Colloidal Films Research Articles

How Particle Deformability Influences the Surfactant Distribution in Colloidal Polymer Films.

The distribution of surfactants in waterborne colloidal polymer films is of significant interest for scientific understanding and defining surface properties in applications including pressure-sensitive adhesives and coatings. Because of negative effects on appearance, wetting, and adhesion, it is desirable to prevent surfactant accumulation at film surfaces. The effect of particle deformation on surfactant migration during film formation was previously investigated by Gromer et al. through simulations, but experimental investigations are lacking. Here, we study deuterium-labeled sodium dodecyl sulfate surfactant in a poly(butyl acrylate) latex model system. The particle deformability was varied via cross-linking of the intraparticle polymer chains by differing extents. The cross-linker concentration varied from 0 to 35 mol % in the copolymer, leading to a transition from viscoelastic to elastic. Ion beam analysis was used to probe the dry films and provide information on the near-surface depth distribution of surfactant. Films of nondeformable particles, containing the highest concentration of cross-linker, show no surfactant accumulation at the top surface. Films from particles partially deformed by capillary action show a distinct surfactant surface layer (ca. 150 nm thick). Films of coalesced particles, containing little or no cross-linker, show a very small amount of surfactant on the surface (ca. 20 nm thick). The observed results are explained by considering the effect of cross-linking on rubber elasticity and applying the viscous particle deformation model by Gromer et al. to elastically deformed particles. We find that partially deformed particles allow surfactant transport to the surface during film formation, whereas there is far less transport when skin formation acts as a barrier. With elastic particles, the surfactant is carried in the water phase as it falls beneath the surface of packed particles. The ability to exert control over surfactant distribution in waterborne colloidal films will aid in the design of new high-performance adhesives and coatings.

Assembly, Properties, and Application of Ordered Group II–VI and IV–VI Colloidal Semiconductor Nanoparticle Films (Adv. Mater. Interfaces 28/2022)

Colloidal Semiconductor Nanoparticle Films In article number 2201039, Maria Wächtler and co-workers summarize recent progress of semiconductor nanoparticle self-assembly into functional thin films for integration in hybrid materials and devices. They compare various deposition strategies and discuss how precise control over nanoparticle order and orientation during assembly can result in unique optoelectronic properties, which can be tailored to potential applications.

Numerical simulation of internal electric field distribution in colloidal photonic crystal thin films

We have computed the internal electric field distribution in monolayer, bilayer, and trilayer colloidal photonic crystal thin films on a glass substrate. The field distributions are evaluated on the top photonic crystal layer under the effects of additional layers along the direction of light propagation We have also investigated the field distributions in photonic crystal thin films at on-resonance and off-resonance incident wavelengths. The calculated reflection and transmission spectra of photonic crystal thin films help us choose the right wavelengths for determining the on-resonance and off-resonance conditions. The electric fields across different regions in the crystal plane are evaluated for a quantitative understanding of the field distribution. We have observed that the internal electric field gets redistributed in the plane of a close-packed periodic array of colloids. Our results reveal that the internal electric field is localized and enhanced in the air void region between the dielectric spheres with nearly the same maximum field values for the on-resonance wavelength. We have observed that at off-resonance wavelength, the field gets localized within the dielectric sphere region for the monolayer and bilayer cases. Contrastingly, the field distribution in the trilayer case forms a connecting network within the photonic crystal plane. Under off-resonance wavelength, the trilayer shows a huge enhancement in maximum field values compared to the monolayer and bilayer thin films. These results promise to employ photonic crystal thin films of colloidal dielectric spheres as templates for sensing applications using the surfaced enhanced Raman scattering technique.

Extracting Bulk-like Semiconductor Parameters from the Characterization of Colloidal Quantum Dot Film Photoconductors

Extracting Bulk-like Semiconductor Parameters from the Characterization of Colloidal Quantum Dot Film Photoconductors

Non-gelated polymeric photonic crystal films

A rapid curing method for the preparation of colloidal photonic crystal films is presented. Firstly, a colloidal crystal array template was prepared by self-assembly of nanospheres, and then a dilute polymer solution was poured into the gap of the template. Then the composite photonic film was obtained as the polymer solution was cured. Such films have good properties in mechanical strength, anti pH interference, rapid solvent response and are easy to preserve. The films show good linear response to ethanol aqueous solutions of different concentrations, and the response equilibrium takes less than 20 s. The films also show long-term stability and reusability, and further functionalization can make the films multi-sensitive.

Real-time kinetics and affinity analysis of the interaction between protein A and immunoglobulins G derived from different species on silica colloidal crystal films

Kinetic and affinity analysis of protein interactions reveals information on their related activities in biological processes. Herein, we established a system for evaluating the kinetics and affinity of the interaction between protein A and various IgG species on the surface of silica spheres of silica colloidal crystal (SCC) films by the extraordinary optical interference capabilities of 190 nm silica spheres after self-assembly. The equilibrium association constant (KA) was calculated by the equilibrium Langmuir model and nonlinear least-squares analysis of time-dependent data. The relative protein A/IgG binding affinity is human > rabbit >cow >goat. In addition, the competitive interaction of distinct species of IgG with protein A at the interface of SCC films was studied and performed. These findings may help with the use of protein A and other recognition components in a number of sensor types. Furthermore, this research might offer a novel approach to determining the kinetics and affinity of proteins on the surface of spheres particles, which may contribute to the development of the application of spheres particles in pharmaceutical science, biomedical engineering, and other techniques.

Application of Polymer Nanocolloid Preparation in Stability Analysis of Motion Mechanics

Nanomaterials are widely used in various fields because of their own advantages. When the particle size of the material is reduced to the nanometer level, it will lead to new characteristics of acoustic, optical, electrical, magnetic, and thermal properties. This will greatly enrich the research content of the material and is expected to get new uses. Therefore, the preparation technology of nanomaterials is one of the current research hotspots and has broad application prospects. At present, the most commonly used preparation techniques are hydrothermal method and sol-gel method, but not all crystals grow in a hydrothermal environment, so this paper adopts the sol-gel method for preparation. However, in the preparation of nanocolloids, agglomeration often occurs between colloidal nanoparticles. In order to avoid the agglomeration between particles, this paper adopts an appropriate method to disperse the particles. In order to further explore the stability of nanocolloids, the colloidal film material prepared by the cadmium sulfide (CdS) sol method was used as the research sample. It also proposes a sol preparation method incorporating ultrasonic waves and further discusses the kinetic characteristics. The research results show that the sedimentation velocity of 1 nm cadmium sulfide nanocolloids in water is 1.3 × 10−12 (m/s) prepared by colloid of cadmium sulfide.

Mie-Resonant Rutile TiO2 Nanospheres for Unclonable Structural Colors

Monodisperse carbon-embedded rutile TiO2 nanospheres were synthesized through the sol–gel reaction of titanium alkoxide in concert with dodecylamine and the subsequent thermal annealing process in inert gas. Since dodecylamine was uniformly carbonized in nanospheres, the growth of rutile crystals was limited to ∼50 nm in the thermal annealing process, which enabled us to keep the spherical shape of nanospheres. Due to the size and shape uniformity, colloidal films of these rutile TiO2 nanospheres showed unique Mie-resonant structural colors, in which multiple scattering was suppressed due to carbon embedded in nanospheres. Due to the high refractive index of rutile TiO2, there were multiple backscattering peaks in the visible region, which make it extremely difficult to replicate the colorimetric information when they were utilized as encryption inks. Finally, we demonstrated that IR-reflective carbon-embedded rutile TiO2 nanospheres can be used as automotive paints, which can be distinguishable at night for light detection and ranging (LiDAR) systems.

Temperature-dependent transition of charge transport in core/shell structured colloidal quantum dot thin films: From Poole–Frenkel emission to variable-range hopping

Solution-processed core/shell quantum dot films are of great significance for light-emitting diodes. It is well known that the operation of core/shell quantum dot-based light-emitting diodes largely relies on charge transport. However, the charge transport mechanism in quantum dot films is still under debate and inconclusive. Herein, the temperature-dependent charge transport properties of light-emitting core/shell colloidal quantum dot thin films are characterized and analyzed across a wide temperature range, and the charge transport mechanism is studied. The results reveal that Poole–Frenkel emission conduction is applicable in the high-temperature range. With the decrease in the temperature, the measured current can be described by the Efros–Shklovskii variable-range hopping model. It is worth noting that, in both cases, trap states and disorders in the quantum dot film play a very important role in charge transport. These findings are of great importance for optimizing quantum dot light-emitting diodes and understanding the effects of charge transport on the device performance.

Ultrafast Fabrication of Large‐Area Colloidal Crystal Micropatterns via Self‐Assembly and Transfer Printing

AbstractColloidal crystals have brought the promise of revolution to modern engineering, yet commonly used fabrication technologies are still limited by the small preparation area, time‐consuming process, and dependence on sophisticated equipment. Here, a surface tension gradient‐driven self‐assembly strategy is proposed for the ultrafast fabrication of large‐area colloidal crystals. The hydrogel loaded with sodium dodecyl sulfate is devised to construct a stable and continuous liquid‐air interfacial tension gradient, and the resulting Marangoni effect can drive the micro‐nano particles to instantaneously form (within several seconds) highly ordered colloidal crystals. Benefiting from the long range of surface tension gradients, the fabrication area of colloidal crystal films is demonstrated to exceed an astonishing 1000 cm2 without compromising their quality, showing great potential in scale‐up manufacture. Moreover, particles of a wide variety of sizes, materials, and functionalities can form close‐packed self‐assembly monolayers and be transferred to various substrates without damage, exhibiting great versatility. Inspired by ink microprinting, an ultrafast nanoparticle transfer printing method is further proposed to convert the close‐packed nanoparticle monolayers into large‐area conformal micropatterns with single‐nanoparticle resolution. The great potential of nanoparticle micropatterns is demonstrated in flexible micro‐electronics/skin electronics. This user‐friendly, efficient self‐assembly, and micropatterning strategy provide promising opportunities for academic and real industrial applications.

Heteroepitaxial Growth of Colloidal Crystals: Dependence of the Growth Mode on the Interparticle Interactions and Lattice Spacing.

Epitaxial growth is one of the most important techniques for the control of crystal growth, especially for growing thin-film semiconductor crystals. Similarly, colloidal epitaxy, a template-assisted self-assembly method, is a powerful technique for controlling the structure of colloidal crystals. In this study, heteroepitaxial growth, which differs from homoepitaxial growth of conventional colloidal epitaxy, using foreign colloidal crystals as a substrate, was used to grow single-component colloidal crystal films. The Frank-van der Merwe (FM), Stranski-Krastanov (SK), and Volmer-Weber (VW) modes were observed, and the mode varied with the lattice-misfit ratio and interparticle interactions between the substrate and epitaxial phase. The transition of the growth mode (from SK to VW) and the coexistence of different growth modes (FM and VW) were observed, and their processes were revealed by in situ observation. Colloidal heteroepitaxy was confirmed to be useful for controlling structure, which will enable exploration of novel colloidal self-assembly structures.

Influence of relative humidity and temperature on human whole blood drying

The drying of human whole blood pools has been studied experimentally to improve the understanding of drops and pools of human whole blood. Here blood is assimilated to a complex fluid made of several bio-colloids. The blood pools were created inside a glove box under controlled conditions. We performed experiments at temperatures of 21C, 29C, and 37C, and relative humidity ranging from 20% to 70%. The air inside the glove box was still, and the evaporation process was mainly purely diffusive. A human blood diffusion coefficient of was obtained. This value is in agreement with literature values for polymers and colloidal films. Moreover, we elaborated an updated model using Page’s model to describe the drying kinetics of human blood pools. The effect of the surface area of pools was considered using a diffusive characteristics time. These findings are of interest for biomedical and forensic applications.

Electron transport mechanism in colloidal SnO2 nanoparticle films and its implications for quantum-dot light-emitting diodes

Charge transport behavior in SnO2 nanoparticle (NP) films is rather crucial to the optoelectronic devices. Temperature-dependent electrical results show that the electron transport in SnO2 NP films is dominated by the Mott variable-range hopping processes, i.e. the electrons are transported between different NPs through surface states rather than the conduction band of the nanocrystals, which is identical to the commonly used ZnO NP solids. Compared with ZnO, SnO2 films exhibit similar electron mobility but lower density of states (DOS). Therefore, we deduce that the low DOS in the SnO2 NP films should be the key factor limiting the device performance in compared with the ZnO as reported in most of the quantum-dot light-emitting diodes (QLEDs). Our work sheds light on optimizing SnO2 NP films for QLEDs. Moreover, we believe that the SnO2 remains a desirable candidate as the electron transport material for the QLEDs due to its excellent physicochemical stability.

Silver Nanoparticle Films Obtained by Convective Self-Assembly for Surface-Enhanced Raman Spectroscopy Analyses of the Pesticides Thiabendazole and Endosulfan.

Pesticides pose a great threat to human health and their rapid detection has become an urgent public safety issue engaging the scientific community to search for fast and reliable detection techniques. In this context, Surface Enhanced Raman Spectroscopy (SERS) has emerged as a valuable detection and analysis tool due to its high sensitivity and selectivity, proving its suitability for the food industry and environmental monitoring applications. Here, we report on the fabrication of colloidal silver nanoparticle (AgNP) films by convective self-assembly (CSA) on solid planar substrate and their use for the SERS analyses of two types of pesticides, the fungicide thiabendazole (TBZ) and the insecticide α-endosulfan (α-ES). Electron microscopy shows that these nanoparticle films are dense, highly compact, and uniform across several mm2 areas. The SERS efficiency of the fabricated AgNP films is evaluated using a well-known Raman probe, p-aminothiophenol, for multiple excitation laser lines (532 nm, 633 nm, and 785 nm). The films exhibit the largest SERS enhancement factors for 785 nm excitation, reaching values larger than 105. Thiabendazole could be readily adsorbed on the AgNPs without any sample surface functionalization and detected down to 10−6 M, reaching the sub-ppm range. Endosulfan, a challenging analyte with poor affinity to metal surfaces, was captured near the metal surface by using self-assembled alkane thiol monolayers (hexanethiol and octanethiol), as demonstrated by the thorough vibrational band analysis, and supported by density functional theory (DFT) calculations. In addition, principal component analysis (PCA) based on SERS spectra offers significant leverage in discrimination of the molecules anchored onto the metallic nanostructured surface. This present study demonstrates the utility of self-assembled colloidal nanoparticle films as SERS substrates for a broad range of analytes (para-aminothiophenol, thiabendazole, α-endosulfan, and alkanethiols) and contributes to the development of SERS-based sensors for pesticides detection, identification and monitoring.

Diagnostic utility of conventional and liquid-based cytology in the management of thyroid lesions; an institutional experience.

Objectives:Liquid-based cytology (LBC) is becoming a useful method in evaluating both gynecological and nongynecological preparations. Many studies have been conducted these days to see the quality of LBC in diagnosing thyroid lesions and its role is yet to be standardized. This study has been done to pick the cytomorphological features of thyroid lesions on LBC and to compare them with the conventional cytology of these lesions.The objective of the study is to evaluate the efficacy of thyroid fine-needle aspiration cytology processed by LBC as compared to conventional smears.Material and Methods:A prospective study has been done on 53 cases of thyroid lesions using standard conventional and LBC techniques.Results:In most of the cases, the cytological features are similar in conventional cytology smears and LBC, but the colloid film and the lymphocytic component show a clear picture on direct smears, whereas nuclear and cytoplasmic details are better evaluated in LBC slides. The preservative solution used in LBC could be efficiently used for the application of immunocytochemical and molecular techniques.Conclusion:LBC does not represent as a valid alternative to conventional cytology but it can be considered as a diagnostic utility process especially due to the possibility of applying additional techniques which enhance the efficacy of the cytological diagnosis of thyroid lesions.

Voltage-controlled reversible modulation of colloidal quantum dot thin film photoluminescence

Active modulation of quantum dot thin film photoluminescence (PL) has been far-reaching potential applications in biomedical and optoelectronic systems, but challenges remain in achieving large PL modulation depth and fast temporal response. Here, we report an efficient voltage-controlled optical down-converter by optically exciting a colloidal quantum dot thin film within a quantum dot light-emitting diode under reverse bias. Utilizing field-induced luminescence quenching, we show that a large electric field can strongly modify carrier dynamics in this nanostructured device, resulting in stable and reversible photoluminescence quenching. The device exhibits photoluminescence reduction of up to 99.5%, corresponding to a contrast ratio of 200:1 under the applied electric field of 3 MV cm−1 with a 300 ns response time. Using excitation wavelength dependent and transient PL spectroscopy, we further show that the high degree of quenching is achieved by a synergistic interplay of quantum-confined Stark effect and field-induced exciton dissociation.

Measurement of volume fraction distribution in a drying film by imaging with a digital camera

We propose a method to measure the evolution of the volume fraction distribution in colloidal films during their drying process, using only the transmitted light intensity recorded with a commercial digital camera. From the Lambert-Beer law, the volume fraction of a dispersion film with a certain film thickness can be measured from the transmitted light intensity. A pseudo-absorption capacity was imparted to the transparent dispersion by adding a red dye, and the ratio of green to blue light absorbance was measured for the liquid film. For any film, the volume fraction increased spatially towards a compaction front, and the volume fraction in this region reached approximately 63.4%. The volume fraction inside the liquid region of the film also increased during drying. These trends are similar to previous results obtained using other methods. The magnification in this method can be easily changed by replacing the imaging equipment, and changes in volume fraction distribution near the drying front were observed at high magnification using different lenses. This method, based on captured photographic images, measure the two-dimensional distribution of the volume fraction, making it easy to observe changes in the volume fraction distribution caused by drying at both macroscopic and microscopic levels. It is likely that the proposed method will be difficult to apply to anisotropic particles or to larger particles which cause significant scattering.

Colloidal exchange flow in ducts

Buoyancy-driven exchange flow, i.e. drainage of a heavy colloidal suspension by an immiscible light pure fluid in a vertical duct, is studied analytically. A lubrication model is developed for two-dimensional channel and axisymmetric pipe geometries. The outcome transport equations are solved numerically to capture the effect of important governing dimensionless parameters: Péclet number, precursor film thickness, Reynolds number, mixtures’ viscosity ratio, initial volume fraction of hard-sphere particles, particles concentration within the precursor film, and capillary number. Recent study of colloidal film flow over a flat surface reveals interface thinning over time due to diffusion impact on suspension viscosity. However, the current model suggests that there is no such thinning phenomenon observed for colloidal flows within confined geometry unless in the absence of surface tension. The extent of the mixed particle zone along the duct grows as the Péclet number is decreased. Precursor film thickness is found to decrease the capillary ridge height forming at heavy suspension front. Either an increase in the Reynolds number or a decrease in light-to-heavy-fluid viscosity ratio extends the interpenetration rate of mixtures. As the bulk volume fraction of particles is increased close to 50 %, interesting secondary fronts emerge within the flow that coincide with the range where the diffusion coefficient is minimized. The heavy suspension layer drains slightly faster when the gradient in particle concentration between the bulk and precursor film regions is maximum. Finally, the non-monotonic impact of surface tension on the extent of mixtures’ exchange zone is uncovered via simulations conducted at various capillary numbers.

Introduction of a planar defect into colloidal photonic-crystal films and their optical properties

A defect mode in the bandgap of photonic crystals is a key factor for potential applications, emission, bandpass filter, sensor, and low throughput laser. A Fabry-Perot type cavity was known as a multilayer film with a planar defect or a one-dimensional colloidal photonic crystal film with a planar defect. In this work, we have developed a simple and easy method by two colloidal crystals, i.e., face center cubic (fcc) structure, films bonded together by hot pressing to form a sandwich structure, and clear defect mode was observed in the photonic bandgap of fcc (111) direction. We have investigated the effect of the thickness of the defect layer in the sandwich structure on the optical properties. A single or double dips appeared in Bragg's diffraction peak at different planar defect thicknesses. In addition, a simulation of the reflection spectra of multilayer film calculation showed the defect mode is much influenced by the planar defect thickness.

Effect of Particle Interactions on the Assembly of Drying Colloidal Mixtures.

The effects of particle interactions on the size segregation and assembly of colloidal mixtures during drying were investigated. A cationic surfactant was added to a binary latex/silica colloidal dispersion that has been shown to self-stratify upon drying at room temperature. Atomic force microscopy was used to show that the change in particle interactions due to the presence of surfactants reduced the degree of stratification and, in some cases, suppressed the effect altogether. Colloidal dispersions containing higher surfactant concentrations can undergo a complete morphology change, resulting instead in the formation of armored particles consisting of latex particles coated with smaller silica nanoparticles. To further prove that armored particles are produced and that stratification is suppressed, cross-sectional images were produced with energy-dispersive X-ray spectroscopy and confocal fluorescence microscopy. The growth of armored particles was also measured using dynamic light scattering. To complement this research, Brownian dynamics simulations were used to model the drying. By tuning the particle interactions to make them more attractive, the simulations showed the presence of armored particles, and the size segregation process was hindered. The prevention of segregation also results in enhanced transparency of the colloidal films. Overall, this research proves that there is a link between particle interactions and size segregation in drying colloidal blends and provides a valuable tool to control the assembly of different film architectures using an extremely simple method.