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.

Experimental and theoretical insights into palladium-mediated polymerization of para-N,N-disubstituted aminostyrene.

Experimental and theoretical insights into polymerization of para-N,N-disubstituted aminostyrene monomers (St-4-NR2, R = Me, Et, Ph) using cationic α-diimine palladium complexes have been initially reported. The effects of the catalyst structure and monomer substituent were studied systematically. Polymerization turnover frequency (TOF) was shown to decrease in the order of monomer substituents Me > Et > Ph, whereas the molecular weight of the produced polymers showed an opposite trend (Me < Et < Ph). Methanol-mediated polymerization of para-N,N-dimethylaminostyrene (DMAS), along with polymer chain-end analysis, and palladium intermediate isolation proved that palladium-initiated DMAS polymerization obeyed a cationic mechanism. Comprehensive theoretical calculations further revealed that the carbocation was generated from the insertion of DMAS into the palladium center rather than the polarization of the methyl palladium intermediate with a coordinated DMAS. The produced amine-functionalized amorphous polystyrenes have low stereoregularity and exhibit good hydrophilic properties. The poly(para-N,N-disphenylaminostyrene) is a luminescent polymer and shows fluorescence properties, rendering this material a promising candidate for versatile potential applications.

A study of potential laser-induced degradation in remote standoff Raman spectroscopy for wall paintings.

A mobile remote standoff Raman spectroscopy system operational at typical distances of 10 m was developed specifically for research of historical sites and wall paintings recently. Here we present an upgrade to that system informed by a thorough experimental investigation of the relevant laser-induced degradation issues. Reflectance spectroscopy as a more sensitive technique than Raman spectroscopy was used for monitoring and a new phenomenon of reversible alterations was detected in many paint samples at very low laser intensities of less than 1 W/cm2 when Raman measurements detected no changes. Contrary to conventional wisdom, the intensity threshold for safe operation was found to decrease significantly for larger incident irradiation area in the case of a vermilion oil paint sample. Damage threshold in intensity for each material needs to be determined for different spot sizes, which can be orders of magnitude lower for 1 mm spot size compared with micro-Raman. Results from this study is also relevant to portable Raman systems which use similarly large spot sizes. However, the larger spot size still generates more Raman photons overall under safe operation than micro-Raman systems. Continuous-wave (CW) lasers are found to be best suited to efficient, that is more Raman signal detected over a given measurement time, and safe Raman operation than ns-pulse lasers at the same wavelength. While the damage threshold in intensity for ns-pulse lasers is much higher than that of CW lasers, the pulse energy allowed in one pulse for safe operation is still too low to allow detection of Raman signal, and the need for multiple pulses makes pulse laser inefficient owing to the low repetition rate necessary to ensure adequate heat dissipation between pulses. The safety of the upgraded system was evaluated and found that no permanent laser-induced degradation was detected within 60 s of laser irradiation for any of the paint samples.

Static and Dynamic Optical Analysis of Micro Wrinkle Formation on a Liquid Surface.

A spatially periodic voltage was used to create a dielectrophoresis induced periodic micro wrinkle deformation on the surface of a liquid film. Optical Coherence Tomography provided the equilibrium wrinkle profile at submicron accuracy. The dynamic wrinkle amplitude was derived from optical diffraction analysis during sub-millisecond wrinkle formation and decay, after abruptly increasing or reducing the voltage, respectively. The decay time constant closely followed the film thickness dependence expected for surface tension driven viscous levelling. Modelling of the system using numerical solution of the Stokes flow equations with electrostatic forcing predicted that wrinkle formation was faster than decay, in accord with observations.

Bathochromic-Shifted Emissions by Postfunctionalization of Nonconjugated Polyketones.

Most nontraditional intrinsic luminescent (NTIL) polymers currently show blue fluorescence. Tuning the emission color of NTIL polymers is of fundamental importance for their applications, but it still remains a scientific challenge. Herein, we initially develop an efficient strategy for bathochromic shifting of NTIL polymers by through-space acceptor-donor charge transfer between the in chain and the side chain. A variety of functionalized polyketones (FPK-R; where R = H, Ph, Me, tBu, F, and Cl) with furan rings built into the polymer chain were prepared by the Paal-Knorr reaction. FPK-R polymers showed bright and bathochromic-shifted fluorescence compared with their counterparts. The emission color could be tuned by changing the postfunctionalization conversion and varying the styrenic monomer substituent. Experimental and theoretical investigations revealed that the color tunability originated from enhanced through-space charge transfer between the side chain phenyl and the in chain furan rings.

Standoff Raman spectroscopy for architectural interiors from 3-15 m distances.

Portable and mobile Raman spectroscopy systems are increasingly being adopted in in situ non-invasive examination of artworks given their high specificity in material identification. However, these systems typically operate within centimeter range working distances, making the examination of large architectural interiors such as wall paintings in churches challenging. We demonstrate the first standoff Raman spectroscopy system for in situ investigation of historic architectural interior at distances > 3 m. The 780 nm continuous wave laser-induced standoff Raman system was successfully deployed for the in situ examination of wall paintings, at distances of 3-15 m, under ambient light. It is able to identify most common pigments while maintaining a very low laser intensity to avoid light induced degradation. It is shown to complement our current method of standoff remote surveys of wall paintings using spectral imaging.

Rapid in situ imaging and whole genome sequencing of biofilm in neonatal feeding tubes: A clinical proof of concept.

The bacterial flora of nasogastric feeding tubes and faecal samples were analysed for a low-birth weight (725 g) neonate EGA 25 weeks in intensive care. Samples were collected at age 6 and 8 weeks of life. Optical coherence tomography (OCT) was used to visualise bacterial biofilms inside the nasogastric feeding tubes. The biofilm was heterogeneously distributed along the tube lumen wall, and had a depth of up to 500 µm. The bacterial biofilm and faecal samples included Enterococcus faecalis and Enterobacter hormaechei. Representative strains, recovered from both feeding tubes and faecal samples, were whole genome sequenced using Illumina, Mi-Seq, which revealed indistinguishable strains, each with less than 28 SNP differences, of E. faecalis and E. hormaechei. The E. faecalis strains were from two sequence types (ST191 and ST211) and encoded for a number of traits related to biofilm formation (BopD), adherence (Epb pili), virulence (cps loci, gelatinase, SprE) and antibiotic resistances (IsaA, tetM). The E. hormaechei were all ST106, and encoded for blaACT-15 ß-lactamase and fosfomycin resistance (fosA). This proof of concept study demonstrates that bacterial flora within the neonatal feeding tubes may influence the bacterial colonisation of the intestinal tract and can be visualised non-destructively using OCT.

Optical coherence tomography and non-linear microscopy for paintings - a study of the complementary capabilities and laser degradation effects.

This paper examines for the first time the potential complementary imaging capabilities of Optical coherence tomography (OCT) and non-linear microscopy (NLM) for multi-modal 3D examination of paintings following the successful application of OCT to the in situ, non-invasive examination of varnish and paint stratigraphy of historic paintings and the promising initial studies of NLM of varnish samples. OCT provides image contrast through the optical scattering and absorption properties of materials, while NLM provides molecular information through multi-photon fluorescence and higher harmonics generation (second and third harmonic generation). OCT is well-established in the in situ non-invasive imaging of the stratigraphy of varnish and paint layers. While NLM examination of transparent samples such as fresh varnish and some transparent paints showed promising results, the ultimate use of NLM on paintings is limited owing to the laser degradation effects caused by the high peak intensity of the laser source necessary for the generation of non-linear phenomena. The high intensity normally employed in NLM is found to be damaging to all non-transparent painting materials from slightly scattering degraded varnish to slightly absorbing paint at the wavelength of the laser excitation source. The results of this paper are potentially applicable to a wide range of materials given the diversity of the materials encountered in paintings (e.g. minerals, plants, insects, oil, egg, synthetic and natural varnish).

Climate change and thermal comfort in Hong Kong.

Thermal comfort is a major issue in cities and it is expected to change in the future due to the changing climate. The objective of this paper is to use the universal thermal comfort index (UTCI) to compare the outdoor thermal comfort in Hong Kong in the past (1971-2000) and the future (2046-2065 and 2081-2100). The future climate of Hong Kong was determined by the general circulation model (GCM) simulations of future climate scenarios (A1B and B1) established by the Intergovernmental Panel on Climate Change (IPCC). Three GCMs were chosen, GISS-ER, GFDL-CM2.1 and MRI-CGCM2.3.2, based on their performance in simulating past climate. Through a statistical downscaling procedure, the future climatic variables were transferred to the local scale. The UTCI is calculated by four predicted climate variables: air temperature, wind speed, relative humidity and solar radiation. After a normalisation procedure, future UTCI profiles for the urban area of Hong Kong were created. Comparing the past UTCI (calculated by observation data) and future UTCI, all three GCMs predicted that the future climate scenarios have a higher mode and a higher maximum value. There is a shift from 'No Thermal Stress' toward 'Moderate Heat Stress' and 'Strong Heat Stress' during the period 2046-2065, becoming more severe for the later period (2081-2100). Comparing the two scenarios, B1 exhibited similar projections in the two time periods whereas for A1B there was a significant difference, with both the mode and maximum increasing by 2°C from 2046-2065 to 2081-2100.