Colloidal Tracers Research Articles

Enhanced Diffusion and Non-Gaussian Displacements of Colloids in Quasi-2D Suspensions of Motile Bacteria

In the real world, active agents interact with surrounding passive objects, thus introducing additional degrees of complexity. The relative contributions of far-field hydrodynamic and near-field contact interactions to the anomalous diffusion of passive particles in suspensions of active swimmers remain a subject of ongoing debate. We constructed a quasi-two-dimensional microswimmer–colloid mixed system by taking advantage of Serratia marcescens’ tendency to become trapped at the air–water interface to investigate the origins of the enhanced diffusion and non-Gaussianity of the displacement distributions of passive colloidal tracers. Our findings reveal that the diffusion behavior of colloidal particles exhibits a strong dependence on bacterial density. At moderate densities, the collective dynamics of bacteria dominate the diffusion of tracer particles. In dilute bacterial suspensions, although there are multiple dynamic types present, near-field contact interactions such as collisions play a major role in the enhancement of colloidal transport and the emergence of non-Gaussian displacement distributions characterized by heavy exponential tails in short times. Despite the distinct types of microorganisms and their diverse self-propulsion mechanisms, a generality in the diffusion behavior of passive colloids and their underlying dynamics is observed.

Tailor-made polymer tracers reveal the role of clay minerals on colloidal transport in carbonate media

HypothesisHost rock weathering and incipient pedogenesis result in the exposition of minerals, e.g., clay minerals in sedimentary limestones. Once exposed, these minerals provide the surfaces for fluid–solid interactions that control the fate of dissolved or suspended compounds such as organic matter and colloids. However, the functional and compositional diversity of organic matter and colloids limits the assessment of reactivity and availability of clay mineral interfaces. Such assessment demands a mobile compound with strong affinity to clay surfaces that is alien to the subsurface. ExperimentWe approached this challenge by using poly(ethylene glycol) (PEG) as interfacial tracer in limestone weathering experiments. FindingsPEG adsorption and transport was dependent on the availability of clay mineral surfaces and carbonate dissolution dynamics. In addition, PEG adsorption featured adsorption–desorption hysteresis which retained PEG mass on clay mineral surfaces. This resulted in different PEG transport for experiments conducted consecutively in the same porous medium. As such, PEG transport was reconstructed with a continuum-scale model parametrized by a Langmuir-type isotherm including hysteresis. Thus, we quantified the influence of exposed clay mineral surfaces on the transport of organic colloids in carbonate media. This renders PEG a suitable model colloid tracer for the assessment of clay surface exposition in porous media.

Temperature stability and enhanced transport properties by surface modifications of silica nanoparticle tracers for geo-reservoir exploration

Tracer tests are an important tool for characterizing and monitoring subsurface reservoir properties. However, they are limited both because of the tracer molecules constraining factors such as irreversible adsorption, retention, and degradations, i.e. interaction processes of fluorophore molecule with surrounding media resulting in a large variation in transport properties. Elaborate tests utilizing more than one tracer to distinguish time or location of injection are complex and interpretation is ambiguous because each tracer interacts differently. In this study, we present an approach to increase tracer stability and enhance the transport uniformity of different tracers, thus making tests utilizing multiple tracers simpler and more feasible. We present this concept of tracer multiplicity by encapsulating an anionic, cationic or amphoteric fluorophore inside mesoporous silica nanoparticle carriers coated with a protective titania layer. Upon encapsulation, increased thermal resistance and drastically lowered sorption affinity towards quartz sand was detected in batch and flow-through experiments. An additional advantage of the presented nanoparticle tracers over molecular tracers is their modularity, which is demonstrated by surface modifications and application of additives that greatly reduce sorption and increase recovery rates in the flow experiments. With the here presented concept of tracer multiplicity, we introduce a new approach for colloidal tracer design that has the potential to expand and enhance measurable parameters, measurement accuracy and simplicity of analysis.

WTP1.6 Unmask Latent Lymphoedema through bipedal Lymphoscintigraphy

Abstract Aim Lymphoscintigraphy (LSG) is the gold standard investigation in Lymphoedema. The study aimed to determine the presence of Latent Lymphoedema (LL) in the unaffected limb when a patient presents with unilateral lymphoedema. Methods All new and follow up patients seen in the Lymphoedema clinic between the January and May 2023 by either a vascular consultant or a lymphoedema nurse were included in the study. Their electronic patient records were retrospectively reviewed. Clinical presentation and findings were compiled from clinic letters and if the signs and symptoms were suggestive of lymphoedema they were then offered an LSG scan. Protocol involved determining the uptake of nano colloid tracer at the ilio-inguinal lymph nodes 2 hours post subcutaneous injection in the 1st web space of each foot. Quantitative uptake of < 8% at 2 hours was considered abnormal. Results 114 patients with a median age of 63.2 (Range 24-83) were seen in the Lymphoedema clinic. Of those, 62 patients had LSG scans. About 37% (n = 23) patients presented with unilateral swelling of lower limb. On LSG scan they had average uptake of 1.6 1.6% in the clinically affected limb. However 57% (n = 13) of these patients had an average of 2.7 2 % uptake in the unaffected Ilio-inguinal lymph nodes, unmasking latent Lymphoedema on that side. Conclusion Presence of lymphatic dysfunction on the unaffected limb should be considered as unmasking of LL. This will allow initiation of preventative measures to prevent development of gross disease in future.

Quantitative 99mTc-albumin colloid nasal mucociliary clearance as an outcome in primary ciliary dyskinesia.

Primary ciliary dyskinesia (PCD) is an inherited disorder in which dyskinetic cilia cause impaired mucociliary clearance of upper and lower airways. Airway ciliary movement can be indirectly tested in vivo after administration of a radiolabelled tracer to the lower airways for assessment of pulmonary mucociliary clearance or to the nose for assessing nasal mucociliary clearance (NMC). With this study, we investigated NMC as a quantifiable study outcome parameter in patients with PCD. This single centre proof-of-concept study on NMC velocity investigated patients with PCD across different genotypes and nasal nitric oxide (nasal NO) levels. Healthy controls were used for comparison. NMC was determined as velocity in mm·min-1 of a nasally applied 99mTc-albumin colloid tracer. Using a gamma camera, repeated dynamic series of images each lasting 30 s were acquired during a 10-minute period and digitally stored. NMC velocity was investigated in seven patients with PCD (aged 9-31 years) and five adult healthy controls. Mean NMC velocity in healthy controls (8.5 mm·min-1) was significantly higher compared with people with PCD (0.00 mm·min-1, p<0.0001). NMC was completely absent in all included patients with PCD across different PCD genotypes and regardless of nasal NO values. The success rate of the test was 100% in both groups. NMC velocity discriminated highly significantly between patients with PCD and healthy controls. We suggest here a fast and feasible set up for NMC measurements that is easily applicable for any clinical trial involving PCD medication aimed for the nasal compartment, a step before or parallel to conducting clinical trials investigating whole-lung ciliary function in PCD.

Lattice Induced Short-Range Attraction between Like-Charged Colloidal Particles.

We perform experiments and computer simulations to study the effective interactions between like-charged colloidal tracers moving in a two-dimensional fluctuating background of colloidal crystal. By a counting method that properly accounts for the configurational degeneracy of tracer pairs, we extract the relative probability of finding a tracer pair in neighboring triangular cells formed by background particles. We find that this probability at the nearest neighbor cell is remarkably greater than those at cells with larger separations, implying an effective attraction between the tracers. This effective attraction weakens sharply as the background lattice constant increases. Furthermore, we clarify that the lattice-mediated effective attraction originates from the minimization of free energy increase from deformation of the crystalline background due to the presence of diffusing tracers.

Microfluidic free interface diffusion: Measurement of diffusion coefficients and evidence of interfacial-driven transport phenomena

We have developed a microfluidic tool to measure the diffusion coefficient D of solutes in an aqueous solution by following the temporal relaxation of an initially steep concentration gradient in a microchannel. Our chip exploits multilayer soft lithography and the opening of a pneumatic microvalve to trigger the interdiffusion of pure water and the solution initially separated in the channel by the valve, the so-called free interface diffusion technique. Another microvalve at a distance from the diffusion zone closes the channel and thus suppresses convection. Using this chip, we have measured diffusion coefficients of solutes in water with a broad size range, from small molecules to polymers and colloids, with values in the range D∈[10−13–10−9] m2/s. The same experiments but with added colloidal tracers also revealed diffusio-phoresis and diffusio-osmosis phenomena due to the presence of the solute concentration gradient. We nevertheless show that these interfacial-driven transport phenomena do not affect the measurements of the solute diffusion coefficients in the explored concentration range.

Anomalous transport of colloids in heterogeneous porous media: A multi-scale statistical theory

HypothesisTransport of suspended colloids in heterogeneous porous media is a multi-scale process that exhibits anomalous behavior and cannot be described by the Fickian dispersion theory. Although many studies have documented colloids’ transport at different length scales, a theoretical basis that links pore- to core-scale observations remains lacking. It is hypothesized that a recently proposed pore-scale statistical kinetic theory is able to capture the results observed experimentally. ExperimentsWe implement a multi-scale approach via conducting core-flooding experiments of colloidal particles in a sandstone sample, simulating particles flowing through a sub-volume of the rock’s digital twin, and developing a core-scale statistical theory for particles’ residence times via upscaling the pore-scale kinetic theory. Experimental and computational results for solute transport are used as benchmark. FindingsBased on good agreement across the scales achieved in our investigation, we show that the macroscopically observed anomalous transport is particle-type dependent and stems from particles’ microscopic dispersion and deposition in heterogeneous flow fields. In particular, we reveal that residence-time distributions (i.e., breakthrough curve) obey a closed-form function that encompasses particles’ microscopic dynamics, which allows investigations of a whole transition from pre-asymptotic to asymptotic behavior. The physical insights attained could be useful for interpreting experimental data and designing colloidal tracers.

Transport characteristics of DNA-tagged silica colloids as a colloidal tracer in saturated sand columns; role of solution chemistry, flow velocity, and sand grain size

In recent years, DNA-tagged silica colloids have been used as an environmental tracer. A major advantage of this technique is that the DNA-coding provides an unlimited number of unique tracers without a background concentration. However, little is known about the effects of physio-chemical subsurface properties on the transport behavior of DNA-tagged silica tracers. We are the first to explore the deposition kinetics of this new DNA-tagged silica tracer for different pore water chemistries, flow rates, and sand grain size distributions in a series of saturated sand column experiments in order to predict environmental conditions for which the DNA-tagged silica tracer can best be employed. Our results indicated that the transport of DNA-tagged silica tracer can be well described by first order kinetic attachment and detachment. Because of massive re-entrainment under transient chemistry conditions, we inferred that attachment was primarily in the secondary energy minimum. Based on calculated sticking efficiencies of the DNA-tagged silica tracer to the sand grains, we concluded that a large fraction of the DNA-tagged silica tracer colliding with the sand grain surface did also stick to that surface, when the ionic strength of the system was higher. The experimental results revealed the sensitivity of DNA-tagged silica tracer to both physical and chemical factors. This reduces its applicability as a conservative hydrological tracer for studying subsurface flow paths. Based on our experiments, the DNA-tagged silica tracer is best applicable for studying flow routes and travel times in coarse grained aquifers, with a relatively high flow rate. DNA-tagged silica tracers may also be applied for simulating the transport of engineered or biological colloidal pollution, such as microplastics and pathogens.

Active microrheology in corrugated channels: Comparison of thermal and colloidal baths

HypothesisThe dynamics of colloidal suspension confined within porous materials strongly differs from that in the bulk. In particular, within porous materials, the presence of boundaries with complex shapes entangles the longitudinal and transverse degrees of freedom inducing a coupling between the transport of the suspension and the density inhomogeneities induced by the walls. MethodColloidal suspension confined within model porous media are characterized by means of active microrheology where a net force is applied on a single colloid (tracer particle) whose transport properties are then studied. The trajectories provided by active microrheology are exploited to determine the local transport coefficients. In order to asses the role of the colloid-colloid interactions we compare the case of a tracer embedded in a colloidal suspension to the case of a tracer suspended in an ideal bath. FindingOur results show that the friction coefficient increases and the passage time distribution widens upon increasing the corrugation of the channel. These features are obtained for a tracer suspended in a (thermalized) colloidal bath as well as for the case of an ideal thermal bath. These results highlight the relevance of the confinement on the transport and show a mild dependence on the colloidal/thermal bath. Finally, we rationalize our numerical results with a semi-analytical model. Interestingly, the predictions of the model are quantitatively reliable for mild external forces, hence providing a reliable tool for predicting the transport across porous materials.

Microrheology of colloidal suspensions via dynamic Monte Carlo simulations

Understanding the rheology of colloidal suspensions is crucial in the formulation of a wide selection of industry-relevant products, such as paints, foods and inks. To characterise the viscoelastic behaviour of these soft materials, one can analyse the microscopic dynamics of colloidal tracers diffusing through the host fluid and generating local deformations and stresses. This technique, referred to as microrheology, links the bulk rheology of fluids to the microscopic dynamics at the particle scale. If tracers are subjected to external forces, rather than freely diffusing, it is called active microrheology. Motivated by the impact of microrheology in providing information on local structure in complex systems such as colloidal glasses, active matter or biological systems, we have extended the dynamic Monte Carlo (DMC) technique to investigate active microrheology in colloidal suspensions. The original DMC theoretical framework, able to accurately describe the Brownian dynamics of colloids at equilibrium, is here reconsidered and expanded to describe the effects of an external force pulling a tracer embedded in isotropic colloidal suspensions at different densities. To this end, we studied the dynamics of a spherical tracer dragged by a constant external force through a bath of spherical and rod-like particles of comparable size. We could extract valuable details on its effective friction coefficient, being constant at small and large values of the external force, but otherwise displaying a nonlinear behaviour that indicates the occurrence of a force-thinning regime. Our DMC simulation results are in excellent quantitative agreement with past Langevin dynamics simulations and theoretical works for the bath of spherical colloids. The bath of rod-like particles is studied in the isotropic phase, and displays an example where DMC is more convenient than Brownian or Langevin dynamics, in this case, in dealing with particle rotation.

On the Time Transition Between Short- and Long-Time Regimes of Colloidal Particles in External Periodic Potentials

The dynamics of colloidal particles at infinite dilution, under the influence of periodic external potentials, is studied hereviaexperiments and numerical simulations for two representative potentials. From the experimental side, we analyzed the motion of a colloidal tracer in a one-dimensional array of fringes produced by the interference of two coherent laser beams, providing in this way an harmonic potential. The numerical analysis has been performedviaBrownian dynamics (BD) simulations. The BD simulations correctly reproduced the experimental position- and time-dependent density of probability of the colloidal tracer in the short-times regime. The long-time diffusion coefficient has been obtained from the corresponding numerical mean square displacement (MSD). Similarly, a simulation of a random walker in a one dimensional array of adjacent cages with a probability of escaping from one cage to the next cage is one of the most simple models of a periodic potential, displaying two diffusive regimes separated by a dynamical caging period. The main result of this study is the observation that, in both potentials, it is seen that the critical timet*, defined as the specific time at which a change of curvature in the MSD is observed, remains approximately constant as a function of the height barrierU0of the harmonic potential or the associated escape probability of the random walker. In order to understand this behavior, histograms of the first passage time of the tracer have been calculated for several height barriersU0or escape probabilities. These histograms display a maximum at the most likely first passage timet′, which is approximately independent of the height barrierU0, or the associated escape probability, and it is located very close to the critical timet*. This behavior suggests that the critical timet*, defining the crossover between short- and long-time regimes, can be identified as the most likely first passage timet′as a first approximation.

Early initiation of insulin attenuates histological and functional changes in the liver of streptozotocin-induced diabetic rats using 99mTc-sulfur colloid functional imaging

This study aimed to investigate the effect of insulin on the reticuloendothelial system (RES) in the liver and spleen in diabetic rats. Sprague Dawley rats were divided into control, diabetic rats (DM) and diabetic rats treated with insulin (IDM) for 2 weeks. Rats were imaged with technetium-99m-sulfur colloid (99mTc-SC) tracer to determine regional distributions of the tracer for all groups by drawing regions of interest and then obtained the ratios as the cumulative counts of heart, liver, and spleen to the whole body (WB). Liver tissue from sacrificed rats from each group was examined by light and electron microscopy. 99mTc-SC uptake ratios showed a lower liver to WB uptake ratio in the DM rats compared to both controls and IDM rats. Electron microscopy showed severe vacuolization of the hepatocytes of DM rats. The IDM rats show complete resolution of the vacuolization. The early administration of insulin for 2 weeks to diabetic rats could significantly resolve the phagocytic RES function and histological changes in the liver.

Diffusion of colloidal particles in model porous media.

Using video microscopy and simulations, we study the long-time diffusion of colloidal tracers in a wide range of model porous media composed of frozen colloidal matrices with different structures. We found that the diffusion coefficient of a tracer can be quantitatively determined by the structures of porous media. In particular, a universal scaling relation exists between the dimensionless diffusion coefficient of the tracer and the structural entropy of the system. This universal scaling relation is an extension of the scaling law previously discovered for the diffusion of colloidal particles in fluctuating media.

Analytical description of colloid behavior in single fractures under irreversible deposition

ObjectivesIrreversible colloid deposition in groundwater-saturated fractures is typically modeled using a lumped deposition coefficient (κ) that reflects the system physiochemical conditions. A mathematical relationship between this coefficient and the physicochemical conditions controlling deposition has not yet been defined in the literature; thus, κ is typically fitted using experimental observations. This research develops, for the first time, an analytical relationship between κ and the fraction of colloids retained in single fractures (Fr). This relationship could be subsequently integrated with available models relating Fr to the system’s physicochemical properties to develop an explicit mathematical relationship between κ and these properties. MethodThe Fr-κ analytical relationship was developed through conceptualizing irreversible deposition as first-order decay, as both lead to permanent mass loss, and coupling this with the advection-dispersion equation. The model estimates of colloid deposition were compared to observations from laboratory-scale colloid tracer experiments. A variance-based global sensitivity analysis was applied to identify the parameters controlling deposition. FindingsThe analytical relationship efficiently replicated the experimental observations, and the global sensitivity analysis revealed that colloid deposition variability is controlled by fracture length, aperture size, and deposition coefficient; this supports the accepted understanding that colloid deposition is controlled by the system’s physicochemical properties.

Electrophoretic origin of long-range repulsion of colloids near water/Nafion interfaces.

One of the most striking properties of Nafion is the formation of a long-range solute exclusion zone (EZ) in contact with water. The mechanism of formation of this EZ has been the subject of a controversial and long-standing debate. Previous studies by Schurr et al. and Florea et al. root the explanation of this phenomenon in the ion-exchange properties of Nafion, which generates ion diffusion and ion gradients that drive the repulsion of solutes by diffusiophoresis. Here we have evaluated separately the electrophoretic and chemiphoretic contributions to multi-ionic diffusiophoresis using differently charged colloidal tracers as solutes to identify better their contribution in the EZ formation. Our experimental results, which are also supported by numerical simulations, show that the electric field, built up due to the unequal diffusion coefficients of the exchanged ions, is the dominant parameter behind such interfacial phenomenon in the presence of alkali metal chlorides. The EZ formation depends on the interplay of the electric field with the zeta potential of the solute and can be additionally modulated by changing ion diffusion coefficients or adding salts. As a consequence, we show that not all solutes can be expelled from the Nafion interface and hence the EZ is not always formed. This study thus provides a more detailed description of the origin and dynamics of this phenomenon and opens the door to the rational use of this active interface for many potential applications.

Percolation theory applied to soil tomography

This study provides insights on the significance of network features of soil macropores on the transport of solutes and colloids, and in the filtrating capacity of the soil. We applied percolation theory and network analysis to the pore network extracted from X-ray computed tomography (imaged porosity) in intact columns sampled from topsoils with different tillage treatments. Moreover, we developed a procedure to extract the backbone, which is the part of the percolation cluster that controls the direct flow between two boundaries in near saturated conditions, using the ImageJ open source imaging software. We also calculated the percolation threshold of each soil, the probability at which the soil starts to percolate (for the resolution considered).Some backbone characteristics (pore volume, wall surface, circularity, fractal dimension, number of loops and tortuosity) showed significant differences between the treatments. Tilled conventional and organic with high earthworm activity exhibited more complex backbones than no-till soil. Backbone volume, surface, fractal dimension and number of loops are correlated with the surface area of pore walls stained by fluorescent microspheres (MS) used as a colloidal tracer. We also found significant correlations between the tortuosity and the number of end-points of the backbone and the transport model parameters for the microspheres and bromide. These findings reinforce the phenomenology between transport in porous media and percolation theory. Moreover, the properties based on percolation theory allow a complete characterization of the complex soil structure and the development of more accurate transport models.

Damped White Noise Diffusion with Memory for Diffusing Microprobes in Ageing Fibrin Gels

From observations of colloidal tracer particles in fibrin undergoing gelation, we introduce an analytical framework that allows the determination of the probability density function for a stochastic process beyond fractional Brownian motion. Using passive microrheology via videomicroscopy, mean square displacements of tracer particles suspended in fibrin at different ageing times are obtained. The anomalous diffusion is then described by a damped white noise process with memory, with analytical results closely matching experimental plots of mean square displacements and probability density function. We further show that the white noise functional stochastic approach applied to passive microrheology reveals the existence of a gelation parameter μ which elucidates the dynamics of constrained tracer particles embedded in a time-dependent soft material. In addition, we found that microstructural heterogeneity of particle environments decreases as the ageing time increases. This study offers experimental insights on the ageing of fibrin gels while presenting a white noise functional stochastic approach that could be applied to other systems exhibiting non-Markovian diffusive behavior.

Nonuniform Crowding Enhances Transport.

The cellular cytoplasm is crowded with macromolecules and other species that occupy up to 40% of the available volume. Previous studies have reported that for high crowder molecule concentrations, colloidal tracer particles have a dampened diffusion due to the higher solution viscosity. However, these studies employed uniform distributions of crowder molecules. We report a scenario, previously unexplored experimentally, of increased tracer transport driven by a nonuniform concentration of crowder macromolecules. In gradients of a polymeric crowder, tracer particles undergo transport several times higher than that of their bulk diffusion rate. The direction of the transport is toward regions of lower crowder concentration. Mechanistically, hard-sphere interactions and the resulting volume exclusion between the tracer and crowder increase the effective diffusion by inducing a convective motion of tracers, which we explain through modeling. Strikingly, soft deformable particles show even greater enhancement in transport in crowder gradients compared to similarly sized hard particles. Overall, this demonstration of enhanced transport in nonuniform distributions of crowders is anticipated to clarify aspects of multicomponent intracellular transport.

Measurement of the zeta-potential of solid surfaces through Laser Doppler Electrophoresis of colloid tracer in a dip-cell: Survey of the effect of ionic strength, pH, tracer chemical nature and size

Measurement techniques for detecting surface charge have gained attention in the last few decades due to the importance of this parameter in many scientific and industrial applications. The estimation of macroscopic surface zeta-potential (ζ) is customarily performed via streaming potential measurements. Recently, a simple and cost-effective Malvern Zetasizer accessory has been introduced which makes use of electroosmotic flow mapping (EOFM) to determine surface charge of hard surfaces. In this work we propose a measurement strategy that considers the key parameters influencing the reliability of surface ζ values. With this purpose, the role of the ionic strength, sample cleaning, tracer particle size and chemical nature on the determination of ζ for borosilicate glass coverslips have been investigated. ζ values obtained in presence of adjusted parameters was finally compared to the equivalent results of streaming potential technique, validating the proposed measurement strategy.