Surface Characteristics of Microporous and Mesoporous Carbons Functionalized with Pentafluorophenyl Groups.

The in situ diazonium reduction reaction is a reliable and well-known approach for the surface modification of carbon materials for use in a range of applications, including in energy conversion, as chromatography supports, in sensors, etc. Here, this approach was used for the first time with mesoporous colloid-imprinted carbons (CICs), materials that contain ordered monodisperse pores (10-100 nm in diameter) and are inherently highly hydrophilic, using a common microporous carbon (Vulcan carbon (VC)), which is relatively more hydrophobic, for a comparison. The ultimate goal of this work was to modify the CIC wettability without altering its nanostructure and also to lower its susceptibility to oxidation, as required in fuel cell and battery electrodes, by the attachment of pentafluorophenyl (-PhF5) groups onto their surfaces. This was shown to be successful for the CIC, with the -PhF5 groups uniformly coating the inner pore walls at a surface coverage of ca. 90% and allowing full solution access to the mesopores, while the -PhF5 groups deposited only on the outer VC surface, likely blocking its micropores. Contact angle kinetics measurements showed enhanced hydrophobicity, as anticipated, for both the -PhF5 modified CIC and VC materials, even revealing superhydrophobicity at times for the CIC materials. In contrast, water vapor sorption and cyclic voltammetry suggested that the micropores remained hydrophilic, arising from the deposition of smaller N- and O-containing surface groups, caused by a side reaction during the in situ diazonium functionalization process.

Wettability of Nafion and Nafion/Vulcan carbon composite films.

The wettability of the Pt/carbon/Nafion catalyst layer in proton exchange membrane fuel cells is critical to their performance and durability, especially the cathode, as water is needed for the transport of protons to the active sites and is also involved in deleterious Pt nanoparticle dissolution and carbon corrosion. Therefore, the focus of this work has been on the first-time use of the water droplet impacting method to determine the wettability of 100% Nafion films, as a benchmark, and then of Vulcan carbon (VC)/Nafion composite films, both deposited by spin-coating in the Pt-free state. Pure Nafion films, shown by SEM analysis to have a nanochanneled structure, are initially hydrophobic but become hydrophilic as the water droplet spreads, likely due to reorientation of the sulfonic acid groups toward water. The wettability of VC/Nafion composite films depends significantly on the VC/Nafion mass ratios, even though Nafion is believed to be preferentially oriented (sulfonate groups toward VC) in all cases. At low VC contents, a significant water droplet contact angle hysteresis is seen, similar to pure Nafion films, while at higher VC contents (>30%), the films become hydrophobic, also exhibiting superhydrophobicity, with surface roughness playing a significant role. At >80% VC, the surfaces become wettable again as there is insufficient Nafion loading present to fully cover the carbon surface, allowing the calculation of the Nafion:carbon ratio required for a full coverage of carbon by Nafion.

Synthesis of hydrophobic derivatives of the G/\C base for rosette nanotube self-assembly in apolar media.

Eleven self-complementary G/\C derivatives bearing hydrophobic moieties were synthesized and characterized. One representative derivative from this family was shown to self-assemble into rosette nanotubes in hexane and form Langmuir-Blodgett films at the air-water interface.

Contact line and contact angle dynamics in superhydrophobic channels.

The dynamics of the wetting and movement of a three-phase contact line confined between two superhydrophobic surfaces were studied using a mean-field free-energy lattice Boltzmann model. Principle features of superhydrophobic surfaces, such as trapped vapor/air between rough microstructures, high contact angles, reduced contact angle hysteresis, and low resistance to fluid flow, were all observed. Movement of the three-phase contact line over a well-patterned superhydrophobic surface displays a periodic stick-jump-slip behavior, while the dynamic contact angle changes accordingly from maximum to minimum. Two regimes were found for the flow velocity as a function of surface roughness and can be related directly to the balance between driving force and flow resistance. This work provides a better understanding of dynamic wetting and fluid flow behaviors over superhydrophobic surfaces and hence could be useful in related applications.

Pressure boundary condition of the lattice Boltzmann method for fully developed periodic flows.

The general periodic boundary condition for the lattice Boltzmann method has been modified to incorporate the pressure difference for fully developed periodic flows. The results demonstrated that, unlike other existing pressure boundary treatments, the proposed procedure/treatment does not generate nonphysical inlet and outlet flow disturbances while preserving the system periodicity. This method is readily applicable to a range of lattice Boltzmann simulations for systems with periodic electric potential and temperature fields.

An improved method for determining zeta potential and pore conductivity of porous materials.

An improved method based on streaming potential and streaming current was proposed to determine zeta potential and surface conductance of porous material simultaneously. In the electrokinetic generation mode, a resistor is connected to the generator and by measuring the voltage drop across resistors with different resistance, a true streaming current can be determined. The zeta potential and surface conductivity can be obtained simultaneously from their relation to streaming potential and streaming current. The electrode and ion concentration polarization effects during the measurement were also discussed. The resistance from channel ends to electrodes, which has typically been ignored in the literature, was shown to have a significant influence on the calculated zeta potential and surface conductance. Ignorance of this resistance would lead to underestimation of both zeta potential and surface conductance values.

The similarity of electric double-layer interaction from the general Poisson-Boltzmann theory.

We studied electric double-layer (EDL) interactions in electrolytes with different valence combinations. Our results show that the interactions are similar for electrolytes with the same co-ion valences and concentrations and such similarity increases with the co-ion valence and surface potential. A scaled surface potential was defined and found to be useful in characterizing the difference in EDL interaction. These results show that co-ions play a more important role than counterions in determining EDL potential and interaction in an electrolyte solution, especially for systems with high co-ion valence and/or high surface potentials.

Transient streaming potential in a finite length microchannel.

Pressure-driven flow of an electrolyte solution in a microchannel with charged solid surfaces induces a streaming potential across the microchannel. Such a flow also causes rejection of ions by the microchannel, leading to different concentrations in the feed and permeate reservoirs connecting the capillary, which forms the basis of membrane based separation of electrolytes. Modeling approaches traditionally employed to assess the streaming potential development and ion rejection by capillaries often present a confusing picture of the governing electrochemical transport processes. In this paper, a transient numerical simulation of electrochemical transport process leading to the development of a streaming potential across a finite length circular cylindrical microchannel connecting two infinite reservoirs is presented. The solution based on finite element analysis shows the transient development of ionic concentrations, electric fields, and the streaming potential over the length of the microchannel. The transient analysis presented here resolves several contradictions between the two types of modeling approaches employed in assessing streaming potential development and ion rejection. The simulation results show that the streaming potential across the channel is predominantly set up at the timescale of the developing convective transport, while the equilibrium ion concentrations are developed over a considerably longer duration.

Electrokinetic power generation by means of streaming potentials: a mobile-ion-drain method to increase the streaming potentials.

We show, by natural occurring phenomena of charge separation near the solid-liquid interface in microchannels, that electricity can be generated by forcing water through a ceramic rod with no moving part and emission. A single hand push on a syringe is our source of power which easily generates a streaming potential of over 20 V and a streaming current of 30 microA. By means of streaming potentials, two capacitors were charged and discharged alternatively to light-up two Light-Emitting-Diodes in every ten seconds. From our specific choice of liquid/solid pair, an efficiency of 0.8% was obtained. A mobile-ion-drain method is also demonstrated to increase the streaming potential.

Surface-ascension of discrete liquid drops via experimental reactive wetting and lattice Boltzmann simulation.

The reactive-wetting technique is employed to move liquid against gravitational force. Experiments have shown that the velocity of an ascending liquid drop is constant, unlike the gradual decrease intuitively linked to objects against gravitation. The ascending velocity decreases for increasing slope. The maximum inclination, or stopping, angle for this particular setup is >25 degrees . Computer simulation of a reactive-wetting drop using the lattice Boltzmann method is also performed. The results indicate that the method employed is suitable for the task, producing most experimentally observable responses. The mass flow of a liquid drop under reactive wetting was studied through simulation results, and a general description of the reactive-wetting phenomenon was deduced.

Investigating the evaporation of metered-dose inhaler formulations in humid air: single droplet experiments.

The effect of excipients and humidity on the evaporation of propellant from metered-dose inhaler (MDI) formulations was examined by means of single, pendant droplet experiments. Droplets of pure hydrofluoroalkane (HFA) 227ea propellant, as well as propellant-ethanol and propellant-ethanol-sorbitan-trioleate mixtures, were suspended by needle into a conditioned viewing chamber. Droplet evaporation was recorded through a microscope-coupled CCD camera for each mixture, with viewing chamber conditions of 37 degrees C and either 100% or <10% relative humidity, over a size range from approximately 4 to approximately 1 microL. Volume versus time data was collected for each droplet through digital processing of image frames, according to Axisymmetric Drop Shape Analysis (ADSA) routines. No significant difference was observed in the rate of change of droplet volume (i.e., evaporation rate) between dry and humid conditions, regardless of the formulation studied. In addition, the rate of propellant evaporation appeared unchanged despite the addition of 15% w/w ethanol to HFA 227ea, while only slight reductions in evaporation rate were observed for a mixture containing 15% ethanol and 0.2% sorbitan trioleate. It was concluded that the rate of evaporation of propellant from HFA-based MDI formulations likely remains unchanged in the presence of high levels of humidity. Therefore, alternative explanations should be explored to explain the increase in MDI particle deposition in highly humid, confined airways.

The dynamics of impacting water droplets on alkanethiol self-assembled monolayers with co-adsorbed CH3 and CO2H terminal groups.

The impact of water droplets (diameter 3.6 mm) at a fixed Weber number of 59 on solid surfaces with precisely tailored surface wettabilities was studied experimentally using a high-speed imaging camera at 2500 frames per second. Solid surface wettability was varied using four fractional mixtures of self-assembled monolayers of 1-octadecanethiol and 16-mercaptohexadecanoic acid. The surfaces so obtained are characterized for contact angle and chemical functionality using the axisymmetric drop shape analysis profile (ADSA-P) technique and Fourier transform infrared spectroscopy (FT-IR). Our results correlate the wetting effects of the impacting droplets with the surface energy and contact angle measurements of the tailored surfaces. Literature models for the maximum spreading diameter are employed and compared with those from our experiments. An equation is also proposed for the maximum spreading diameter which makes use of the correct contact angles and results in the least error among the models considered. As a consequence of Young's equation, the correct contact angles to be used for droplet impact dynamics should be the corresponding advancing angles on a smooth substrate of interest. We also conclude that accurate examination of literature models requires careful experimentation on impact dynamic data on well-prepared and characterized surfaces such as those presented here.

On the surface conductance, flow rate, and current continuities of microfluidics with nonuniform surface potentials.

The characteristics of electrokinetic flow in a microchannel depend on both the nature of surface potentials, that is, whether it is uniform or nonuniform, and the electrical potential distribution along the channel. In this paper, the nonlinear Poisson-Boltzmann equation is used to model the electrical double layer and the lattice Boltzmann model coupled with the constraint of current continuity is used to simulate the microfluidic flow field in a rectangular microchannel with a step variation of surface potentials. This current continuity, including surface conduction, convection, and bulk conduction currents, has often been neglected in the literature for electroosmotic flow with nonuniform (heterogeneous) microchannels. Results show that step variation of ion distribution caused by step variation surface potential will influence significantly the electrical potential distribution along the channel and volumetric flow rate. For the system considered, we showed that the volumetric flow rate could have been overestimated by as much as 70% without consideration of the current continuity constraint.

Tradeoff between mixing and transport for electroosmotic flow in heterogeneous microchannels with nonuniform surface potentials.

Electroosmotic flow (EOF) is a phenomenon associated with the movement of an aqueous solution induced by the application of an electric field in microchannels. The characteristics of EOF depend on the nature of the surface potential, i.e., whether it is uniform or nonuniform. In this paper, a lattice Boltzmann model (LBM) combined with the Poisson-Boltzmann equation is used to simulate flow field in a rectangular microchannel with nonuniform (step change) surface potentials. The simulation results indicate that local circulations can occur near a heterogeneous region with nonuniform surface potentials, in agreement with those by other authors. Largest circulations, which imply a highest mixing efficiency due to convection and short-range diffusion, were found when the average surface potential is zero, regardless of whether the distribution of the heterogeneous patches is symmetric or asymmetric. In this work, we have illustrated that there is a trade-off between the mixing and liquid transport in EOF microfluidics. One should not simply focus on mixing and neglect liquid transport, as performed in the literature. Excellent mixing could lead to a poor transport of electroosmotic flow in microchannels.

On the maximum spreading diameter of impacting droplets on well-prepared solid surfaces.

This paper presents a systematic study of liquid droplet impact on three polymer surfaces: poly(methyl methacrylate), poly(methyl methacrylate/n-butyl methacrylate), and poly(n-butyl methacrylate). Changing from one surface to the next represents an incremental variation in solid surface tensions of 5-6 mJ/m2. These surfaces were prepared through careful experimental procedures that were used for the determination of solid surface tensions from contact angles. Our data for the maximum spreading diameter of water and formamide impacting on these surfaces were compared with those predicted from literature models. Of the models selected, we modified the model of Pasandideh-Fard et al. [Phys. Fluids 1996, 8, 650] and the results yielded a least error of only 5.09 +/- 5.05% in the determination of the maximum spreading diameter. The improved model was also compared with literature data, and good agreement was found. Of course, any such comparisons would rely on accurate experimental impact dynamics data on carefully prepared surfaces.

On the validity of the Cassie equation via a mean-field free-energy lattice Boltzmann approach.

We studied wetting phenomena on heterogeneous surfaces by a mean-field free-energy lattice Boltzmann method recently proposed [Phys. Rev. E 69 (2004) 32,602]. Our results suggest that the Cassie equation in macroscopic contact angle measurements is in general not valid. It was found that the Cassie equation is valid only when the patch size is on the same order of the liquid-vapor interfacial thickness. We also demonstrated that contact angle manifests itself from local surface properties near the contact point and does not result from the specific solid-liquid interactions across the contact area.

Apparent slip over a solid-liquid interface with a no-slip boundary condition.

We studied solid-liquid slip by a mean-field free-energy lattice Boltzmann approach recently proposed [Phys. Rev. E 69, 032602 (2004)]. With a general bounce-back no-slip boundary condition applied to the interface, liquid slip was observed because of the specific solid-fluid interactions. Our work relates interfacial slip to a more realistic solid-fluid interaction and hence contact angle. The kinetic nature of LBM is manifested in this interfacial study. A small negative slip length can also be produced with a stronger solid-fluid attraction.

Wettability interpretation of oxygen plasma modified poly(methyl methacrylate).

Poly(methyl methacrylate) (PMMA) has been modified via a dc pulsed oxygen plasma for different treatment times. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), optical profilometer, zeta potential, and advancing contact angle measurements. The measured advancing contact angles of water decreased considerably as a function of discharge. Several oxygen-based functionalities (carbonyl, carboxyl, carbonate, etc.) were detected by XPS, while zeta potential measurements confirmed an increase in negative charge for the treated PMMA surface. Evaluating the correlation between the concentration of polar chemical species and zeta potential, we found that increase in surface hydrophilicity results from the coeffect due to incorporation of oxygen functional groups and creation of charge states. The electrical double layer (EDL) effect was also considered in contact angle interpretation by introducing an additional surface tension term into Young's equation. We also found that EDL contribution to the solid-liquid interfacial tension is negligible and can be safely ignored for the systems considered here.

Dynamic interfacial effect of electroosmotic slip flow with a moving capillary front in hydrophobic circular microchannels.

Miniaturization of chemical analysis using microfabrication is an emerging technology. The use of polymeric materials as opposed to conventional glass substrate is also a promising alternative. As most polymeric materials are hydrophobic relative to glass, we describe here the implication for the loading process of electroosmotic flow (EOF) when a three-phase (solid-liquid-vapor) contact line exists. The presence of these interfaces can result in a large Laplace pressure that resists EOF and hence hinders its flow performance. This effect depends on the phenomenological contact angle at the solid-liquid interface. In our model for EOF, we considered simultaneously the presence of an electric double layer, liquid slips via a weaker solid-liquid interaction and Laplace pressure across a liquid-vapor interface.

Lattice boltzmann study on the contact angle and contact line dynamics of liquid-vapor interfaces.

The moving contact line problem of liquid-vapor interfaces was studied using a mean-field free-energy lattice Boltzmann method recently proposed [Phys. Rev. E 2004, 69, 032602]. We have examined the static and dynamic interfacial behaviors by means of the bubble and capillary wave tests and found that both the Laplace equation of capillarity and the dispersion relation were satisfied. Dynamic contact angles followed the general trend of contact line velocity observed experimentally and can be described by Blake's theory. The velocity fields near the interface were also obtained and are in good agreement with fluid mechanics and molecular dynamics studies. Our simulations demonstrated that incorporating interfacial effects into the lattice Boltzmann model can be a valuable and powerful alternative in interfacial studies.