Diffusing Wave Spectroscopy Research Articles

Light-Scattering Analysis of Drying Behavior in Suspension Droplets with Silica and Polystyrene Particles and a Hydrosoluble Polymer.

The coffee-ring structure, which is the final drying pattern of a sessile suspension droplet, is a key factor in controlling the uniformity of the particulate deposits in various coatings. Two light-scattering methods, diffusing wave spectroscopy (DWS) and multispeckle DWS (MSDWS), were used to quantitatively distinguish temporal changes in particle mobility in evaporating suspension droplets containing micrometer-sized silica and polystyrene (PS) particles. The characteristic particle mobility was measured in terms of the mean square displacement in the early stage of drying, and the local particle dynamics around the edge and center regimes of the droplets during drying were analyzed using MSDWS. Hydroxyethyl cellulose (HEC), a hydrosoluble polymer, was added to the silica and PS suspensions to further investigate its role in suppressing or enhancing coffee-ring patterns based on particle-polymer interactions. Consequently, dried microstructures can be directly correlated with real-time drying dynamics, as well as the interactions between solutes by comprehensive light-scattering methods.

Decoding diffuse light scattering dynamics in layered tissues: path length versus fluctuation time scale.

Dynamic multiple light scattering (DMLS) has found numerous applications, including soft matter physics and biomedical optics. Yet biological tissues may have complex internal geometries, presenting a challenge for noninvasive measurements. Deciphering laminar dynamics is crucial to accurately interpret tissue or organ physiology. Seminal DMLS work noted that one can probe deeper layers indirectly by analyzing light fluctuations on shorter time scales. Recent technologies have enabled probing deeper layers directly by analyzing fluctuations at longer path lengths. The following question arises: are the indirect and direct approaches synergistic or redundant? Here, by adding an optical switch to path-length-filtered interferometric diffusing wave spectroscopy, we experimentally address this question in the context of a forearm occlusion study. We find that both approaches afford better distinction of light scattering dynamics in layered tissues than either approach alone. This motivates further development of methods that integrate both decorrelation time scale and light path length to probe layered tissues.

Characterization of nano-silica vegetable grease with diffusing wave spectroscopy DWS and Raman spectroscopy

The diffusing wave spectroscopy (DWS) method made it possible to identify changes in the dynamics of the free movement of particles in the grease under the influence of temperature, which changed the viscoelastic properties of the grease. Changes in the parameters determined by DWS method influenced changes in the chemical structure, which was confirmed by Raman spectroscopy, determining the integral intensity of the unsaturated to saturated bond bands found in the grease. The article presents the results of the influence of temperature on changes in the viscoelastic states of vegetable grease evaluated on the basis of properties determined by DWS (diffusing wave spectroscopy). The following parameters were used to evaluate the viscoelastic states: the intensity correlation function (ICF), the correlation function of mean square displacement (MSD), the elastic modulus G′ and the viscosity modulus G″. A significant effect of temperature on changes in the microstructure of vegetable grease was observed, which was reflected in the viscoelastic parameters. The dynamics of the free movement of molecules in the grease was changed, which affected the elasticity of the system and the displacement of the G′ and G″ modules towards higher frequencies.

Light Scattering through a Drying Coating

Drying coatings undergo internal dynamic densification and rearrangement, which are challenging to discern with optical techniques due to their multiple scatterings of light. Experiments such as diffusing wave spectroscopy (DWS) and laser speckle imaging (LSI) leverage these multiple scatterings to reveal the in situ dynamics of the coating. In such experiments, the knowledge of the sample volume that can be accessed and therefore studied is fundamental, especially in cases of micrometer-scale coating thicknesses. In this paper, we present a robust and reliable method using transmission geometry to calculate the parameter l*, defined as the transport light mean-free path, which is strongly related to the volume of the sample that light explores in DWS and LSI experiments. We show how this dynamic parameter can be measured for liquid and solid film samples and, crucially, in the case of time-evolving samples, such as drying coatings of paint or ink, which has not been previously explored. We present a series of model ink samples and discuss the evolution of their densification during drying through quantification of dynamic l*.

Two-point microrheology and diffusing wave spectroscopy

The dynamics of charged and hard-sphere silica colloidal suspensions are measured using diffusing wave spectroscopy and are interpreted by the Generalized Stokes–Einstein Relation (GSER). At high concentration, the resulting moduli are in good qualitative agreement with bulk rheology, including the frequency response, but are higher by a quantitative factor. We show that the “two-point” GSER, which derives from the correlated motion of the colloidal particles, provides a better quantitative agreement between bulk and microrheology. The two-point GSER applies generally when the average scattering vector sampled by the multiple scattering events corresponds to a length scale greater than the scatterer diameter. We discuss applications for characterizing colloid interactions by measuring the high-frequency moduli of suspensions and extend the two-point analysis to tracer particle microrheology measurements of a semiflexible biopolymer network.

Effect of small molecule surfactant structure on the stability of water-in-lubricating oil emulsions

During automotive engine operation, water may contaminate engine oil, inhibiting its role in maintaining safe engine operation. In many cases, engine oil must be capable of emulsifying any water contamination to avoid such problems. This study focuses on the impact of small molecule surfactant concentration structure and concentration in emulsions comprised of engine oil, water, and E85 fuel to understand the effects on emulsion stability and formulation optimization. Three small molecule surfacatants were tested; glycerol dioleate (GDO), glyceryl monooleate (GMO), and oleamide (OA). Three characterization methods were used to investigate their effects; the current state of the art, ASTM D7563, microscopy, and diffusing wave spectroscopy (DWS). We found that DWS could yield insights into mechanisms of emulsion stability that are otherwise inaccessible through other experimental techniques. Specifically, utilizing DWS, we are able to extract specific emulsion stability mechanisms associated directly with molecular features for the three surfactants examined.

Interaction between potassium iodide and bovine serum albumin, ovalbumin and lysozyme under different temperature induction

In this study, the interaction between potassium iodide and protein molecules under different temperature induction was studied, taking potassium iodide (KI) and protein molecules as a model system. The effects of KI on protein conformation, size, surface charge, binding constant, and binding site were analyzed by fluorescence spectrum, infrared spectrum, and diffusing wave spectroscopy. The results revealed that bovine serum albumin (BSA)/ovalbumin (OVA) and I−1 formed the 1: 1 complex and significantly affected the hydrodynamic radius and spatial structure. This could be attributed to the exposure of tyrosine residues inside the proteins to the polar conditions under increased temperature. The unfolding of protein structures induced the interaction between KI/KCl and proteins. As for BSA and OVA, the particle size and surface charge of the complex increased significantly in the presence of KI/KCl. KI had a strong static quenching effect on the fluorescence of BSA and OVA. Overall, these results provide insights into the physiological effects of iodine ions.

Developing concentrated emulsion gel hybrids structured by natural food fibres

The integration of healthy unsaturated fat into plant-based meat alternatives is being advocated worldwide but remains technically challenging, due to limitations in the textural qualities and processability of edible oil. One way to improve the rheological behaviour of oil is via emulsion gel technology. However, adaptable solutions for real-life food processing are few and far between. On this basis, we formulated physically stable emulsion gel hybrid templates with 50–83% sunflower oil load, mostly oil-in-water microstructure, and structured solely by natural food fibres without using oleogelators. The fibres included citrus fibre and soy polysaccharides as emulsifying fibres, and curdlan and deacetylated konjac glucomannan as structuring fibres. Upon heating, they developed stiffness up to 10E4 Pa in storage modulus G′. Here, we present the first characterisation of such fibre-based emulsion gel hybrids through analysis by epifluorescence microscopy, oscillatory rheology, and diffusing wave spectroscopy, and report underlying composition-structure-rheology-thermal behaviour relations. The key findings are that strength in the emulsion gel hybrid could come from combinations of the following observed mechanisms: (i) tiny droplets of dispersed phase droplets with high curvature, (ii) emulsifying fibre of high molecular weight, (iii) controlled interference from emulsifying fibre on the viscoelastic network development of structuring fibre, and (iv) for additional shear stability, highly aggregated state of dispersed phase droplets without coalescence. Promising templates bearing close resemblance to animal fat references such as pork belly fat (in terms of G′), and shear stability with potential toward extrusion and food printing application have been identified.

Heat-induced gelation, rheology and stability of oil-in-water emulsions prepared with patatin-rich potato protein

The heat treatment after non-aseptic homogenization is sufficient to ensure the microbiological safety of the product, but such a sequence of processes may affect the technological suitability of protein-stabilized emulsions. In this study, we investigated the heat-induced gelation of oil-in-water emulsions prepared with patatin-rich potato proteins (EPs) and the effect of heating to 85 °C on their final properties (the appearance, rheology and stability). Low-fat and high-fat EPs of 3.2 and 30% (w/v) oil, respectively, were prepared in a two-stage high-pressure homogenizer using pressures of 50 MPa and 5 MPa in the first and second stages before heating with or without 0.1 M NaCl addition. Multi-speckle diffusing wave spectroscopy and oscillatory rheology techniques were used to monitor the molecular and macroscopic dynamics of heat-induced EPs gelation. The results showed that both, the oil content and the salt addition play a critical role in the dynamics of EPs gelation as well as in creating properties of heated and unheated emulsions, regardless of the cycles number (one or three passes through a two-stage homogenizer) of HPH pre-processing. The protein denaturation temperature in EPs ranged from 58.1 °C to 60.2 °C, but it was shifted towards lower values in the presence of salt. Heating the low-fat EPs with 3.2% (w/v) oil resulted in their transformation at 69 °C into weak gels of homogeneous structure, strong shear thinning and thixotropy and good stability against creaming. In contrast, flocculated high-fat EPs with 30% (w/v) oil formed already at 64°C64heterogeneous gels of viscoelastic solid character and poor stability due to the tendency to syneresis.

Effect of nano-TiO2 particle size on the bonding performance and film-forming properties of starch-based wood adhesives

The effect of nano-TiO2 particle size on the properties of starch-based wood adhesives was studied in this work. Our findings indicate that a smaller size of nano-TiO2 particles corresponds with a larger specific surface area and more hydroxyl sites on the particle surface that interact with latex molecules, forming a more compact network structure. Therefore, the bonding performance and water resistance of the adhesive were enhanced. In addition, rheology results showed that the adhesive behaves as a pseudoplastic fluid. Small-angle X-ray scattering and energy dispersive spectroscopy confirmed the good compatibility and dispersion of nano-TiO2 in the adhesive films. Diffusing wave spectroscopy and scanning electron microscopy showed that smaller TiO2 particles were more favorable for the formation of smoother and denser films. These results are of great significance for improving the structure and properties of starch-based wood adhesives and preparing high-performance environmentally friendly biobased adhesives.

Complex optical transport, dynamics, and rheology of intermediately attractive emulsions

Introducing short-range attractions in Brownian systems of monodisperse colloidal spheres can substantially impact their structures and consequently their optical transport and rheological properties. Here, for size-fractionated colloidal emulsions, we show that imposing an intermediate strength of attraction, well above but not much larger than thermal energy (approx 5.6k_{textrm{B}}T), through micellar depletion leads to a striking notch in the measured inverse mean free path of optical transport, 1/ell ^*, as a function of droplet volume fraction, phi. This notch, which appears between the hard-sphere glass transition, phi _{textrm{g}}, and maximal random jamming, phi _{textrm{MRJ}}, implies the existence of a greater population of compact dense clusters of droplets, as compared to tenuous networks of droplets in strongly attractive emulsion gels. We extend a prior decorated core-shell network model for strongly attractive colloidal systems to include dense non-percolating clusters that do not contribute to shear rigidity. By constraining this extended model using the measured 1/ell ^*(phi ), we improve and expand the microrheological interpretation of diffusing wave spectroscopy (DWS) experiments made on attractive colloidal systems. Our measurements and modeling demonstrate richness and complexity in optical transport and shear rheological properties of dense, disordered colloidal systems having short-range intermediate attractions between moderately attractive glasses and strongly attractive gels.

Interferometric diffusing wave spectroscopy imaging with an electronically variable time-of-flight filter.

Diffuse optics (DO) is a light-based technique used to study the human brain, but it suffers from low brain specificity. Interferometric diffuse optics (iDO) promises to improve the quantitative accuracy and depth specificity of DO, and particularly, coherent light fluctuations (CLFs) arising from blood flow. iDO techniques have alternatively achieved either time-of-flight (TOF) discrimination or highly parallel detection, but not both at once. Here, we break this barrier with a single iDO instrument. Specifically, we show that rapid tuning of a temporally coherent laser during the sensor integration time increases the effective linewidth seen by a highly parallel interferometer. Using this concept to create a continuously variable and user-specified TOF filter, we demonstrate a solution to the canonical problem of DO, measuring optical properties. Then, with a deep TOF filter, we reduce scalp sensitivity of CLFs by 2.7 times at 1 cm source-collector separation. With this unique combination of desirable features, i.e., TOF-discrimination, spatial localization, and highly parallel CLF detection, we perform multiparametric imaging of light intensities and CLFs via the human forehead.

The influence of KCl concentration on the gelation of myofibrillar protein giant squid (Dosidicus gigas) due to molecular conformation change.

Protein gelation process is of importance in food industry. The objective of this study is to investigate the influence of salt concentration variation, which induced protein conformation change, on protein's intermolecular interactions and its gelation process. Paramyosin has been separated and purified from myofibrillar protein extracted from giant squid. Then Giant squid's paramyosin molecular mass and intermolecular interactions were quantified by means of light scattering techniques. Finally, the micro-rheology study via diffusing wave spectroscopy (DWS) technique revealed that this conformation change dramatically affected myofibrillar protein gelation process. The obtained apparent molecular weight (ca 2 × 105 g/mol) suggested that protein molecules existed as dimers, while the second virial coefficient A2 significantly reduced from -3.98456 × 10-5 to -5.07575 × 10-4 ml mol/g2 when KCl concentrated from 0.15 to 1 mol/L. Light scattering data also suggest that paramyosin dimers are stiff, with a persistence length of 120 nm, almost the length of a molecule and independent of salt concentration. Mean-square displacement (MSD) of tracer particles at 5 temperatures with 4 salt concentrations displayed that this conformation change had dramatic effect. Therefore, G' and G" were remarkably altered with at least one order of magnitude difference owing to this event occurrence. Paramyosin conformation change due to KCl concentrated enhances attractive interactions with apparent molecular mass increase, which resulted in majority paramyosin molecules (> 99%) in dimeric form and promoted aggregates formation. DWS technique revealed that the conformation change dramatic affected this process characterized by the correlation functions, MSD, and G' and G". This study brings forward data on understanding the effect of a major salt supplement, KCl, on the chemical physics of a major muscle protein.

Thermal Creep on Mars: Visualizing a Soil Layer under Tension

At low ambient pressure, temperature gradients in porous soil lead to a gas flow called thermal creep. In this regard, Mars is unique as the conditions for thermal creep to occur in natural soil only exist on this planet in the solar system. Known as a Knudsen compressor, thermal creep induces pressure variations. In the case of Mars, there might be a pressure maximum below the very top dust particle layers of the soil, which would support particle lift and might decrease threshold wind velocities necessary to trigger saltation or reduce angles of repose on certain slopes. In laboratory experiments, we applied diffusing wave spectroscopy (DWS) to trace minute motions of grains on the nanometer scale in an illuminated simulated soil. This way, DWS visualizes pressure variations. We observe a minimum of motion, which we attribute to the pressure maximum ∼2 mm below the surface. The motion above but especially below that depth characteristically depends on the ambient pressure with a peak at an ambient pressure of about 3 mbar for our sample. This is consistent with earlier work on the ejection of particle layers and is in agreement with a thermal creep origin. It underlines the supporting nature of thermal creep for particle lift, which might be especially important on Mars.

Multiscale investigation of viscoelastic properties of aqueous solutions of sodium alginate and evaluation of their biocompatibility.

In order to get better knowledge of mechanical properties from microscopic to macroscopic scale of biopolymers, viscoelastic bulk properties of aqueous solutions of sodium alginate were studied at different scales by combining macroscopic shear rheology (Hz), diffusing-wave spectroscopy microrheology (kHz-MHz) and Brillouin spectroscopy (GHz). Structural properties were also directly probed by small-angle X-ray scattering (SAXS). The results demonstrate a change from polyelectrolyte behavior to neutral polymer behavior by increasing polymer concentration with the determination of characteristic sizes (persistence length, correlation length). The viscoelastic properties probed at the phonon wavelength much higher than the ones obtained at low frequency reflect the variation of microscopic viscosity. First experiments obtained by metabolic activity assays with mouse embryonic fibroblasts showed biocompatibility of sodium alginate aqueous solutions in the studied range of concentrations (2.5-10 g L-1) and consequently their potential biomedical applications.

Effect of scatterer interactions on photon transport in diffusing wave spectroscopy.

We calculate the effect of particle size, concentration, and interactions on the photon transport mean-free path l^{*} that characterizes the multiple light scattering in diffusing wave spectroscopy (DWS). For scatterers of sufficient size, such that the first peak of the suspension structure factor S(q_{max}) remains in the range of accessible scattering vectors, neither repulsive nor attractive interactions between scatterers contribute strongly to l^{*}; its values are bounded by those for hard spheres and scatterers without interactions. However, for scatterers smaller than the wavelength of light, crowding induced by attraction or repulsion can lead to nonmonotonic behavior in l^{*} with increasing scatterer concentration. The effect is strongest for repulsive particles.

Heat induced interactions in whey protein depleted milk concentrates: Comparison of ultrafiltration and microfiltration

The physical and chemical changes of 4x milk concentrates prepared by ultrafiltration (UF) and microfiltration (MF) combined with diafiltration were studied after heating in situ to 85 °C and subsequent cooling. Skim milk (1x) was analyzed as control. The concentrates had comparable pH, colloidal casein and colloidal calcium and phosphate, but differed in whey proteins and serum calcium and phosphate, allowing investigating the effect of soluble phase composition on viscosity changes and protein aggregation during heating. In heated samples, there was a decrease in the apparent diameter of the protein particles, as well as a decrease in bulk viscosity compared to unheated concentrates. On the other hand, the turbidity increased. Particles’ diffusivity measured by diffusive wave spectroscopy was slower in UF concentrate compared to the MF counterpart, both before and after heating, demonstrating the effect of protein aggregates in the soluble phase in changing the bulk properties of the concentrates. In this work, by comparing 4x MF and UF fresh concentrates with reduced calcium, and with similar processing history, it was demonstrated that the presence of whey proteins drives casein dissociation. MF concentrates showed increased heat stability compared to UF concentrates, with lower viscosity after heating. It was also observed that cooling kinetics affect the dissociation of caseins in heated skim milk. A fine control of diafiltration for ions and whey protein depletion is critical for control of the technological quality of the concentrates.

Multispeckle diffusing wave spectroscopy as a tool to study heterogeneous mechanical behavior in soft solids

Multiple speckle diffusing wave spectroscopy (MSDWS) can be applied to measure spatially heterogeneous mechanical behavior in soft solids, with high sensitivity to deformation and both spatial and temporal resolution. In this paper, we discuss the mathematical approach behind the quantification of the deformation rate from MSDWS data and provide guidelines for optimizing the selection of experimental parameters in measurements. After validating the method in extensional tests on an elastomer, we provide an example of the potentiality of MSDWS by measuring the spatial distribution of the deformation rate during shear debonding of adhesive tapes. We quantitatively characterize the deformation rate distribution related to shearing and peeling under loading. A highly heterogeneous deformation rate distribution is observed, and time-dependent measurements reveal an increase in deformation localization hundreds of seconds before full debonding. This behavior, previously predicted by theory and simulation, is demonstrated here experimentally for the first time.

Rheological behavior and drying characteristics of ceria-based suspensions under various pH conditions

Microstructural changes in ceria-based suspensions at different pH values and corresponding rheological and drying properties were correlated. Ceria-based suspensions were classified into four zones based on their pH values. Well-dispersed or aggregated states of ceria particles in suspensions were mainly discerned by the magnitude of zeta-potential with pH variations and additional condensation reaction at relatively high pH. Suspensions with well-dispersed particles in a particular pH range exhibited transition between shear-thinning and Newtonian behavior along the shear rate and weak thixotropic features, whereas suspensions with agglomerated particles in different pH ranges showed significant shear-thinning and thixotropic behaviors with shear rate. The motion of particles within suspension droplets in each zone was quantified in terms of autocorrelation functions during drying using multi-speckle diffusing wave spectroscopy. Suspension droplets with well-dispersed particles showed conspicuous coffee-ring patterns compared to those with aggregated particles, owing to the rapid Brownian motion of well-dispersed particles. • The dispersion state of ceria particles depends on the pH condition. • A densely packed structure at high pH originates from the condensation reaction • Rheological properties and drying characteristics are correlated with the microstructures.

Ju et al.: Multispeckle diffusing wave spectroscopy as a tool to study heterogeneous mechanical behavior in soft solids

Ju <i>et al</i>.: Multispeckle diffusing wave spectroscopy as a tool to study heterogeneous mechanical behavior in soft solids