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1.
J Colloid Interface Sci ; 581(Pt A): 135-147, 2021 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-32771725

RESUMO

HYPOTHESIS: Molecular architecture and composition of amphiphilic bottlebrush copolymers will dictate the dominant interfacial relaxation modes and the corresponding dilatational rheology for adsorbed layers at oil/water interfaces in a way that will correlate with the emulsifying efficiency of different bottlebrush copolymers. EXPERIMENTS: Amphiphilic, xylene-soluble poly(ethylene oxide)-poly(n-butyl acrylate) (PEO-PBA) heterografted bottlebrush copolymers with controlled differences in backbone length, hydrophilicity and arm length were synthesized by atom transfer radical polymerization. Dilatational rheology of adsorbed layers at the xylene/water interface was probed via pendant drop tensiometry by measuring the interfacial stress response to either large-amplitude strain cycling or small-amplitude strain oscillation. The rheological response was recorded as a function of interfacial pressure for adsorbed layers under different compression states. Emulsifying efficiency was determined as the lowest copolymer concentration that yielded water-in-xylene emulsions with at least one-month stability against coalescence. FINDINGS: The more hydrophilic copolymers with longer PEO arms exhibited non-hysteretic stress-strain response curves in large-amplitude strain cycling and a tendency for the modulus to increase with increasing interfacial pressure. These were more efficient emulsifiers than less hydrophilic copolymers that exhibited hysteretic interfacial rheology. Mere existence of significant moduli did not correlate with high emulsifying efficiency, while an increase in modulus with increasing interfacial pressure did so.

2.
Langmuir ; 2020 Sep 03.
Artigo em Inglês | MEDLINE | ID: mdl-32830506

RESUMO

Depletion forces were measured between a silica sphere and a silica plate in solutions containing nonionic Pluronic P123 poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymers and anionic sodium dodecyl sulfate (SDS) surfactants using colloidal probe atomic force microscopy. Prior research established synergistic depletion force enhancement in solutions containing SDS and unimeric Pluronic F108 block copolymers via formation of large pseudo-polyelectrolyte complexes. The current work addresses a more complex system where the polymer is above its critical micelle concentration, and surfactant binding alters not only the size and charge of the micelles but also the number of polymers per micelle. Force profiles were measured in 10 000 ppm P123 (1 wt %, corresponding to 1.72 mM based on average molar mass) solutions containing SDS at concentrations up to 64 mM and compared to micellar P123 solutions and to P123-free SDS solutions. Whereas force profiles in the SDS-free micellar P123 solutions were purely repulsive, P123/SDS complexation produced synergistic depletion force enhancement for SDS concentrations between 2 and 32 mM. The synergism that occurred within a finite SDS concentration range was explained by comparing the hydrodynamic size, molar mass, charge, and concentration of depletants in P123/SDS mixtures and their respective single-component solutions obtained with the aid of dynamic light scattering, static light scattering, and dodecyl sulfate ion-selective electrode measurements. These measurements showed that complexation produced effects that would be mutually counteracting with respect to depletion forces: decreasing the mixed micelle hydrodynamic diameter relative to SDS-free P123 micelles would tend to weaken depletion forces, while adding charge and decreasing the aggregation number of polymers per micelle (thereby increasing the number concentration of micellar depletants) would tend to strengthen depletion forces.

3.
ACS Nano ; 14(9): 10954-10965, 2020 09 22.
Artigo em Inglês | MEDLINE | ID: mdl-32628009

RESUMO

Climate change is increasing the severity and length of heat waves. Heat stress limits crop productivity and can make plants more sensitive to other biotic and abiotic stresses. New methods for managing heat stress are needed. Herein, we have developed ∼30 nm diameter poly(acrylic acid)-block-poly(N-isopropylacrylamide) (PAA-b-PNIPAm) star polymers with varying block ratios for temperature-programmed release of a model antimicrobial agent (crystal violet, CV) at plant-relevant pH. Hyperspectral-Enhanced Dark field Microscopy was used to investigate star polymer-leaf interactions and route of entrance. The majority of loaded star polymers entered plant leaves through cuticular and epidermis penetration when applied with the adjuvant Silwet L-77. Up to 43 wt % of star polymers (20 µL at 200 mg L-1 polymer concentration) applied onto tomato (Solanum lycopersicum) leaves translocated to other plant compartments (younger and older shoots, stem, and root) over 3 days. Without Silwet L-77, the star polymers penetrated the cuticle, but mainly accumulated at the epidermis cell layer. The degree of the star polymer temperature responsiveness for CV release in vitro in the range of 20 to 40 °C depends on pH and the ratio of the PAA to PNIPAm blocks. Temperature-responsive release of CV was also observed in vivo in tomato leaves. These results underline the potential for PAA-b-PNIPAm star polymers to provide efficient and temperature-programmed delivery of cationic agrochemicals into plants for protection against heat stress.

4.
Soft Matter ; 16(1): 238-246, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31777903

RESUMO

We analytically calculate the one-dimensional advective-diffusive spreading of a point source of diffusiophoretic (DP) colloids, driven by the simultaneous diffusion of a Gaussian solute patch. The spreading of the DP colloids depends critically on the ratio of the DP mobility, M (which can be positive or negative), to the solute diffusivity, Ds. For instance, we demonstrate, for the first time, that solute-repelling colloids (M < 0) undergo long-time super-diffusive transport for M/Ds < -1. In contrast, the spreading of strongly solute-attracting colloids (M/Ds≫ 1) can be spatially arrested over long periods on the solute diffusion timescale, due to a balance between colloid diffusion and DP under the evolving solute gradient. Further, a patch of the translating solute acts as a "shuttle" that rapidly transports the colloids relative to their diffusive timescale. Finally, we use numerical computations to show that the above behaviors persist for three-dimensional, radially symmetric DP spreading. The results presented here could guide the use of DP colloids for microscale particle sorting, deposition, and delivery.

5.
Langmuir ; 35(48): 15937-15947, 2019 Dec 03.
Artigo em Inglês | MEDLINE | ID: mdl-31446760

RESUMO

Depletion and structural forces were measured between a silica sphere and plate in solutions containing sodium polyacrylate (Na-PAA) anionic polyelectrolyte and sodium dodecyl sulfate (SDS) anionic surfactant using colloidal probe atomic force microscopy, at high pH where the two species are electrostatically repelling from each other and from the silica surfaces. Measurements were performed for a range of SDS and Na-PAA concentrations to span conditions where only one of the species or both of the species would exert a detectable depletion or structural force when present in a single-component solution. In mixed solutions, conditions were identified (i) where depletion attraction was synergistically enhanced or antagonistically weakened relative to single component solutions; (ii) where the range of the depletion attraction was significantly extended and the repulsive structural force barrier was eliminated, due to simultaneous depletion of both species over different length scales; and (iii) where one species was the dominant depletant and forces in mixtures were indistinguishable from those in a single component solution of the dominant depletant. Force measurements were interpreted with the aid of pyrene solubilization assays of SDS micellization and dynamic light scattering investigation of the state of assembly of the polyelectrolyte or surfactant. The variety of colloidal force effects were attributed to ionic strength and excluded volume effects of Na-PAA on SDS micellization, ionic strength effects of SDS on Na-PAA chain clustering in solution, and ionic strength effects on the counterion contribution to polyelectrolyte osmotic pressure. While prior studies have shown that depletion force synergism occurs when polymers and surfactants form mixed complexes, this work shows that it can occur in noncomplexing mixtures as well, and it indicates the variety of effects that should be taken into account when attempting to predict forces in such mixtures.

6.
J Colloid Interface Sci ; 553: 436-450, 2019 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-31228758

RESUMO

HYPOTHESIS: Charge, size and concentration of depletants control the magnitude and range of depletion and structural forces. Noncovalent association of nonionic polymers with ionic surfactants may therefore synergistically enhance these forces to an extent that depends on the structure and composition of the resulting complexes. EXPERIMENTS: Forces were measured between a silica sphere and a silica plate in solutions of Pluronic F108 nonionic poly(ethylene oxide - block - propylene oxide - block - ethylene oxide) triblock copolymers and anionic sodium dodecylsulfate (SDS) surfactants using colloidal probe atomic force microscopy as a function of polymer, surfactant and NaCl background electrolyte concentrations. Trends in the magnitudes of the depletion attraction minimum and the first repulsive maximum in the oscillatory structural force were interpreted with the aid of pyrene solubilization assays, sodium ion-selective electrode analysis, and dynamic light scattering measurements that characterized the formation, charge, and hydrodynamic radius of F108/SDS complexes respectively. FINDINGS: Synergistic enhancement of the depletion force and first repulsive maximum occurred within a finite range of SDS concentrations, to an extent that depended on F108 and NaCl concentrations. This was due mainly to charging of F108/SDS complexes. Size effects were important at low NaCl concentration. Forces measured above the synergistic SDS concentration range were indistinguishable from those in polymer-free SDS solutions due to the appearance of excess unbound SDS micelles.

7.
J Colloid Interface Sci ; 553: 136-147, 2019 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-31202050

RESUMO

HYPOTHESIS: Surfactant-driven Marangoni flow on liquid films is predicted to depend on subphase depth and initial surface tension difference between the subphase and deposited surfactant solution drop. Changes in flow behavior will impact transport of soluble species entrained in the Marangoni flow along the surface. In extreme cases, the subphase film may rupture, limiting transport. Understanding this behavior is important for applications in drug delivery, coatings, and oil spill remediation. EXPERIMENTS: A trans-illumination optical technique measured the subphase height profiles and drop content transport after drop deposition when varying initial subphase depth, surfactant concentration, and subphase viscosity. FINDINGS: Three distinct flow regimes were identified depending on the subphase depth and surfactant concentration and mapped onto an operating diagram. These are characterized as a "central depression" bounded by an outwardly traveling ridge, an "annular depression" bounded by a central dome and the traveling ridge, and an "annular dewetting" when the subphase ruptures. Well above the critical micelle concentration, transitions between regimes occur at characteristic ratios of gravitational and initial surface tension gradient stresses; transitions shift when surfactant dilution during spreading weakens the stress before the completion of the spreading event. Drop contents travel with the ridge, but dewetting hinders transport.

8.
Colloids Surf A Physicochem Eng Asp ; 546: 266-275, 2018 Jun 05.
Artigo em Inglês | MEDLINE | ID: mdl-30416264

RESUMO

Traditionally, an interface is defined as a boundary between immiscible phases. However, previous work has shown that even when two fluids are completely miscible, they maintain a detectable "effective interface" for long times. Miscible interfaces have been studied in various systems of two fluids with a single boundary between them. However, this work has not extended to the three-phase system of a fluid droplet placed on top of a miscible pool. We show that these three-phase systems obey the same wetting conditions as immiscible systems, and that their drop shapes obey the Augmented Young-Laplace Equation. Over time, the miscible interface diffuses and the shape of the drop evolves. We place 2-microliter drops of water atop miscible poly(acrylamide) solutions. The drop is completely wetted by the subphase, and then remains detectable beneath the surface for many minutes. An initial effective interfacial tension can be approximated to be on the order of 0.5 mN/m using the capillary number. Water and poly(acrylamide) are completely miscible in all concentrations, and yet, when viewed from the side, the drop maintains a capillary shape. Study of this behavior is important to the understanding of effective interfaces between miscible polymer phases, which are pervasive in nature.

9.
Curr Opin Colloid Interface Sci ; 36: 58-69, 2018 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-30147429

RESUMO

Understanding the fundamentals of surface transport on thin viscous films has important application in pulmonary drug delivery. The human lung contains a large-area interface between its complex fluid lining and inhaled air. Marangoni flows driven by surface tension gradients along this interface would promote enhanced distribution of inhaled therapeutics by carrying them from where they are deposited in the upper airways, along the fluid interface to deeper regions of the lung. Motivated by the potential to improve therapies for acute and chronic lung diseases, we review recent progress in modeling and experimental studies of Marangoni transport induced by the deposition of surfactant-containing microliter drops and liquid aerosols (picoliter drops) onto a fluid interface. The roles of key system variables are identified, including surfactant solubility, drop miscibility with the subphase, and the thickness, composition and surface properties of the subphase liquid. Of particular interest is the unanticipated but crucial role of aerosol processing to achieve Marangoni transport via phospholipid vesicle dispersions, which are likely candidates for a biocompatible delivery system. Progress in this field has the potential to not only improve outcomes in patients with chronic and acute lung diseases, but also to further our understanding of surface transport in complex systems.

10.
Soft Matter ; 14(31): 6485-6495, 2018 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-30043771

RESUMO

Smart polymer hydrogels, which can undergo structural and volume phase transitions in response to external stimuli, have gained much attention for their widespread technological applications. Compared to linear polymers, branched chains offer more extensive opportunities to rationally design functional materials, since they permit more extensive structural tunability-for instance by adjusting the balance between hydrophobic and hydrophilic units, the grafting fraction of backbone monomers, or the side chain length, topology, and solubility. Here we conduct coarse-grained molecular dynamics simulations to assess how well generic physical principles capture this complex interplay of tuning parameters, specifically when building networks from complex branched chains with a hydrophobic backbone. Swollen chains collapse upon reducing side chain solubility, length, and grafting density, but neither the sharpness of this transition nor its dynamic range, if measured via chain extension, depends monotonically on these parameters. Networks comprising such chains are more swollen and exhibit even sharper transitions, but their higher responsiveness goes along with a swelling ratio that falls behind that of single chains.

11.
J Colloid Interface Sci ; 526: 114-123, 2018 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-29723792

RESUMO

HYPOTHESIS: Adsorption of polyelectrolyte brush-grafted nanoparticles (BGNPs) produces a heterogeneous interface with sub-monolayer surface coverage resulting from lateral electrostatic repulsions that limit packing. As a result, the interaction forces between opposing BGNP layers include an adhesive cross-surface BGNP-substrate bridging force that depends on the interparticle spacing, particle size, and strength of electrostatic interactions. We hypothesize that BGNPs with pH-responsive, annealed polyelectrolyte brushes can undergo controlled changes in surface area coverage through post-adsorption swelling or de-swelling into non-equilibrium layer conformations and that such changes in surface coverage can switch off or switch on particle intercalation, bridging attractions, and enhanced energy dissipation upon sliding. This work aims to characterize the nature of surface forces in heterogeneous BGNP adsorbed layers and to utilize pH-sensitive bridging forces as a mechanism to tune friction and adhesion. EXPERIMENTS: Colloidal probe atomic force microscopy (CP-AFM) is used to measure normal and lateral forces between negatively charged silica surfaces with adsorbed pH-responsive cationic BGNPs. The BGNPs are poly(2-dimethylaminoethyl methacrylate) brush-grafted silica nanoparticles. Adhesion force and friction analysis is complemented by simultaneous quartz-crystal microbalance and ellipsometry measurements under conditions that render the particles strongly charged and swollen (acidic) or weakly charged and de-swollen (basic). FINDINGS: Adsorbed BGNPs can be swollen or de-swollen via pH rinses, enabling direct control of surface coverage and bridging interactions. Transitions from adhesive bridging contacts with high friction to non-adhesive contacts with low friction forces occur when adsorbed BGNP layers are switched from a de-swollen/weakly charged state to a swollen/highly charged state. The ability to controllably shift the character of normal and lateral forces via coverage-mediated bridging interactions is a unique feature of adsorbed nanoparticulate brush constructs and highlights their potential to condition surfaces with additional functionality compared to dense, planar homopolymer brushes.

12.
Langmuir ; 34(16): 4852-4860, 2018 04 24.
Artigo em Inglês | MEDLINE | ID: mdl-29566490

RESUMO

Motivated by the proposed use of cationic protein-modified sand for water filtration in developing nations, this study concerns the adsorption of Moringa oleifera seed proteins to silica surfaces. These proteins were prepared in model waters of varying hardness and underwent different levels of fractionation, including fatty acid extraction and cation exchange chromatography. Adsorption isotherms were measured by ellipsometry, and the zeta potentials of the resulting protein-decorated surfaces were measured by the rotating disk streaming potential method. The results indicate that the presence of fatty acids has little effect on the M. oleifera cationic protein adsorption isotherm. Adsorption from the unfractionated extract was indistinguishable from that of the cationic protein isolates at low concentrations but yielded significantly greater extents of adsorption at high concentrations. Adsorption isotherms for samples prepared in model hard and soft fresh waters were indistinguishable from each other over the measured bulk solution concentration range, but adsorption from hard or soft water was more extensive than adsorption from deionized water at moderate protein concentrations. Streaming potential measurements showed that adsorption reversed the net sign of the zeta potential of silica from negative to positive for all protein fractions and water hardness conditions at protein bulk concentrations as low as 0.03 µg/mL. This suggests that sands can be effectively modified with M. oleifera proteins using small amounts of seed extract under various local water hardness conditions. Finally, ellipsometry indicated that M. oleifera proteins adsorb irreversibly with respect to rinsing in these model fresh waters, suggesting that the modified sand would be stable on repeated use for water filtration. These studies may aid in the design of a simple, effective, and sustainable water purification device for developing nations.


Assuntos
Moringa oleifera/química , Proteínas de Plantas/química , Sementes/química , Dióxido de Silício/química , Purificação da Água/instrumentação , Purificação da Água/métodos , Água/química , Adsorção , Ácidos Graxos/química , Minerais/química , Proteínas de Plantas/isolamento & purificação
13.
J Aerosol Med Pulm Drug Deliv ; 31(4): 212-220, 2018 08.
Artigo em Inglês | MEDLINE | ID: mdl-29053080

RESUMO

BACKGROUND: Secondary lung infections are the primary cause of morbidity associated with cystic fibrosis lung disease. Aerosolized antibiotic inhalation is potentially advantageous but has limited effectiveness due to altered airway aerodynamics and deposition patterns that limit drug access to infected regions. One potential strategy to better reach infected areas is to formulate aerosols with surfactants that induce surface tension gradients and drive postdeposition drug dispersal via Marangoni transport along the airway surface liquid (ASL). Since this relies on surfactant-induced surface tension reduction, the presence of endogenous lipid monolayers may hinder drug dispersal performance. METHODS: Tobramycin solutions were formulated with dipalmitoylphosphatidylcholine (DPPC), a major component of endogenous pulmonary surfactant, to drive postdeposition aerosol dispersal across a model ASL based on a liquid layer or "subphase" of aqueous porcine gastric mucin (PGM) solution with predeposited DPPC monolayers to mimic the endogenous surfactant. In vitro subphase samples were collected from regions outside the aerosol deposition zone and assayed for tobramycin concentration using a closed enzyme donor immunoassay. The motion of a tracking bead across the subphase surface and the corresponding decrease in surface tension on aerosol deposition were tracked both with and without a predeposited DPPC monolayer. The surface tension/area isotherm for DPPC on PGM solution subphase was measured to aid in the interpretation of the tobramycin dispersal behavior. RESULTS AND CONCLUSIONS: Transport of tobramycin away from the deposition region occurs in aerosols formulated with DPPC whether or not predeposited lipid is present, and tobramycin concentrations are similar in both cases across biologically relevant length scales (∼8 cm). When DPPC is deposited from an aerosol, it induces ultralow surface tensions (<5 mN/m), which drive Marangoni flows, even in the presence of a dense background layer of DPPC. Therefore, aerosolized phospholipids, such as DPPC, will likely be effective spreading agents in the human lung.


Assuntos
1,2-Dipalmitoilfosfatidilcolina/química , Antibacterianos/farmacocinética , Pulmão/metabolismo , Tobramicina/farmacocinética , Aerossóis , Animais , Transporte Biológico , Mucinas/química , Tensão Superficial , Suínos , Tobramicina/administração & dosagem
14.
Environ Sci Pollut Res Int ; 25(8): 7157-7169, 2018 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26233743

RESUMO

For in situ groundwater remediation, polyelectrolyte-modified nanoscale zerovalent iron particles (NZVIs) have to be delivered into the subsurface, where they degrade pollutants such as trichloroethylene (TCE). The effect of groundwater organic and ionic solutes on TCE dechlorination using polyelectrolyte-modified NZVIs is unexplored, but is required for an effective remediation design. This study evaluates the TCE dechlorination rate and reaction by-products using poly(aspartate) (PAP)-modified and bare NZVIs in groundwater samples from actual TCE-contaminated sites in Florida, South Carolina, and Michigan. The effects of groundwater solutes on short- and intermediate-term dechlorination rates were evaluated. An adsorbed PAP layer on the NZVIs appeared to limit the adverse effect of groundwater solutes on the TCE dechlorination rate in the first TCE dechlorination cycle (short-term effect). Presumably, the pre-adsorption of PAP "trains" and the Donnan potential in the adsorbed PAP layer prevented groundwater solutes from further blocking NZVI reactive sites, which appeared to substantially decrease the TCE dechlorination rate of bare NZVIs. In the second and third TCE dechlorination cycles (intermediate-term effect), TCE dechlorination rates using PAP-modified NZVIs increased substantially (~100 and 200%, respectively, from the rate of the first spike). The desorption of PAP from the surface of NZVIs over time due to salt-induced desorption is hypothesized to restore NZVI reactivity with TCE. This study suggests that NZVI surface modification with small, charged macromolecules, such as PAP, helps to restore NZVI reactivity due to gradual PAP desorption in groundwater.


Assuntos
Ácido Aspártico/química , Água Subterrânea/química , Ferro/química , Nanopartículas Metálicas/química , Tricloroetileno/química , Poluentes Químicos da Água/química , Purificação da Água/métodos , Adsorção , Halogenação , Tricloroetileno/isolamento & purificação , Poluentes Químicos da Água/isolamento & purificação
15.
Phys Chem Chem Phys ; 19(35): 23854-23868, 2017 Sep 13.
Artigo em Inglês | MEDLINE | ID: mdl-28726899

RESUMO

Interfacial tension reduction, dynamic dilatational elasticity and extent of adsorption were investigated for linear poly(ethylene oxide) (PEO) chains of varying molecular weight and for PEO star polymers with an average of 64 arms per star at air/water, xylene/water, and cyclohexane/water interfaces. Adsorption on planar interfaces was monitored by ellipsometry, while interfacial tension and dilatational elasticity were measured separately by pendant drop tensiometry. Previously reported to be efficient emulsifiers, PEO stars are shown here to also be more effective foaming agents than linear PEO. Accordingly, PEO stars adsorb to a greater extent and produce larger interfacial tension reduction and greater dynamic dilatational moduli than linear PEO. The more extensive adsorption and greater interfacial tension reduction for PEO stars are attributed to their compactness. More mass is introduced per unit area of interface, and more interfacial penetration is achieved, upon their adsorption than for adsorption of linear polymers that adopt the conformation of loops, trains and tails. Whereas cyclohexane is a non-solvent for PEO, xylene is a good solvent. Dispersing PEO stars in the xylene phase yields greater interfacial tension reduction at the xylene/water interface than occurs when initially dispersing PEO stars in the aqueous phase. In contrast, the interfacial tension for linear PEO shows no dependence on the phase from which it adsorbs. Ellipsometry confirms the path-dependent extent of adsorption to the xylene/water interface, but also reveals additional complexity. When adsorbing from xylene, thick interfacial films result that likely contain dispersed water, as suggested by the observation of spontaneous water-in-xylene emulsification when PEO stars are initially dispersed in xylene. This is tentatively attributed to shear provided by Marangoni flow. Spontaneous emulsification occurs only when PEO stars are initially dispersed in the xylene phase. Linear PEO produces neither thick interfacial films nor spontaneous emulsification.

16.
Colloids Surf A Physicochem Eng Asp ; 521: 49-60, 2017 May 20.
Artigo em Inglês | MEDLINE | ID: mdl-28479673

RESUMO

Marangoni flows offer an interesting and useful means to transport particles at fluid interfaces with potential applications such as dry powder pulmonary drug delivery. In this article, we investigate the transport of partially wetted particles at a liquid/vapor interface under the influence of Marangoni flows driven by gradients in the surface excess concentration of surfactants. We deposit a microliter drop of soluble (sodium dodecyl sulfate aqueous solution) surfactant solution or pure insoluble liquid (oleic acid) surfactant on a water subphase and observe the transport of a pre-deposited particle. Following the previous observation by Wang et al. [1] that a surfactant front rapidly advances ahead of the deposited drop contact line initiates particle motion but then moves beyond the particle, we now characterize the two dominant, time- and position-dependent forces acting on the moving particle: 1) a surface tension force acting on the three-phase contact line around the particle periphery due to the surface tension gradient at the liquid/vapor interface which always accelerates the particle and 2) a viscous force acting on the immersed surface area of the particle which accelerates or decelerates the particle depending on the difference in the velocities of the liquid and particle. We find that the particle velocity evolves over time in two regimes. In the acceleration regime, the net force on the particle acts in the direction of particle motion, and the particle quickly accelerates and reaches a maximum velocity. In the deceleration regime, the net force on the particle reverses and the particle decelerates gradually and stops. We identify the parameters that affect the two forces acting on the particle, including the initial particle position relative to the surfactant drop, particle diameter, particle wettability, subphase thickness, and surfactant solubility. We systematically vary these parameters and probe the spatial and temporal evolution of the two forces acting on the particle as it moves along its trajectory in both regimes. We find that a larger particle always lags behind the smaller particle when placed at an equal initial distance from the drop. Similarly, particles more deeply engulfed in the subphase lag behind those less deeply engulfed. Further, the extent of particle transport is reduced as the subphase thickness decreases, due to the larger velocity gradients in the subphase recirculation flows.

17.
J Hazard Mater ; 322(Pt A): 136-144, 2017 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-27250869

RESUMO

The effect of nZVI mass loading and groundwater velocity on the tetrachloroethylene (PCE) dechlorination rate and the hydrogen evolution rate for poly(maleic acid-co-olefin) (MW=12K) coated nZVI was examined. In batch reactors, the PCE reaction rate constant (3.7×10-4Lhr-1m-2) and hydrogen evolution rate constant (1.4 nanomolLhr-1m-2) were independent of nZVI concentration above 10g/L, but the PCE dechlorination rate decreased and the hydrogen evolution rate increased for nZVI concentration below 10g/L. The nonlinearity between nZVI mass loading and PCE dechlorination and H2 evolution was explained by differences in pH and Eh at each nZVI mass loading; PCE reactivity increased when solution Eh decreased, and the H2 evolution rate increased with decreasing pH. Thus, nZVI mass loading of <5g/L yields lower reactivity with PCE and lower efficiency of Fe° utilization than for higher nZVI mass loading. The PCE dechlorination rate increased with increasing pore-water velocity, suggesting that mass transfer limits the reaction at low porewater velocity. Overall, this work suggests that design of nZVI-based reactive barriers for groundwater treatment should consider the non-linear effects of both mass loading and flow velocity on performance and expected reactive lifetime.

18.
Environ Sci Technol ; 51(3): 1754-1763, 2017 02 07.
Artigo em Inglês | MEDLINE | ID: mdl-27997146

RESUMO

Silver nanoparticles (AgNPs) released from consumer products may enter the environment and possibly harm microbial communities. Prior research showed that surface-adherent AgNPs inhibit bacterial surface colonization, a precursor to biofilm formation, only when planktonic bacterial inoculum concentrations are less than a threshold level ( Wirth and co-workers, J. Colloid Interface Sci. 2016 , 467 , 17 - 27 ). This inoculum effect is due to a decrease in free silver ion concentration associated with sublethal binding to bacteria. Natural organic matter can be an additional silver sink in environmental systems. Using Pseudomonas fluorescens as a model biofilm-forming bacterium, we find significant increases in minimum bactericidal concentrations for AgNP suspensions and Ag+ in solution when adding humic acid (HA) to bacterial suspensions. When HA is present, planktonic bacteria survive and colonize AgNP-laden glass surfaces at lower bacterial inoculum concentrations than were needed for survival and colonization in its absence. This occurs despite the observed tendency of HA to inhibit colonization on bare glass surfaces when silver is absent. Results are interpreted through equilibrium Ag+ binding isotherms to HA and suspended bacteria. These results indicate that silver ion sinks may lessen AgNP impacts on natural microbial ecology relative to the disruption observed in pristine laboratory conditions.


Assuntos
Substâncias Húmicas , Prata/farmacologia , Íons/farmacologia , Nanopartículas Metálicas , Plâncton/efeitos dos fármacos
19.
Langmuir ; 32(44): 11440-11447, 2016 11 08.
Artigo em Inglês | MEDLINE | ID: mdl-27734683

RESUMO

This work investigates surface forces that result from adsorbed layers of silica nanoparticles with grafted pH-responsive, cationic poly(2-(dimethylamino)ethyl methacrylate) brushes (SiO2-g-PDMAEMA) and how adhesive bridging forces can be suppressed and friction forces reduced by "backfilling" these heterogeneous adsorbed layers with nonionic poly(ethylene oxide) star copolymers (Star PEO45MA). Adsorption of SiO2-g-PDMAEMA and Star PEO45MA to silica is measured as a function of pH by quartz crystal microbalance with dissipation (QCM-D) in order to evaluate the electrostatically driven adsorption of SiO2-g-PDMAEMA and hydrogen-bonding-driven adsorption of Star PEO45MA. Force measurements performed using colloidal probe force microscopy show the strong role that limited surface coverage plays in adhesive bridging forces between silica with adsorbed SiO2-g-PDMAEMA, while Star PEO45MA adsorption produces purely repulsive steric interactions. Bridging between SiO2-g-PDMAEMA-coated surfaces produces frictional forces that tend to be larger than those acting between bare surfaces at similar normal loads, while friction is consistently decreased by Star PEO45MA adsorption. Sequential adsorption of SiO2-g-PDMAEMA and Star PEO45MA generates high-coverage mixed nanoparticulate brush layers with uniformly repulsive normal forces and reduced friction forces. Adsorption and force measurements reveal that Star PEO45MA not only adsorbs to silica but also binds to SiO2-g-PDMAEMA. The latter allows sequential adsorption of the two components to produce mixed multilayers. The mixed SiO2-g-PDMAEMA/Star PEO45MA multilayers exhibit larger layer thicknesses, no bridging, and sustained smooth friction, highlighting their potential usefulness as aqueous boundary lubricant layers.

20.
J Colloid Interface Sci ; 484: 270-278, 2016 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-27623189

RESUMO

It has long been known that deposited drops of surfactant solution induce Marangoni flows at air-liquid interfaces. These surfactant drops create a surface tension gradient, which causes an outward flow at the fluid interface. We show that aqueous phospholipid dispersions may be used for this same purpose. In aqueous dispersions, phospholipids aggregate into vesicles that are not surface-active; therefore, drops of these dispersions do not initiate Marangoni flow. However, aerosolization of these dispersions disrupts the vesicles, allowing access to the surface-active monomers within. These lipid monomers do have the ability to induce Marangoni flow. We hypothesize that monomers released from broken vesicles adsorb on the surfaces of individual aerosol droplets and then create localized surface tension reduction upon droplet deposition. Deposition of lipid monomers via aerosolization produces surface tensions as low as 1mN/m on water. In addition, aerosolized lipid deposition also drives Marangoni flow on entangled polymer solution subphases with low initial surface tensions (∼34mN/m). The fact that aerosolization of phospholipids naturally found within pulmonary surfactant can drive Marangoni flows on low surface tension liquids suggests that aerosolized lipids may be used to promote uniform pulmonary drug delivery without the need for exogenous spreading agents.


Assuntos
1,2-Dipalmitoilfosfatidilcolina/química , Resinas Acrílicas/química , Dimiristoilfosfatidilcolina/química , Mucinas/química , Surfactantes Pulmonares/química , Lipossomas Unilamelares/química , Aerossóis , Animais , Transição de Fase , Reologia , Soluções , Tensão Superficial , Suínos , Água/química
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