Dynamics of drying colloidal suspensions, measured by optical coherence tomography.

Colloidal suspensions are the basis of a wide variety of coatings, prepared as liquids and then dried into solid films. The processes at play during film formation, however, are difficult to observe directly. Here, we demonstrate that optical coherence tomography (OCT) can provide fast, non-contact, precise profiling of the dynamics within a drying suspension. Using a scanning Michelson interferometer with a broadband laser source, OCT creates cross-sectional images of the optical stratigraphy of a sample. With this method, we observed the drying of colloidal silica in Hele-Shaw cells with 10 µm transverse and 1.8 µm depth resolution, over a 1 cm scan line and a 15 s sampling period. The resulting images were calibrated to show how the concentration of colloidal particles varied with position and drying time. This gives access to important transport properties, for example, of how collective diffusion depends on particle concentration. Looking at early-time behaviours, we also show how a drying front initially develops, and how the induction time before the appearance of a solid film depends on the balance of diffusion and evaporation-driven motion. Pairing these results with optical microscopy and particle tracking techniques, we find that film formation can be significantly delayed by any density-driven circulation occurring near the drying front.

Position-dependent rates of film growth in drying colloidal suspensions on tilted air-water interfaces.

Suspended particles in a solvent form a packed film when the solvent evaporates. We investigated film growth rates in a narrow channel on a tilted drying interface and observed clear differences in the rates of film growth. Films grew faster at one end and slower at the other; thus, the slope of the packing front, i.e., the boundary between the packed film and the drying suspension, changed as drying proceeded. However, the difference in film growth rates became smaller as the slope of the packing front changed and the rates of film growth at the either end ultimately became identical. We found that the differences in the rates of film growth were proportional to cosθ, where θ is the angle defined by the slope of the packing front. We constructed a mathematical description to successfully express the time evolution of both the difference in the growth rates and the packing front angle θ. Relationships between drying-induced flow of bulk suspensions and transport of suspended particles to the tilted packing front are discussed.

Correction: Drying-induced back flow of colloidal suspensions confined in thin unidirectional drying cells.

[This corrects the article DOI: 10.1039/D0RA02837A.].

Buckling and Drying Kinetics of Particle-Stabilized Water Droplets Fully or Partially Immersed in an Oil Layer.

We have investigated the effect of buckling of particle-stabilized water droplets on the drying kinetics. Particle-stabilized water droplets in an oil phase were prepared and the shrinking modes of the droplets during drying were controlled by the wettability of the particles. We obtained water droplets with and without buckling and used them in drying experiments. The drying times were comparable when the droplets were fully immersed in a thick oil layer. However, when the thickness of the oil layer was smaller than the droplet diameter, the buckled droplets showed faster drying. Observation of the reflection images around the droplets suggested that the buckled droplets preferentially shrank in the height direction, while the droplets without buckling isotropically shrank. Mathematical models that assumed diffusion of dissolved water molecules in the oil layer showed good agreement with the experimental data. The effective water-oil interfacial area was constant in the buckled droplets, whereas it shrank in the droplets without buckling. This would be a reason for the faster drying of the partially immersed buckled droplets. Particulate shells on liquid droplets could be used to enhance droplet drying.

Scaling law for the kinetics of water imbibition in polydisperse foams.

We investigated the kinetics of water imbibition in polydisperse foams. We used a Hele-Shaw cell, and horizontal imbibition was observed for a timescale of up to 103 s in which the gravity effect was negligible. While several papers have reported kinetics for imbibition in foams, imbibition kinetics in polydisperse foams and its variations in longer timescales are not well understood. The tip position of imbibition was proportional to the square root of time in the initial stage of imbibition, but it showed plateauing in the late stage of imbibition. We evaluated the proportional constant A in the initial stage of imbibition as a kinetic constant for the time-dependent increase in the tip position, which showed a clear dependency on the initial and final water volume fractions in the foams. Conversely, the mean initial radius of the curvature and the channel length in the Plateau borders did not show any clear correlations with A, although both valuables are frequently used in modeling for liquid imbibition in foams. On the basis of the t 1/2 dependence, the correlation of A with the water volume fraction and the increase in the water volume fraction during imbibition, we proposed a simple equation to describe the tip position over the entire period of imbibition. We used them to scale all of the experimental data, which showed good agreement with the theoretical line. This clearly showed that the water volume fraction in the foams during imbibition was the key factor to quantitatively describe the rate of water imbibition. Features in the kinetics of imbibition were discussed.

Evaporation kinetics of continuous water and dispersed oil droplets.

Drying of volatile oil droplets immersed in a continuous water phase was observed and analysed. Drying sample solutions were sandwiched between two glass plates and the water and oil phases were observed by confocal microscopy. In the initial stage of drying, evaporation of water was dominant and drying of the oil droplets was negligible. However, the rate of water evaporation decreased when the oil droplets were compressed. Comparison of experimental data with a diffusion model of water vapour showed that the decline in drying rates occurred earlier in the experiment than in the theoretical prediction. This implies that compression and narrowing of water paths caused the decline in the rate of water evaporation. After most water had evaporated, evaporation of the oil droplets occurred. The oil droplets did not shrink isotropically and the air-liquid interface invaded into the drying oil droplets. Cross-sectional observation by z-scanning revealed direct exposure of the oil droplets and they were pinned by the residual water phase. The water network between the oil droplets collapsed after the oil droplets had evaporated. The correlation between changes in structures and drying kinetics in both liquid phases was discussed.

Drying-induced back flow of colloidal suspensions confined in thin unidirectional drying cells.

A clear back flow was observed in the thin unidirectional drying cell of a colloidal suspension. Flow around the colloidal-particle packing front was more complex than expected, even though a colloidal suspension was confined in a narrow space with a submillimeter-scale or shorter gap height. We propose that an increase in particle concentration around the packing front induces downward flow, which is the origin for back flow inside the cell. A mathematical model, which considered both a drying induced horizontal flow and a circulation flow caused by a concentration gradient of particles, showed a reasonable agreement with experimental data for the width of the back-flow region. The concentration gradient of particles was not negligible and it generated a rather complicated flow even in a thin drying liquid film.

Positive and negative birefringence in packed films of binary spherical colloidal particles.

We have investigated the birefringence in packed films of binary spherical colloidal particles. Particulate films were obtained by drying a mixed suspension of colloidal particles with two different diameters. We observed positive and negative birefringence depending on the diameters and volume ratios of the large and small particles. When the diameters of the large and small particles were similar, the films showed positive birefringence. However, negative birefringence or weakening of positive birefringence was observed in films with a large diameter ratio and an optimal volume fraction of large particles. The large particles were embedded in packed small particles in the negative and weakened positive birefringent films. We propose a packing structure in which a single shell layer of small particles formed around a large particle. Using this model, we estimated the required volume ratio of large particles, and it was in good agreement with the optimal volume fraction. The relation between the packing structure of the binary colloidal particles and the birefringence is discussed.

Flow of condensed particles around a packing front visualized by drying colloidal suspensions on a tilted substrate.

A gravity effect was demonstrated for 10 nm particles drying in colloidal suspensions. The particles were well-dispersed and did not sediment. However, when a suspension was dried on a tilted directional cell, a clear downward flow of particles was observed around the packing front, which was the boundary between the packed particles layer and the suspension. Three particle sizes (10-110 nm) were examined, with the most pronounced effect being on the 10 nm particles. The primary origin of the downflow was attributed to condensation of particles near the packing front and the subsequent increase in the overall density of the condensed layer. Because of the flow, the packing front was not parallel to the drying interface and tilted cracks formed in the packed layer. A mathematical model was proposed that considered conservation of the suspended particles in the condensed layer. Three competing factors of particle transport (advection, particle consumption by packing, and particle transport by the downward flow) were used to explain the experimental results. Overall, the results suggested that simple substrate tilting would be useful to evaluate whether suspended particles are easily packed or not during drying.

A quantitative study of enhanced drying flux from a narrow liquid-air interface of colloidal suspensions during directional drying.

We evaluate an enhancement of the drying flux from a narrow drying interface. We examine drying of colloidal suspensions in a directional drying cell with various combinations of width and height to change the area of the drying interface. The drying flux changes depending on the height or width of the drying interface. A simple scaling law describes the experimental data quantitatively. Using this effect, we systematically change the drying flux and analyze the formation kinetics of particulate films. There is a threshold drying flux for immediate film formation after the beginning of drying. In addition, the threshold drying flux depends on the initial volume fraction of particles. A kinetics model based on the conservation of particles explains the threshold well.

Drying kinetics of water droplets stabilized by surfactant molecules or solid particles in a thin non-volatile oil layer.

We have investigated drying of water droplets stabilized by solid particles or surfactant molecules in a thin oil layer. The surfactant-stabilized droplets isotropically shrink, whereas the droplets stabilized by spherical particles severely deform during drying because of buckling of the particulate shells. However, buckling of the shells hardly affects droplet drying. The drying times for complete evaporation are almost the same for water droplets with the same initial diameter and the drying time is independent of the type of surface stabilizer (particles or surfactant). The drying kinetics of the water droplets is well described by mathematical models, in which diffusion of water molecules in the oil phase to the oil-air interface is proposed as the rate-determining process. Droplets with a diameter comparable with the thickness of the oil layer shrink faster than small droplets because of the short diffusion length from the water droplets to the oil-air interface. We also investigated drying of water droplets stabilized by plate-like mica particles. The droplets also buckled but larger shells of mica particles remained compared with those of spherical particles. In addition, a longer drying time is necessary for some droplets stabilized by mica particles. These results indicate the possible effect of the particle morphology on the buckling and drying kinetics of particle-stabilized water droplets.

Aggregates of silicon quantum dots as a drug carrier: selective intracellular drug release based on pH-responsive aggregation/dispersion.

Using amine-modified silicon quantum dots (Si-QDs) with visible photoluminescence as a building block, drug-loaded Si-QD aggregates were assembled. The aggregates were designed to break down in response to the endosomal pH decrease, which enabled the selective intracellular release of the loaded drugs.

Optical anisotropy in packed isotropic spherical particles: indication of nanometer scale anisotropy in packing structure.

We investigated the origin of birefringence in colloidal films of spherical silica particles. Although each particle is optically isotropic in shape, colloidal films formed by drop drying demonstrated birefringence. While periodic particle structures were observed in silica colloidal films, no regular pattern was found in blended films of silica and latex particles. However, since both films showed birefringence, regular film structure patterns were not required to exhibit birefringence. Instead, we propose that nanometer-scale film structure anisotropy causes birefringence. Due to capillary flow from the center to the edge of a cast suspension, particles are more tightly packed in the radial direction. Directional packing results in nanometer-scale anisotropy. The difference in the interparticle distance between radial and circumferential axes was estimated to be 10 nm at most. Nanometer-scale anisotropy in colloidal films and the subsequent optical properties are discussed.

Real time observation and kinetic modeling of the cellular uptake and removal of silicon quantum dots.

The time courses of uptake and removal of silicon quantum dots (Si-QDs) by human umbilical endothelial cells (HUVECs) were observed via confocal laser scanning microscope. Si-QDs were internalized via endocytosis and transported to late endosomes/lysosomes. The number of internalized Si-QDs increased with time and gradually reached a plateau value. When Si-QD-internalized HUVECs were subsequently washed and exposed to fresh culture medium, HUVECs removed internalized Si-QDs via exocytosis. The number of internalized Si-QDs decreased with time and gradually reached a plateau value. Not all internalized Si-QDs were removed from the cell interior but large numbers of internalized Si-QDs remained accumulated inside cells. A kinetic model based on the mass balance of Si-QDs and receptors in a cell was proposed to describe the cellular uptake and removal of Si-QDs. Model calculation fitted well with experimental results. Using this model, the dissociation constant between receptors and Si-QDs in the endosome, K(d,in), was found to be a determinant factor for Si-QD accumulation in cells after the removal process.

Selective labeling of the endoplasmic reticulum in live cells with silicon quantum dots.

A simple and novel approach was developed to obtain water-dispersible silicon quantum dots (Si-QDs) of low toxicity that were able to selectively label the endoplasmic reticulum (ER) in live cells. A block copolymer (Pluronic F127) was used to coat the surface of Si-QDs. Si-QDs form aggregates with diameters of 20-40 nm and show an outstanding optical stability upon UV irradiation. Our F127-treated Si-QDs would be a powerful tool for long-term real-time observation of the ER in live cells.

Characterization of flowerlike silicon particles obtained from chemical etching: visible fluorescence and superhydrophobicity.

Flowerlike silicon particles are obtained by chemical etching of polycrystalline silicon polyhedrons using a mixture of hydrofluoric acid and nitric acid. The etched flowerlike particles show stable bright red photoluminescence under UV irradiation. The formation of pores with diameters of 3, 5.5, and 20 nm is revealed during etching. The etched particles exhibit superhydrophobic behavior with a contact angle of 158 degrees because of the sharp tips of their "petals". The source silicon polyhedrons are shown to possess radial grain boundaries. Preferential etching along the radial grain boundaries of the polyhedrons is thought to be the key reason for the formation of flowerlike porous silicon particles.

Formation of optically anisotropic films from spherical colloidal particles.

Colloidal silica films, formed by the drop evaporation method, showed birefringent spherulite optical properties. They displayed a Maltese cross pattern under crossed polarizers, and interference colors, such as blue and orange-red, under crossed polarizers with a compensator. The difference in refractive index was estimated to be 9x10(-4) from the interference colors. Scanning electron microscopy (SEM) results revealed anisotropic structures in the colloidal films. Particles formed radially ordered hexagonal arrays. The drop evaporation method used in this report, which dries from the edge to the center, resulted in a radially ordered colloidal film. When a colloidal silica film was prepared using a unidirectional drying method, particles were packed in an ordered structure corresponding to the drying direction and the resulting film showed different birefringent optical properties. Our results show that a variety of birefringent films can be obtained from spherical colloidal dispersions through control of the drying method.

Spectroscopic study of laser-induced phase transition of gold nanoparticles on nanosecond time scales and longer.

The pulsed laser induced phase transition of gold nanoparticles in aqueous solution was observed via a transient absorption on nanosecond time scales and longer. Gold nanoparticles were excited with an intense picosecond laser pulse (355 nm, 30 ps), and the subsequent changes were monitored using two continuous wave laser wavelengths (488 and 635 nm). On the nanosecond time scale, below 6.3 mJ cm(-2), no change was observed; however, in the low fluence region between 6.3 and 17 mJ cm(-2), gold nanoparticles produced a bleach signal (488 nm) attributed to the melting of the gold nanoparticles, which decreased linearly with increasing laser fluence. Laser fluences above 17 mJ cm(-2) resulted in a strong absorption at both wavelengths, which is ascribed to vaporization of gold nanoparticles rather than solvated electrons (ejected from gold nanoparticles) or light scattering. The decay of both signals was faster than the 5 ns time resolution used in our experimental system. On the microsecond time scale, increase in absorbance at 635 nm was observed with a time constant of 1.0 micros, while no change was observed at 488 nm. It is considered that this increase is attributed to the formation of smaller gold nanoparticles resulting from pulsed laser induced size reduction of initial gold nanoparticles.

Laser-induced shape transformation of gold nanoparticles below the melting point: the effect of surface melting.

Relatively large gold nanoparticles (mean diameter of major axis 38.2 nm, mean aspect ratio 1.29) in aqueous solution were found to undergo shape transformations from ellipsoids to spheres at ca. 940 degrees C, which is much lower than their melting point, ca. 1060 degrees C. The shape transformation of gold nanoparticles induced by a single pulse of a Nd:YAG laser (lambda = 355 nm, pulse width = 30 ps) was directly observed by a transmission electron microscope (TEM). Analysis of the experimental data showed that the threshold energy for photothermally induced shape transformation was on the order of 40 fJ for a particle, which is smaller than the energy, 67 fJ, required for its complete melting. Estimations based on the heat balance and surface melting model revealed that the temperature which particles reach after a single laser pulse was about 940 degrees C, with the thickness of the liquid layer on the surface of the solid core being 1.4 nm. We also examined thermally induced shape transformation of gold nanoparticles on Si substrates; above 950 degrees C they changed their shapes to spheres, which supported our estimation. Due to the surface melting of particles, their shape transformation occurs at a temperature much lower than their melting point.

Bimodal Size Distribution of Gold Nanoparticles under Picosecond Laser Pulses.

The evolution of size distributions of gold nanoparticles under pulsed laser irradiation (Nd:YAG, lambda = 355 nm, pulse width 30 ps) was carefully observed by transmission electron microscopy. Interestingly, the initial monomodal size distribution of gold nanoparticles turned into a bimodal one, with two peaks in the number of particles, one at 6 nm and the other at 16-24 nm. The sizes for small particles depended very little on the irradiated laser energy. This change is attributed to laser-induced size reduction of the initial gold nanoparticles followed by the formation of small particles. In our analysis, we extracted a characteristic value for the size-reduction rate per one pulse and revealed that laser-induced size reduction of gold nanoparticles occurred even below the boiling point. When laser energy is insufficient for the boiling of particles, formation of gold vapor around liquid gold drops is thought to cause the phenomenon. With enough laser energy for the boiling, the formation of gold vapor around and inside liquid gold drops is responsible for the phenomenon. We also observed particles with gold strings after one pulse irradiation with a laser energy of 43 mJ cm(-2) pulse(-1), which is sufficient energy for the boiling. It is considered that such particles with gold strings are formed by the projection of gaseous gold from liquid gold drops with some volume of liquid gold around the bubble. On the basis of comparison with previous work, picosecond laser pulses are thought to be the most efficient way to cause laser-induced size reduction of gold nanoparticles.