel and antimony metal are used as substrates to compare growth surfaces that are inert with those that form alloys with lithium. Ex situ SEM is used to explore the morphology evolution at temperatures ranging from -20 °C to 20 °C. In order to better track the particle size evolution over time, we use an electrolyte solution tailored to cause Li to deposit as spherical particles (a 1:1 (v/v) 1,3-dioxolane (DOL) and 1,2 dimethoxyethane (DME) mixture with 1M LiTFSI and 0.8M LiNO3). We find that as temperature increases, the particle size also increases. Furthermore, growth on lithiated antimony films results in larger average particle sizes than on stainless steel. Our experiments also show that the extent and nature of the initial SEI growth on the substrate plays a controlling role in determining the Li morphology and uniformity of growth, and prolonged growth of the SEI was found to increase the Coulombic efficiency during the first few cycles. These results highlight the sensitivity of Li to cycling conditions and temperature, and they indicate that careful control is necessary to attain high-efficiency cycling in Li metal batteries.

Multiscale Simulation of the Formation of Platinum-Particles on Alumina Nanoparticles in a Spray Flame Experiment

Platinum decorated alumina particles have the potential of being a highly (cost-)effective catalyst. The particles are synthesized from platinum(II) acetylacetonate dissolved in a mixture of isopropanol and acetic acid with dispersed alumina carriers. The process is simulated by means of large eddy simulation with reaction kinetics and aerosol dynamics modeling. A two mixture fraction approach for tabulated chemistry with a thickened flame model is used to consider the complex reaction kinetics of the solvent spray combustion. Diffusion is described followings Ficks law with a unity Lewis number for the gas phase species, whereas the particle diffusion coefficients are calculated according to the kinetic theory. An extended model for aerosol dynamics, capable of predicting deposition rate and surface particle growth, is derived from the classical sectional technique. The simulations are compared and validated with product particle characteristics obtained from the experimental observations. Distributions for different locations within the simulation domain show the evolution of particle sizes deposited on the alumina particle surface, and transmission electron microscopy (TEM) images of the composite particles are shown in comparison to 3D particles ballistically reconstructed from simulation data. The ratio of deposited platinum on the alumina carrier particles and the mean diameters of the deposited particles are in good agreement with the experimental observation. Overall, the new method has demonstrated to be suitable for simulating the particle decoration process.

Role of interfacial adhesion on minimum wear particle size and roughness evolution.

Adhesion between two bodies is a key parameter in wear processes. At the macroscale, strong adhesive bonds are known to lead to high wear rates, as observed in clean metal-on-metal contact. Reducing the strength of the interfacial adhesion is then desirable, and techniques such as lubrication and surface passivation are employed to this end. Still, little is known about the influence of adhesion on the microscopic processes of wear. In particular, the effects of interfacial adhesion on the wear particle size and on the surface roughness evolution are not clear and are therefore addressed here by means of molecular dynamics simulations. We show that, at short timescales, the surface morphology and not the interfacial adhesion strength dictates the minimum size of wear particles. However, at longer timescales, adhesion alters the particle motion and thus the wear rate and the surface morphology.

Dynamics of aerosol generation and release – Dry powder inhaler performance considerations

The main objective of the work was to investigate how the dynamics of inhalation affect aerosol release process in real-time, with an unsteady airflow through passive dry powder inhalers (DPIs) of comparable airflow resistance, but different principle of powder release. It consists of two complementary studies, concerning dynamics of controlled aerosol release (emission kinetics) and dynamics of aerosol generation (aerosol size characteristics in time series).Aerosol behavior of the powder was studied experimentally at the outlet of the inhalers: RS01 inhaler (Plastiape Spa, Osnago LC, Lombardy, Italy) and Turbospin (Ph&T, Milano, Lombardy, Italy). Two breathing profiles were analyzed throughout the study. Timing of aerosol emission was done by light scattering. Aerosol characterization was done by laser diffraction. It resulted in time evolution of volume concentration and volumetric particle size distribution.Both the design of the passive DPI inhaler and the dynamics of the inhalation (along with its intensification) influence the course of the aerosol generation and emission. The process of aerosol emission occurs only in the initial part of the inhalation process, before the peak inspiratory flow rate is established. This part is strongly responsible for the quality of the aerosol emitted from the device and consequently will potentially influence the drug delivery.

Effect of co-flow velocity ratio on evolution of poly-disperse particles in coaxial turbulent jets: A large-eddy simulation study

In the present work, the particle-laden coaxial turbulent jet flow is studied using large-eddy simulation (LES). An Eulerian–Lagrangian framework is used to study the interaction between the continuous phase (air) and the discrete phase (glass bead particles). The solver is validated, using single-phase and particle-laden simulations, with reference data from experiments. A good match is observed between the present results and the reference data, for centerline velocity decay and radial profiles of axial velocity. Simulations are performed for three co-flow velocity ratios of 0, 1, and 1.5. The results pertaining to particle characteristics are presented for three different particle size-classes. The effect of the co-flow velocity ratio on the particle size–velocity correlation and velocity statistics of both phases are studied with an emphasis on understanding the differences in the particle dispersion due to co-flow around the central jet. It is observed that the particle size–velocity correlation is negative in the potential core region, and it becomes positive as one moves downstream. For heavy particles, the axial distance required to attain the same velocity as that of air increases with an increase in the co-flow velocity ratio. The size-conditioned particle number density profiles along the axial and radial directions of coaxial jets showed some interesting trends that could be explained based on the particle Stokes number effect. Significant radial dispersion of particles is realized when the corresponding Stokes number (StL), defined based on large-scale turbulent eddies, is of the order of one. The axial evolution of the characteristic particle size exhibited non-monotonic trends for all co-flow ratios. Overall, the present work demonstrates potential application of LES for an in-depth study of dispersion of poly-disperse particles in turbulent coaxial jets.

Limits on Protoplanet Growth by Accretion of Small Solids

Abstract This paper identifies constraints on the growth of a small planetary core (0.3 M ⊕) that accretes millimeter-sized pebbles from a gaseous disk. We construct time-dependent spherical envelope models that capture physical processes that are not included in existing global hydrodynamic simulations, including particle size evolution, dust transport, and realistic gas equations of state. We assume a low enough disk density that pebbles are marginally coupled to the gas and are trapped efficiently near the core Bondi radius. Pebbles then drift rapidly enough to experience erosion by sandblasting, mutual collisions, and sublimation of water ice. We find that pebble fragmentation is more efficient than dust resticking. Therefore the high pebble accretion rate needed to build a core of mass &gt;M ⊕ leads to a high envelope metallicity and grain opacity. Above yr−1, and without other luminosity sources, convective motions expand near the Bondi radius. The warm, dusty, and turbulent envelope buffers the inward drift of pebble debris: given a turbulent concentration factor f turb ≳ 1 near the lower convective boundary, the core growth rate is limited to 1 × 10−7 f turb M ⊕ yr−1 and the e-folding time 3/f turb Myr. The remainder of the solid debris is expelled as highly processed silicates. Pebble ice never reaches the core, and the envelope contains comparable amounts of H2/He and metals. We interpret our results using simpler steady models and semianalytical estimates. Future global simulations incorporating the processes modeled here are needed to understand the influence of rotation and vertical disk structure.

Evolution of grinding energy and particle size during dry ball-milling of silica sand

The energy efficiency of dry fine grinding process of powders dramatically declines with decreasing particle size, and represents a growing concern in various cutting-edge technical applications for pharmaceutical and cosmetic products, advanced-ceramics, and food industry. In this paper, we report on a detailed experimental analysis of the long-term grinding behavior of silica sand as a model material in an oscillatory ball mill for a broad range of the values of vibration frequency and amount of powder. We show that the re-agglomeration of fine particles unambiguously explains the anomalous increase of grinding power consumption. We also find that the agglomeration phenomena are correlated with the volume fraction of the class of finest particles. A nonlinear comminution model is shown to capture this effect for the prediction of the evolution of the specific surface area.

Reverse Coarsening and the Control of Particle Size Distribution through Surfactant

The minimization of surface area, as a result of the minimization of (positive) surface energy, is a well-known driving force behind the spontaneous broadening of (nano) particle size distribution. We show that surfactant molecules binding to particle surfaces effectively decrease the surface energy and may change its sign. In this case, contrary to the expected broadening behavior, a minimum of free energy is achieved at the maximum surface area for all particles, i.e., when the particles are identical. Numerical simulations based on the classical Lifshitz–Slyozov–Wagner theory with surfactant-induced surface energy renormalization confirm the collapse of the particle size distribution. As the particle size evolution is much slower than particle nucleation and growth, the manipulation of surface energy with in-situ replacement of surfactant molecules provides a method for controlling particle size distribution with great potential for creating mono-disperse nanoparticles, a key goal of nanotechnology.

Effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions

The effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions was studied by single particle experiments and modelling. Experiments of pine and beech wood char conversion were carried out in a single particle combustor under conditions of 1473-1723 K, 0.0-10.5% O2, and 25-42% H2O. A comprehensive progressive char conversion model, including heterogeneous reactions (char oxidation and char gasification with CO2 and H2O), homogeneous reactions (CO oxidation, water-gas shift reaction, and H2 oxidation) in the particle boundary layer, particle shrinkage, and external and internal heat and mass transfer, was developed. The modelling results are in good agreement with both experimental char conversion time and particle size evolution in the presence of oxygen, while larger deviations are found for the gasification experiments. The modelling results show that the char oxidation is limited by mass transfer, while the char gasification is controlled by both mass transfer and gasification kinetics at the investigated conditions. A sensitivity analysis shows that the CO oxidation in the boundary layer and the gasification kinetics influence significantly the char conversion time, while the water-gas shift reaction and H2 oxidation have only a small effect. Analysis of the sensitive parameters on the char conversion process under a typical pulverized biomass combustion condition (4% O2, 13% CO2, 13% H2O), shows that the char gasification reactions contribute significantly to char conversion, especially for millimeter-sized biomass char particles at high temperatures.

Mechanistic Investigation of Surfactant-Free Emulsion Polymerization Using Magnetite Nanoparticles Modified by Citric Acid as Stabilizers.

Fe3O4-armored latexes were successfully synthesized by using modified Fe3O4 (IO) nanoparticles as stabilizers without a surfactant. The particle size, conversion, and particle number density of latex particles during the formation process were studied in detail. The surface charge density and the particle size evolutions of latexes were studied by dynamic light scattering. The use of scanning electron microscopy confirmed that IO nanoparticles were adsorbed on the polymer particle surface. Furthermore, the efficiency of iron oxide incorporation (IE) was evaluated by thermogravimetric analysis. The effect of pH, solid content, and zeta potential of IO nanoparticles on the results of polymerization was also discussed in detail. Attempts were made to explain the change of latex particle surface charge density by using Guy-Chapman-Stern's electric double layer theory. In addition, the effect of ionic strength of ammonium sulfate on particle number density of latex particles was described using P. John Feeney's equation. Finally, the mechanistic insights were discussed by studying polymerization kinetics.

Scaling behavior of particle breakage in granular flows inside rotating drums.

We perform systematic particle dynamics simulations of granular flows composed of breakable particles in a 2D rotating drum to investigate the evolution of the mean particle size and specific surface as a function of system parameters such as drum size, rotation speed, filling degree, and particle shape and size. The specific surface increases at a nearly constant rate up to a point where particle breakage begins to slow down. The rates of particle breakage for all values of system parameters are found to collapse on a master curve when the times are scaled by the characteristic time defined in the linear regime. We determine the characteristic time as a function of all system parameters, and we show that the rate of particle breakage can be expressed as a linear function of a general scaling parameter that incorporates all our system parameters. This scaling behavior provides a general framework for the upscaling of drum grinding process from laboratory to industrial scale.

In Situ Resistance Monitoring during Flame-Aerosol Deposition of Metal Oxide Gas Sensing Layers

Introduction Chemo-resistive sensors are highly promising for the detection of trace-level compounds in medical diagnostics[1] or indoor air monitoring[2]. In particular, gas sensors made by direct flame-aerosol synthesis are attractive due to their highly porous and crack-free sensing films[3]. They are composed of high-surface area nanoparticles with diverse composition tuned for ppb-level sensitivity and high selectivity towards key analytes such as Si-doped WO3 for acetone[1] or Ti-doped ZnO for isoprene[4]. Fabrication development has also led to the ability to process flame-made sensors by techniques compatible with standard silicon-wafer micromachining[5]. This enables the preparation of low power sensors that are attractive for portable handheld devices. So far, the development of such sensors has been focused mostly on material composition and, to a lesser extent, on film morphology and design. However, optimizing the latter could improve the sensor sensitivity as well as response and recovery times. Measuring the resistance during flame-deposition of nanostructured films in situ offers fabrication control by providing immediate feedback when an interconnected film is created[6]. This ultimately allows the fabrication of sensing films of minimal thickness and optimal resistance in a reproducible and economical manner. In situ resistance monitoring This work focuses on in situ monitoring the resistance of nanostructured films fabricated by direct deposition of flame-made Sb-doped SnO2 nanoparticles[6]. The sensing film morphology was tuned by a substrate-impinging particle flame, fed by a precursor solution containing varying metal ion concentrations. In situ resistance monitoring enabled control over final film resistance as well as direct insight into network formation and nanoparticle growth, necking and coalescence. The performance of these films as ethanol vapor sensors was assessed and related to their in situ resistance and nanoparticle film characteristics. Method Gas sensing nanoparticle films were prepared by direct deposition of flame-made Sb-doped (12 at%) SnO2 onto alumina substrates with interdigitated platinum electrodes. Deposition was carried-out for up to 6 min. Substrate preheating, film deposition and cooling to room temperature was closely monitored by the in situ resistance (Ri) between the interdigitated electrodes with a multimeter (Tektronix DMM4050). Nanoparticle sensing films were imaged by scanning electron microscopy (Hitachi S-4800 FE-SEM) after particle deposition for 2 min. Results and Conclusions Figure 1a shows the formation of Sb-doped SnO2 nanoparticles in a flame, followed by direct deposition onto the sensor substrate and in situ film resistance (Ri) read out. Figure 1b shows the in situ film resistance as a function of deposition duration using 1 (red squares), 10 (blue circles) or 100 (green triangles) mM FSP precursor solution concentrations (PSC). At low PSC (1 mM), the Ri remains at first (up to ca. 1 min) rather indifferent to nanoparticle deposition as it takes some time to build the interconnected particle bridges. But as deposition is prolonged (t >1.5 min), the Ri starts to drop. This clearly indicates the formation of an interconnected network of nanoparticles. Also quite notable is the strong oscillation of Ri by more than an order of magnitude during network formation. This stems from the constant formation (by deposition) and break-up of necked nanoparticles (by coalescence) on the substrate (i.e., repeatedly from lace-like to cauliflower-like structures) during flame-deposition. Increasing the PSC from 1 to 10 mM or 100 mM leads naturally to a higher generation rate of nanoparticles and, therefore, faster drop in resistance. This is confirmed by SEM imaging after 2 minute deposition time for 1, 10 and 100 mM (Figure 2 a-c). As expected, nanoparticles are assembled more densely at higher PSC. As a result, the film formation and morphology of Sb-doped SnO2 can be monitored during direct flame-deposition. This monitored Ri is correlated to its evolving particle size and necking, enabling the ability to optimize material quantity, fabrication time and the final film resistance. This leads to the ability to rapidly synthesize thin flame-made gas sensors with improved sensitivity and reduced response/recovery times.

Investigation of thermal, structural, morphological and photocatalytic properties of CuxCo1-xFe2O4 (0 ≤ x ≤ 1) nanoparticles embedded in SiO2 matrix

In the present work, Cu2+ substituted Co ferrite (CuxCo1-xFe2O4, x = 0.00, 0.25, 0.50, 0.75 and 1.00) nanoparticles embedded in SiO2 matrix were synthesized via a modified sol-gel method followed by high temperature annealing. The reaction progress was investigated by thermal analysis coupled with mass spectrometry and Fourier transformed infrared spectroscopy (FT-IR), while the formation of CuxCo1-xFe2O4/SiO2 nanocomposites (NCs) and of secondary phases as a result of the variation of Cu/Co molar ratios were investigated by powder X-ray diffraction analysis (PXRD) and FT-IR. The particle size and shape were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). By varying the Cu/Co molar ratios, the dependence of phase specific parameters (crystallite size, cell parameter, density, porosity) and the evolution of particle shape and size at 1000 °C were investigated by TEM. The photocatalytic activity of the NCs by increasing the Cu content was evaluated through the degradation of Rhodamine B under UV-Vis light irradiation.

The effect of alloying elements on cementite coarsening during martensite tempering

The effects of anti-segregating elements (Si and Al) and segregating elements (Cr and Mo) on cementite precipitation during tempering of Fe-C-Mn martensites have been quantitatively investigated in terms of both cementite chemistry and particle size evolution. Si and Al additions both accelerate the Mn partitioning kinetics and decelerate the particle growth kinetics. They are trapped in cementite during the initial stage of precipitation but are rejected from cementite after prolonged tempering. By comparison, small Cr and Mo additions do not strongly affect the Mn partitioning kinetics or particle growth kinetics, even though both Cr and Mo partition to cementite. The experimental measurements of cementite coarsening with time-dependent chemistry evolution are compared with a cementite coarsening model recently proposed by Wu et al. that has been generalised to multicomponent steels. The model can describe well the partitioning kinetics of segregating elements Mn/Cr/Mo and cementite particle growth kinetics simultaneously in a range of higher order systems. At 600 °C, the model works very well under local equilibrium (LE) and volume diffusion assumptions. The retardation by Si/Al on cementite coarsening and resulting accelerated Mn partitioning can be understood by the kinetic barrier from the migration of Si/Al spikes in front of growing cementite. At lower temperatures of 500 °C and 400 °C, the model can also reasonably well describe the Mn/Cr/Mo partitioning kinetics (and particle growth kinetics). However, to better describe the particle growth kinetics, the interfacial condition has to be treated with a deviation from LE due to Si/Al solute trapping. The Cr and Mo mobilities in cementite are enhanced by the same factor as for Mn mobility in cementite previously tuned by Wu et al., and the simulated Mn/Cr/Mo partitioning kinetics indicate these substitutional diffusion data are reasonably well estimated, which is further demonstrated in the simulation of Al rejection kinetics.

Spatio-temporally resolved investigations of layered particle growth in a reactive argon-acetylene plasma

The growth of nanometer-sized particle populations is studied in a reactive argon-acetylene plasma. Particular attention is paid to the spatio-temporal evolution of nested particle populations. For this purpose, a new non-invasive 2D imaging ellipsometry diagnostic is applied in combination with laser extinction measurements. In this way, the spatio-temporal evolution of particle size and density is obtained. The temporal evolution of the particle clouds during the first and subsequent growth cycles reveals a complex interplay between the different particle generations, the plasma and the resulting dust confinement. It is demonstrated that the electrostatic pressure needs to be considered in the force balance of systems with high dust densities. The relationship between the growth of particles and characteristic transition events visible in the dynamic of the void is discussed.

Using A Spin-Coater to Capture Adhesive Species during Polydopamine Thin-Film Fabrication.

Spin-coating was evaluated as a technique to study events that occur during polydopamine (PDA) thin-film formation. The reaction variants studied included type of oxidant, dopamine (DA) concentration, pH, adhesion time prior to spin, substrate chemistry, and notably, DA solution aging time. A strong oxidant, sodium periodate (SP), and a weak oxidant, atmospheric oxygen were chosen. It was found that reactions in solution were much faster and produced much thicker PDA films with SP than with oxygen. PDA thickness correlated positively with DA concentration, SP solution pH, and adhesion time. DA oxidation and aggregation is a dynamic process, which is reflected in the DA aging-time parameter. PDA film thickness reached a maximum value as DA solution aged. Color photography, UV-vis spectroscopy, and dynamic light scattering indicated that the optimal DA aging time for PDA adhesion is the result of the evolution of PDA particle size and chemistry over time. The capture of the optimal aging-time window was identified as the critical parameter for preparing PDA films with continuity and appreciable thickness. When these conditions were applied in a modified dip-coating method, comparable PDA films were fabricated as those obtained from spin-coating. Native silicon wafers (SiO2) as well as wafers that were modified with polydimethylsiloxane (PDMS) and amine-containing polydimethylsiloxane (PADMS) were chosen to represent a wide range of substrates with different substrate-PDA interactions. The main effect of substrate structural difference was on PDA film morphology. "Island" morphologies were obtained on PDMS where only hydrophobic interactions are responsible for PDA adhesion, while "speck" morphologies were observed on SiO2 and PADMS. The stabilities of the fabricated PDA films were tested in 0.1 M HCl and DMSO. The SP-derived PDA films exhibited very little mass loss compared to those fabricated using either the conventional dip-coating method or oxygen as an oxidant. Choosing a strong oxidant, understanding the DA reaction dynamics, and taking advantage of the optimal DA aging time are important in the fabrication of stable PDA films on a variety of substrates.