Transparency measurement of thin films with one-sided optical access using fluorescence imaging.

Optical transparency can be determined by passing light through an object and comparing the intensity of the transmitted light with the intensity of the incident light. This measurement needs optical access to both sides of the object for illumination and measurement. Optical access to both sides of the object may be restricted in some situations due to the presence of an opaque obstacle, lack of physical access, etc. A novel technique of transmittance measurement is presented that is able to determine the transparency of thin-film objects with optical access limited to just one side. This method involves mounting the object on a fluorescent substrate, illuminating the object at an excitation wavelength, and observing the light radiated from the object at the fluorescence wavelength. The observed intensity of the light at the fluorescence wavelength is directly related to the transmittance of the object at the excitation and fluorescence wavelengths. This optical configuration eliminates the spurious effect of reflection of the incident light by filtering the excitation wavelength before reaching the optical detector. The technique was used to measure the transmittance of neutral density filters, which were also measured using a conventional transmittance configuration. The difference between the transmittance measured using one-sided optical access and the conventional two-sided configuration was 2.4% or less. As an example of the utility of the one-sided measurement technique, the transmittance of paper was measured during drying, while the paper sample was sandwiched between a woven dryer fabric and a heater. The relationship between the optical transmittance of paper and its moisture content has been determined previously, and this relationship was used to infer the moisture content of the paper during drying. The moisture content distribution during the drying process is shown to be spatially correlated with the structure of the dryer fabric.

A micromechanical comparison of human and porcine skin before and after preservation by freezing for medical device development.

Collecting human skin samples for medical research, including developing microneedle-based medical devices, is challenging and time-consuming. Researchers rely on human skin substitutes and skin preservation techniques, such as freezing, to overcome the lack of skin availability. Porcine skin is considered the best substitute to human skin, but their mechanical resemblance has not been fully validated. We provide a direct mechanical comparison between human and porcine skin samples using a conventional mechano-analytical technique (microindentation) and a medical application (microneedle insertion), at 35% and 100% relative humidity. Human and porcine skin samples were tested immediately after surgical excision from subjects, and after one freeze-thaw cycle at -80 °C to assess the impact of freezing on their mechanical properties. The mechanical properties of fresh human and porcine skin (especially of the stratum corneum) were found to be different for bulk measurements using microindentation; and both types of skin were mechanically affected by freezing. Localized in-plane mechanical properties of skin during microneedle insertion appeared to be more comparable between human and porcine skin samples than their bulk out-of-plane mechanical properties. The results from this study serve as a reference for future mechanical tests conducted with frozen human skin and/or porcine skin as a human skin substitute.

Optical transparency of paper as a function of moisture content with applications to moisture measurement.

Accurate measurement of the moisture content of paper is essential in papermaking and is also important in some paper-based microfluidic devices. Traditional measurement techniques provide very limited spatiotemporal resolution and working range. This article presents a novel method for moisture content measurement whose operating principle is the strong correlation between the optical transparency of paper and its moisture content. Spectrographic and microscopic measurement techniques were employed to characterize the relation of moisture content and relative transparency of four types of paper: hardwood chemi-thermomechanical pulp paper, Northern bleached softwood kraft paper, unbleached softwood kraft paper, and General Electric(®) Whatman™ grade 1 chromatography paper. It was found that for all paper types, the paper transparency increased monotonically with the moisture content (as the ratio of the mass-of-water to the mass-of-dry-paper increased from 0% to 120%). This significant increase in relative transparency occurred due to the refractive index matching role of water in wet paper. It is further shown that mechanical loading of the paper has little impact on the relative transparency, for loadings that would be typical on a paper machine. The results of two transient water absorption experiments are presented that show the utility and accuracy of the technique.

Single Layer Deposition of Polystyrene Particles onto Planar Polydimethylsiloxane Substrates.

This work investigates the deposition of polystyrene particles onto cross-linked polydimethylsiloxane (PDMS) substrates by using an impinging jet flow cell for different concentrations of sodium chloride in solution. Particle tracking reveals that particles near the substrate can be immobilized to different degrees. An attempt is made to classify the mobility of the particles close to the surface by distinguishing between weakly immobilized and strongly immobilized particles where only the latter ones are considered as deposited. Subsequently, the measured initial deposition rates for different concentrations of sodium chloride in solution are compared to the commonly applied theory based on the convective diffusion equation in which different surface interaction potentials were considered. With currently available data on the surface properties of PDMS, the extended Derjaguin-Landau-Verwey-Overbeek (extended DLVO) theory gave a better description of the observed deposition rates as compared to the DLVO theory; however, in either case, the presence of significant surface charge heterogeneity had to be assumed in order to capture the observed trend of the deposition rates with respect to the electrolyte concentration. Careful analysis of the more weakly immobilized particles through particle displacement step analysis reveals that there is a buildup of a particle accumulation layer near the substrate in which particle motion parallel to the substrate is hindered by nonhydrodynamic effects. Possible reasons for the reduced particle motion in the accumulation layer are discussed. As a result, the presence of lateral surface interaction forces resulting from charge heterogeneity and surface roughness of the PDMS substrate is found to be the most plausible explanation for the hindered particle motion in the accumulation layer. This suggests that particles associated with the secondary minimum of the surface interaction potential may not always be freely mobile in any direction parallel to the substrate.

Concentration Field Evolution during the Drying of a Thin Polymer Solution Film near the Contact Line.

An experimental study is performed for polymer concentration field measurements during the drying of an aqueous poly(vinyl alcohol) solution inside a shallow cavity near a vertical side wall. The measurements are based on optical techniques such as 3D confocal microscopy for laser-induced fluorescence analysis. The results reveal a significant concentration heterogeneity across the film near the meniscus during the drying process. The concentration at the solution-air interface remains higher compared to the bulk, and it increases toward the pinned contact line and also over time. A skin layer starts forming as the surface concentration reaches the glass-transition concentration, after which the evaporation rate starts decreasing. Regardless of the cavity depth and the initial polymer concentration, the drying film undergoes a similar concentration evolution during the evaporation process, although minor differences can be recognized. For instance, a low local capillary number at the surface is associated with a wavy surface concentration profile while at higher capillary numbers disturbances are damped and a much more uniform concentration profile is observed.