Photothermal Atomic Force Microscopy Coupled with Infrared Spectroscopy (AFM-IR) Analysis of High Extinction Coefficient Materials: A Case Study with Silica and Silicate Glasses.

Photothermal atomic force microscopy coupled with infrared spectroscopy (AFM-IR) brings significant value as a spatially resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret AFM-IR spectra, since the existing theory described for organic materials does not work for materials with high extinction coefficients for infrared (IR) absorption. This paper describes theoretical calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample. This calculation explains the differences in peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IR applied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes and surface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layers through aqueous corrosion.

Scale-up issues for commercial depth filters in bioprocessing.

Significant increases in cell density and product titer have led to renewed interest in the application of depth filtration for initial clarification of cell culture fluid in antibody production. The performance of these depth filters will depend on the local pressure and velocity distribution within the filter capsule, but these are very difficult to probe experimentally, leading to challenges in both process design and scale-up. We have used a combination of carefully designed experimental studies and computational fluid dynamics (CFD) to examine these issues in both lab-scale (SupracapTM 50) and pilot-scale (StaxTM ) depth filter modules, both employing dual-layer lenticular PDH4 media containing diatomaceous earth. The SupracapTM 50 showed a more rapid increase in transmembrane pressure and a more rapid DNA breakthrough during filtration of a Chinese Hamster Ovary cell culture fluid. These results were explained using CFD calculations which showed very different flow distributions within the modules. CFD predictions were further validated using measurements of the residence time distribution and dye binding in both the lab-scale and pilot-plant modules. These results provide important insights into the factors controlling the performance and scale-up of these commercially important depth filters as well as a framework that can be broadly applied to develop more effective depth filters and depth filtration processes.

Chemically Controlled Spatiotemporal Oscillations of Colloidal Assemblies.

We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters, the size of which changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase-shifted synchrony. The oscillatory behavior is governed by an electrolytic self-diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag3 PO4 particles form a non-oscillating core with an oscillating shell composed largely of silica microparticles.

Effect of Gravity on the Configuration of Droplets on Two-Dimensional Physically Patterned Surfaces.

Wetting of solid surfaces is important for many potential applications, including the design of low-drag and antifouling/self-cleaning surfaces, and it is usually quantified by the contact angle and by contact angle hysteresis. Both the chemistry and the physical patterning of the surface are known to affect the contact angle. In studying the wetting of such surfaces, most models focus on the small Bond number (Bo) limit in which the effect of gravity is negligible, which simplifies free energy calculations. In this work, we employ a thermodynamic model for surfaces patterned with two-dimensional asperities, which remains applicable for nonzero Bo. We employ two versions of the model: one in which we require the liquid-vapor interface to remain a circular cap, and another in which we allow the liquid-vapor interface to deform. We find that the effects of gravity are twofold. First, drops with larger Bo tend to flatten and spread across the surface relative to the same size drops with Bo = 0. Second, gravity makes it more favorable for drops to penetrate surface asperities compared to the case of Bo = 0, which also tends to lower the contact angles. The main effect of droplet deformation is to produce larger contact angles for the same wetting configuration. Finally, we compare our model predictions with relevant experimental observations. We find very close agreement with the experiments, thereby validating our theoretical model.

Effects of Hierarchical Surface Roughness on Droplet Contact Angle.

Superhydrophobic surfaces often incorporate roughness on both micron and nanometer length scales, although a satisfactory understanding of the role of this hierarchical roughness in causing superhydrophobicity remains elusive. We present a two-dimensional thermodynamic model to describe wetting on hierarchically grooved surfaces by droplets for which the influence of gravity is negligible. By creating wetting phase diagrams for droplets on surfaces with both single-scale and hierarchical roughness, we find that hierarchical roughness leads to greatly expanded superhydrophobic domains in phase space over those for a single scale of roughness. Our results indicate that an important role of the nanoscale roughness is to increase the effective Young's angle of the microscale features, leading to smaller required aspect ratios (height to width) for the surface structures. We then show how this idea may be used to design a hierarchically rough surface with optimally high contact angles.

Kinetics of droplet wetting mode transitions on grooved surfaces: forward flux sampling.

The wetting configuration of a liquid droplet on a rough or physically patterned surface is typically characterized by either the Cassie wetting mode, in which the droplet resides on top of the roughness, or the Wenzel mode, in which the droplet penetrates into the roughness. For a fixed surface topology and droplet size, one of these modes corresponds to the global free-energy minimum. However, the other state is often metastable and long-lived due to a free-energy barrier that hinders the transition between the two wetting states. Metastable wetting states have been observed experimentally, and we also observe them in molecular dynamics (MD) simulations of a droplet on a grooved surface. Using forward flux sampling, we study the kinetics of the Cassie to Wenzel and Wenzel to Cassie transitions for two-dimensional droplets on periodically grooved substrates. The global-minimum wetting states that emerge from our nanoscale MD approach are consistent with those predicted by a macroscopic model based on free energy minimization. We find that the free-energy barriers for these transitions depend on the droplet size and surface topology. A committor analysis indicates that the transition-state ensemble consists of droplets that are on the verge of initiating/breaking contact with the substrate at the bottom of the grooves.

Enzyme Catalysis Causes Fluid Flow, Motility, and Directional Transport on Supported Lipid Bilayers.

The dynamic interplay between the composition of lipid membranes and the behavior of membrane-bound enzymes is critical to the understanding of cellular function and viability, and the design of membrane-based biosensing platforms. While there is a significant body of knowledge about how lipid composition and dynamics affect membrane-bound enzymes, little is known about how enzyme catalysis influences the motility and lateral transport on lipid membranes. Using enzyme-attached lipids in supported bilayers (SLBs), we provide direct evidence of catalysis-induced fluid flow that underlies the observed motility on SLBs. Additionally, by using active enzyme patches, we demonstrate the directional transport of tracer particles on SLBs. As expected, enhancing the membrane viscosity by incorporating cholesterol into the bilayer suppresses the overall movement. These are the first steps in understanding diffusion and transport on lipid membranes due to active, out-of-equilibrium processes that are the hallmark of living systems. In general, our study demonstrates how active enzymes can be used to control diffusion and transport in confined 2-D environments.

Wetting on physically patterned solid surfaces: the relevance of molecular dynamics simulations to macroscopic systems.

We used molecular dynamics (MD) simulations to study the wetting of Lennard-Jones cylindrical droplets on surfaces patterned with grooves. By scaling the surface topography parameters with the droplet size, we find that the preferred wetting modes and contact angles become independent of the droplet size. This result is in agreement with a mathematical model for the droplet free energy at small Bond numbers for which the effects of gravity are negligible. The MD contact angles for various wetting modes are in good agreement with those predicted by the mathematical model. We construct phase diagrams of the dependence of the wetting modes observed in the MD simulations on the topography of the surface. Depending on the topographical parameters characterizing the surface, multiple wetting modes can be observed, as is also seen experimentally. Thus, our studies indicate that MD simulations can yield insight into the large-length-scale behavior of droplets on patterned surfaces.

Critical conditions for development of a second pair of Dean vortices in curved microfluidic channels.

The centrifugal force in flow through a curved channel initiates a hydrodynamic instability that results in the development of Dean vortices, a pair of counter-rotating roll cells across the channel that deflect the high velocity fluid in the center toward the outer (concave) wall. When this secondary flow toward the concave (outer) wall is too strong to be dissipated by viscous effects, an additional pair of vortices emerges near the outer wall. Combining numerical simulation and dimensional analysis, we find that the critical condition for the onset of the second vortex pair depends on γ^{1/2}Dn (γ: channel aspect ratio; Dn: Dean number). We also investigate the development length for the additional vortex pair in channels with different aspect ratios and curvatures. The larger centrifugal force at higher Dean numbers produces the additional vortices further upstream, with the required development length being inversely proportional to the Reynolds number and increasing linearly with the radius of curvature of the channel.

Confined high-pressure chemical deposition of hydrogenated amorphous silicon.

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.

A theory for the morphological dependence of wetting on a physically patterned solid surface.

We present a theoretical model for predicting equilibrium wetting configurations of two-dimensional droplets on periodically grooved hydrophobic surfaces. The main advantage of our model is that it accounts for pinning/depinning of the contact line at step edges, a feature that is not captured by the Cassie and Wenzel models. We also account for the effects of gravity (via the Bond number) on various wetting configurations that can occur. Using free-energy minimization, we construct phase diagrams depicting the dependence of the wetting modes (including the number of surface grooves involved in the wetting configuration) and their corresponding contact angles on the geometrical parameters characterizing the patterned surface. In the limit of vanishing Bond number, the predicted wetting modes and contact angles become independent of drop size if the geometrical parameters are scaled with drop radius. Contact angles predicted by our continuum-level theoretical model are in good agreement with corresponding results from nanometer-scale molecular dynamics simulations. Our theoretical predictions are also in good agreement with experimentally measured contact angles of small drops, for which gravitational effects on interface deformation are negligible. We show that contact-line pinning is important for superhydrophobicity and that the contact angle is maximized when the droplet size is comparable to the length scale of the surface pattern.

Simultaneous Screening of Nitrofuran Metabolites in Honey Using Biochip Array Technology: Validation Study According to the Decision 2002/657/EC of the European Union.

Background: Although no authorization is available for antibiotics to treat bee diseases, some veterinary compounds are used by beekeepers, and each country sets its own thresholds. Inappropriate and excessive use of these drugs can cause allergic reactions and antibiotic resistance in humans who consume the remaining antibiotic residues in honey and its products. It is, therefore, relevant to monitor the presence of antibiotic residues in this matrix. Objectives: A rapid method for the simultaneous screening of nitrofuran metabolite residues in honey was validated according to Commission Decision 2002/657/EC (C.D 657) and the European guideline for the validation of screening methods for veterinary medicines. Methods: This multi-analytical screening method enables the simultaneous determination of four nitrofuran metabolites [3-amino-2-oxazolidone (AOZ), 3-amino-5-morpholinomethyl-2-oxazolidinone (AMOZ), 1-Aminohydantoin HCl (ADH), and semicarbazide (SEC)] from a single honey sample. Thirty-five honey samples were collected randomly as real samples for screening from Tehran, IR Iran, Germany, and the Netherlands in 2018. Results: For all four antibiotic residues, the positivity threshold T was higher than the cut-off value Fm, and no false-positive results were obtained for three antibiotics (AOZ, AMOZ, and SEC). Detection capabilities (CCß) of all compounds were under the minimum required performance limit (MRPL) authorized by the European Commission (currently 1 µg/kg). The screening results of 15 domestic and 20 imported honey samples showed that the levels of AOZ in 6.66% and 10% of the samples, the level of AMOZ in 13.33% and 0% of the samples, and the level of SEC in 33.33% and 40% of the samples were less than the cut-off ([in relative light units (RLUs)], respectively. Conclusions: This study found that this technique is valid for detecting and quantifying three antibiotic residues in honey samples at the measured validation levels. This method was simple, rapid, and capable of simultaneously screening three nitrofuran metabolites from a single honey sample.

Shear-induced unidirectional deposition of bacterial cellulose microfibrils using rising bubble stream cultivation.

In crystalline cellulose I, all glucan chains are ordered from reducing ends to non-reducing ends. Thus, the polarity of individual chains is added forming a large dipole within the crystal. If one can engineer unidirectional alignment (parallel packing) of cellulose crystals, then it might be possible to utilize the material properties originating from polar crystalline structures. However, most post-synthesis manipulation methods reported so far can only achieve the uniaxial alignment with bi-directionality (antiparallel packing). Here, we report a method to induce the parallel packing of bacterial cellulose microfibrils by applying unidirectional shear stress during the synthesis and deposition through the rising bubble stream in a culture medium. Driving force for the alignment is explained with mathematical estimation of the shear stress. Evidences of the parallel alignment of crystalline cellulose Iα domains were obtained using nonlinear optical spectroscopy techniques.

Interaction of fluids with physically patterned solid surfaces.

We derive coarse-grained potentials for the interaction of a fluid atom with a solid substrate patterned with cylindrical or rectangular pillars, as well as with rectangular grooves. These potentials rely on the assumption of pairwise Lennard-Jones (LJ) (12-6) interactions of a fluid atom with a surface that has a uniform density of solid atoms. In the limit of a flat, semi-infinite surface, this potential becomes the LJ 9-3 potential. Simplified forms of the potentials are presented that are valid for sufficiently large pillars or grooves. The potentials can be truncated for greater computational efficiency while retaining similar accuracy to the full form. Both the full and simplified potentials exhibit the same features as a full summation of LJ (12-6) interactions, and these may be used in simulation studies to resolve phenomena associated with the interaction of fluids with patterned surfaces.

A Volume-Corrected Wenzel Model.

The Wenzel model, commonly used for predicting the equilibrium contact angle (CA) of drops which penetrate the asperities of a rough surface, does not account for the liquid volume stored in the asperities. Interestingly, many previous experimental and molecular dynamics studies have noted discrepancies between observed CAs and those predicted by the Wenzel model because of this neglected liquid volume. Here, we apply a thermodynamic model to wetting of periodically patterned surfaces to derive a volume-corrected Wenzel equation in the limit of small pattern wavelength (compared to drop size). We show that the corrected equilibrium CA is smaller than that predicted by the Wenzel equation and that the reduction in CA can be significant when the liquid volume within the asperities becomes non-negligible compared to the total droplet volume. In such cases, the corrected CAs agree reasonably well with experimental observations and results of molecular dynamics simulations reported in previous studies.

Ozone Uptake During Inspiratory Flow in a Model of the Larynx, Trachea and Primary Bronchial Bifurcation.

Three-dimensional simulations of the transport and uptake of a reactive gas such as O(3) were compared between an idealized model of the larynx, trachea, and first bifurcation and a second "control" model in which the larynx was replaced by an equivalent, cylindrical, tube segment. The Navier-Stokes equations, Spalart-Allmaras turbulence equation, and convection-diffusion equation were implemented at conditions reflecting inhalation into an adult human lung. Simulation results were used to analyze axial velocity, turbulent viscosity, local fractional uptake, and regional uptake. Axial velocity data revealed a strong laryngeal jet with a reattachment point in the proximal trachea. Turbulent viscosity data indicated that jet turbulence occurred only at high Reynolds numbers and was attenuated by the first bifurcation. Local fractional uptake data affirmed hotspots previously reported at the first carina, and suggested additional hotspots at the glottal constriction and jet reattachment point in the proximal trachea. These laryngeal effects strongly depended on inlet Reynolds number, with maximal effects (approaching 15%) occurring at maximal inlet flow rates. While the increase in the regional uptake caused by the larynx subsided by the end of the model, the effect of the larynx on cumulative uptake persisted further downstream. These results suggest that with prolonged exposure to a reactive gas, entire regions of the larynx and proximal trachea could show signs of tissue injury.

Gas uptake in a three-generation model geometry with a flat inlet velocity during steady inspiration: comparison of axisymmetric and three-dimensional models.

Mass transfer coefficients were predicted and compared for uptake of reactive gas system using an axisymmetric single-path model (ASPM) with experimentally predicted values in a two-generation geometry and with a three-dimensional computational fluid dynamics model (CFDM) in a three-generation model geometry at steady inspiratory flow with a flat inlet velocity profile. The flow and concentration fields in the ASPM were solved using Galerkin's finite element method and in the CFDM using a commercial finite element software FIDAP. ASPM predicted average gas phase mass transfer coefficients within 25% of the experimental values. Numerical results in terms of overall mass transfer coefficients from the two models within each bifurcation unit were compared for two different inlet flow rates, wall mass transfer coefficients, and bifurcation angles. The overall mass transfer coefficients variation with bifurcation unit from the ASPM and CFDM compared qualitatively and quantitatively closely at lower wall mass transfer coefficients for both 40 degree and 70 degree bifurcation angles. But at higher wall mass transfer coefficients, quantitatively they were off in the range of 2-10% for 40 degree bifurcation angle and in the range of 4-15% for 70 degree bifurcation angle. Both CFDM and ASPM predict the same trends of increase in mass transfer coefficients with inlet flow, wall mass transfer coefficients, and during inspiration compared to expiration. Higher mass transfer coefficients were obtained with a flat velocity profile compared to a parabolic velocity profile using ASPM. These results validate the simplified ASPM and the complex CFDM.

Gas uptake in a three-generation model geometry during steady expiration: comparison of axisymmetric and three-dimensional models.

Mass transfer coefficients were predicted and compared for uptake of a formaldehyde-air gas system using an axisymmetric single path model (ASPM) and a three-dimensional computational fluid dynamics model (CFDM) in three-generation model geometry at steady expiratory flow. The flow and concentration fields in the ASPM were solved using Galerkin's finite-element method and in the CFDM using a commercial finite-element software, FIDAP. Numerical results were compared for two different inlet flow rates, wall mass transfer coefficients, and bifurcation angles. The mass transfer coefficients variation with bifurcation unit from the ASPM and CFDM compared qualitatively and quantitatively closely at all flows and lower wall mass transfer coefficients for both 40 degrees and 70 degrees bifurcation angles. However, at higher wall mass transfer coefficients, quantitatively they were within 40% for both the bifurcation angles. Also, at higher flow and wall mass transfer coefficients, they were off qualitatively for a 70 degrees bifurcation angle although the uptake compared qualitatively. This is due to the normalization of uptake within a bifurcation unit with the average of inlet and outlet average concentrations. Both CFDM and ASPM predict the same trends of increase in mass transfer coefficients with inlet flow and wall mass transfer coefficients. Also, the local values of the mass transfer coefficients compared closely at all conditions. These results validate the simplified ASPM and the complex CFDM. Mass transfer coefficients increase with bifurcation angles and with a flat inlet velocity profile compared to a parabolic velocity profile since the flow is non-fully developed and hence, the uptake increases.

Thermocapillary convection in double-layer fluid structures within a two-dimensional open cavity.

Thermocapillary convection within a differentially-heated open rectangular cavity containing two immiscible liquid layers is considered in the absence of gravitational effects. The temperature and flow fields in the two layers are computed using domain mapping in conjunction with a finite-difference scheme on a staggered grid. The melt-encapsulant and air-encapsulant interfaces are allowed to deform, with the contact lines pinned on the solid boundaries. The presence of a free surface at the top leads to increased convection in the encapsulant phase while retarding thermocapillary flow in the melt. The intensity of thermocapillary convection in the encapsulated layer is reduced as the viscosity of the encapsulant is increased or the thickness of the encapsulant layer is decreased. Choosing an encapsulant with a greater sensitivity of interfacial tension to temperature (as compared to that of the melt phase) can almost completely suppress thermocapillary convection in the melt. Deformations of the melt-encapsulant interface in an open cavity are found to be larger than those in a closed cavity with a rigid top surface, due to higher pressure gradients realized in the encapsulant phase. In contrast to interface deformation behavior reported earlier for a double-layer system in a closed cavity, the shape of the melt-encapsulant interface is qualitatively similar for all values of the viscosity ratio, with the interface dipping into the melt near the cold wall, and into the encapsulant near the hot wall. For the double-layers considered in this study, a free surface at the top of the encapsulant layer was found to be more effective than a rigid top in reducing the intensity of thermocapillary convection in the melt.

Conducting Polymer Microcups for Organic Bioelectronics and Drug Delivery Applications.

An ideal neural device enables long-term, sensitive, and selective communication with the nervous system. To accomplish this task, the material interface should mimic the biophysical and the biochemical properties of neural tissue. By contrast, microfabricated neural probes utilize hard metallic conductors, which hinder their long-term performance because these materials are not intrinsically similar to soft neural tissue. This study reports a method for the fabrication of monodisperse conducting polymer microcups. It is demonstrated that the physical surface properties of conducting polymer microcups can be precisely modulated to control electrical properties and drug-loading/release characteristics.