We study experimentally the effect of added salt in the phoretic motion of chemically driven colloidal particles. We show that the response of passive colloids to a fixed active colloid, be it ...attractive or repulsive, depends on the ionic strength, the ζ potential, and the size of the passive colloids. We further report that the direction of self-propulsion of Janus colloids can be reversed by decreasing their ζ potential below a critical value. By constructing an effective model that treats the colloid and ions as a whole subjected to the concentration field of generated ions and takes into account the joint effect of both generated and background ions in determining the Debye length, we demonstrate that the response of the passive colloids and the velocity of the Janus colloids can be quantitatively captured by this model under the ionic diffusiophoresis theory beyond the infinitely-thin-double-layer limit.
In this Review, we describe the synthesis of high‐quality colloidal nanoparticles in organic solvents, the mechanisms by which they can be transferred into aqueous solution, and some of their ...applications in biology. In particular, we will place emphasis on the creation of multifunctional nanoparticles or nanoparticle assemblies.
The nanomaterials of the future will be designed with the aim of performimg specific functions, as dictated by their synthetic manufacture. This Review examines one particular type of material, colloidal nanoparticles, whose potential in a number of medical and biological applications is already beginning to be realized (the image shows MDA‐MB‐43s and MCF‐7 cells labeled with green and red fluorescent silica‐coated CdSe/ZnS nanocrystals, respectively).
This paper examines the critical role of surface roughness (both nano- and micro-scale) on the processes of colloid retention and release in porous media under steady-state and transient chemical ...conditions. Nanoscale surface roughness (NSR) in the order of a few nanometers, which is common on natural solid surfaces, was incorporated into extended-DLVO calculations to quantify the magnitudes of interaction energy parameters (e.g. the energy barrier to attachment, ΔΦa , and detachment, ΔΦd , from a primary minimum). This information was subsequently used to explain the behavior of colloid retention and release in column and batch experiments under different ionic strength (IS) and pH conditions. Results demonstrated that the density and height of NSR significantly influenced the interaction energy parameters and consequently the extent and kinetics of colloid retention and release. In particular, values of ΔΦa and ΔΦd significantly decreased in the presence of NSR. Therefore, consistent with findings of column experiments, colloid retention in the primary minimum was predicted to occur at some specific locations on the sand surface, even at low IS conditions. However, NSR yielded a much weaker primary minimum interaction compared with that of smooth surfaces. Colloid release from primary minima upon decreasing IS and increasing pH was attributed to the impact of NSR on the values of ΔΦd . Pronounced differences in the amount of colloid retention in batch and column experiments indicated that primary minimum interactions were weak even at high IS conditions. Negligible colloid retention in batch experiments was attributed to hydrodynamic torques overcoming adhesive torques, whereas significant colloid retention in column experiments was attributed to nano- and micro-scale roughness which would dramatically alter the lever arms associated with hydrodynamic and adhesive torques.
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•Density and height of nanoscale roughness significantly influenced colloid retention and release processes.•Colloid retention in the primary energy minimum occurred at low solution ionic strength.•Fractional colloid release from primary minima upon decreasing ionic strength and increasing pH was observed.•Microscopic roughness caused significant colloid retention in column experiments.
The transport of ferrihydrite colloid (FHC) through porous media is influenced by anions (e.g., PO4 3–) and cations (e.g., Ca2+) in the aqueous environment. This study investigated the cotransport of ...FHC with P and P/Ca in saturated sand columns. The results showed that P adsorption enhanced FHC transport, whereas Ca loaded onto P–FHC retarded FHC transport. Phosphate adsorption provided a negative potential on the FHC, while Ca added to P-FHC led to electrostatic screening, compression of the electric double layer, and formation of Ca5(PO4)3OH followed by heteroaggregation at pH ≥ 6.0. The monodentate and bidentate P surface complexes coexisted, and Ca mainly formed a ternary complex with bidentate P ((FeO)2PO2Ca). The unprotonation bidentate P at the Stern 1-plane had a considerable negative potential at the Van der Waals molecular surface. Extending the potential effect to the outer layer of FHC, the potential at the Stern 2-plane and zeta potential exhibited a corresponding change, resulting in a change in FHC mobility, which was validated by comparison of experimental results, DFT calculations, and CD-MUSIC models. Our results highlighted the influence of P and Ca on FHC transport and elucidated their interaction mechanisms based on quantum chemistry and colloidal chemical interface reactions.
Active particles such as swimming bacteria or self-propelled colloids spontaneously self-organize into large-scale dynamic structures. The emergence of these collective states from the motility ...pattern of the individual particles, typically a random walk, is yet to be probed in a well-defined synthetic system. Here, we report the experimental realization of tunable colloidal motion that reproduces run-and-tumble and Lévy trajectories. We utilize the Quincke effect to achieve controlled sequences of repeated particle runs and random reorientations. We find that a population of these random walkers exhibit behaviors reminiscent of bacterial suspensions such as dynamic clusters and mesoscale turbulentlike flows.
Microplastics (MPs) can act as vectors for various contaminants in the aquatic environment. Although some research has investigated the adsorption characteristics and influencing factors of ...metals/organic molecules on MPs, the effects of dissolved organic matter (DOM) (which are ubiquitous active species in ecosystems) on metal oxyanions such as Cr(VI) capture by MPs are largely unknown. This study explored the adsorption behaviors and mechanisms of Cr(VI) oxyanions onto polystyrene (PS) MPs using batch adsorption experiments and multiple spectroscopic methods. The effects of representative DOM components (i.e., humic acid (HA), fulvic acid (FA) and tannic acid (TA)) on Cr(VI) capture by PS were particularly studied. Results revealed a significantly enhanced adsorption of Cr(VI) on PS in the presence of TA. The Cr(VI) adsorption capacity was increased from 2876 μg g−1 to 4259 μg g−1 and 5135 μg g−1 when the TA concentrations raised from 0 to 10 and 20 mg L−1, respectively. Combined microscopic and spectroscopic investigations revealed that Cr(VI) was reduced to Cr(III) by TA and formed stable Cr(OH)3 colloids on PS surfaces. Contrarily, HA and FA inhibited Cr(VI) adsorption onto PS, especially at pH > 2.0 and higher DOM concentrations, due to site competition and electrostatic repulsion. Increase in pH was found to reduce zeta potentials of MPs, resulting in inhibited Cr(VI) adsorption. The adsorbed Cr(VI) declined with increasing ionic strength, implying that outer-sphere surface complexation affected the adsorption process in the presence of DOM. These new findings improved our fundamental understanding of the fate of Cr(VI) and MPs in DOM-rich environmental matrices.
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•Dissolved organic matter had different roles on Cr(VI) capture by PS microplastics.•TA promoted Cr(VI) adsorption while HA/FA inhibited its adsorption on PS.•Increase in pH and salinity hindered Cr(VI) adsorption on PS.•Cr(VI) was reduced to Cr(III) by TA and precipitated as Cr(OH)3 on PS surface.•SEM observation and XPS spectrum confirmed the deposition of Cr(OH)3 on PS surface.
The physico‐chemical properties of colloidal particles determine their uptake into cells. For a series of microparticles only one parameter, the mechanical stiffness, was varied, whereas other ...parameters such as size, shape, and charge were kept constant. The uptake was monitored in situ by analyzing individual particle trajectories including the progress of endocytosis, derived from local pH measurements around each particle. Evidence is presented that soft particles with low stiffness are transported faster to lysosomes than stiffer ones.
Stiffness matters: The properties of colloidal particles determine their uptake into cells. Only the mechanical stiffness of microparticles was varied, whereas other parameters were kept constant. The uptake was monitored in situ by analyzing individual particle trajectories including the progress of endocytosis. Soft particles with low stiffness are transported faster to lysosomes than stiffer ones.
Clear understanding of pore-scale mechanisms that control transport and retention of colloids in porous media at different physicochemical conditions is critical to improve design and efficient ...cleanup methodologies of filter beds. The objective of this work was to investigate the impact of hydrophobicity, solution ionic strength, and pH on colloid retention mechanisms in single-phase and two-phase flow in porous media systems. A series of experiments were conducted using a geometrically representative micromodel. Hydrophilic and hydrophobic colloids were dispersed in water at different solution ionic strength and pH conditions. Findings indicate that hydrophilic colloids exhibit high filtration efficiency as the colloids interact attractively with other colloids and solid-water-interface irrespective of the solution chemistry. However, for hydrophobic colloids, changes in solution chemistry significantly increase colloid retention where the colloid interaction become attractive with the increase in ionic strength and decrease in pH values. Colloids attached to the collector surfaces mobilized by the strong capillary forces induced by the moving gas-water interface and transported along with the interface. However, hydrophilic colloids redeposited on gas-water-solid interfaces or thin water films because of their greater capillary potential. Therefore, greater filtration efficiency is achieved with the hydrophilic colloids compared to the hydrophobic colloids for which the efficiency can be improved by changing the solution chemistry. Moreover, the removal efficiency by the moving gas-water interface was observed to be more for hydrophobic colloids compared to hydrophilic colloids for which the efficiency can be improved by lowering the ionic strength or increasing the pH value. This study indicates that the coupled effects of solution chemistry and colloid hydrophobicity should be taken into account while investigating efficient filtration and cleaning practices for the filter beds.
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•Applying a geometrically representative micromodel to study colloid retention mechanisms.•The filtration of hydrophobic colloids has been improved by altering solution chemistry.•Use of moving gas-water interface to clean filter beds with hydrophobic colloids.•Impact of solution chemistry differ for hydrophilic and hydrophobic colloids.•Incorporating coupled effects of ionic strength and pH on colloid transport mechanisms.
Phoretic Self-Propulsion Moran, Jeffrey L; Posner, Jonathan D
Annual review of fluid mechanics,
01/2017, Letnik:
49, Številka:
1
Journal Article
Recenzirano
Odprti dostop
It is well-known that micro- and nanoparticles can move by phoretic effects in response to externally imposed gradients of scalar quantities such as chemical concentration or electric potential. A ...class of active colloids can propel themselves through aqueous media by generating local gradients of concentration and electrical potential via surface reactions. Phoretic active colloids can be controlled using external stimuli and can mimic collective behaviors exhibited by many biological swimmers. Low-Reynolds number physicochemical hydrodynamics imposes unique challenges and constraints that must be understood for the practical potential of active colloids to be realized. Here, we review the rich physics underlying the operation of phoretic active colloids, describe their interactions and collective behaviors, and discuss promising directions for future research.
Colloidal Self Assembly
In article number 2211197, Dwaipayan Chakrabarti and co‐workers report on the self‐assembly of enantiomorphic single colloidal gyroid crystals in silico from designer chiral ...patchy spheres. Each of these crystals is shown to support a complete photonic bandgap, whilst also exhibiting rich chiroptical properties. This computational study introduces single colloidal gyroids as attractive targets for chiral photonic crystals operating at optical frequencies. Image credit: Wesley Flavell.
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