Biomolecules adsorb at fluid-fluid interfaces and can form a cohesive interfacial network imparting distinctive local interfacial mechanics. This modification of interfacial behavior is empirically known to significantly affect the stability and flow behavior of foams and emulsions. Droplet formation in the presence of interfacial networks is investigated in a flow-focusing microfluidic device using designed peptide surfactants, which allow decoupled control of interfacial rheology and interfacial tension. The influence of interfacial elasticity on droplet breakup, satellite droplet formation and droplet size are reported. The presence of high interfacial elasticity strongly affects the mechanism of droplet breakup by delaying neck thinning and altering interfacial shape at the point of droplet detachment, resulting in the suppression of satellite droplet formation and a decrease in droplet size. We report a correlation between dimensionless droplet size and a new dimensionless grouping which combines flow-rate ratio with the ratio of interfacial tension and interfacial elasticity. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1669-1677, 2011

Both micro- and nanofluidics are finding increasing use in the growing toolbox of nanotechnology; for the production of nanoparticles, and as micro-reactors for carefully controlled chemical reactions. These laboratories-on-a-chip hold vast potential for industrial application, however, only the most simple are truly starting to emerge as commercially viable, particularly in the area of droplet formation and emulsion creation. In order to automate droplet production with a desired size and dispersity, we have designed a microfluidic-based technology utilizing elementary microchannel geometries in combination with a closed loop feedback system to control the continuous- and dispersed-phase flow rates. Both the device geometry and control system have been optimized to allow for the production of a tunable emulsion. By utilizing discrete linear control theory, the device is able to produce the desired results with little to no prior knowledge of the fluid material properties to be used in either phase. We present our results from initial development using flow-focusing microfluidic geometry for droplet formation, computer-tethered syringe pumps to individually control the continuous and dispersed phase flow rates, a high-speed camera, and a controller and driver system for the optical measurements and pumps, respectively. We will show the efficacy of this technique for Newtonian and viscoelastic liquids, with and without the presence of surfactants. It can be envisioned that through careful control optimization, such a system can be developed to a point that will allow the production of "designer" emulsions with droplets eventually reaching the nanoscale.

A series of laboratory tests have been conducted on several different batches of Vaisala RS80 radiosondes to understand and develop methods to correct six humidity measurement errors, including chemical contamination, temperature dependence, basic calibration model, ground check, sensor aging, and sensor arm heating. The contamination and temperature-dependence (TD) errors dominate total errors. The chemical contamination error produces a dry bias, and is due to the occupation of binding sites in the sensor polymer by nonwater molecules emitted from the sonde packaging material. The magnitude of the dry bias depends on sensor polymer type (RS80-A and RS80-H), age of the sonde, relative humidity (RH), and temperature, and it exists throughout the troposphere. The contamination error generally increases with age and RH, and is larger for the RS80-H than the RS80-A. It is similar to2% and similar to10% at saturation for 1-yr-old RS80-A and RS80-H sondes, respectively. The TD error for the RS80-A results from an approximation of a linear function of temperature to the actual nonlinear temperature dependence of the sensor, and also introduces a dry bias. The TD error mainly exists at temperatures below -20degreesC, increases substantially with decreasing temperatures below -30degreesC, and is much larger for the RS80-A than the RS80-H. The RS80-A's TD correction (C-TA) dominates the total correction at temperatures below -40degreesC and has a correction factor [C-TA = (RH) (C-TA-factor)] of 0.15, 0.75, and 2.3 at -40degrees, 60degrees, and -80degreesC, respectively. The correction methods are applied to 8129 Vaisala RS80 soundings collected during the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) and are applicable to RS80 radiosonde data from other field experiments and historical and operational radiosonde datasets. The methods are validated by examining various summary plots of the TOGA COARE data and comparing them with other independent data. The corrections greatly improve the accuracy of the TOGA COARE radiosonde dataset. These correction methods have their own uncertainties and may not correct all errors in Vaisala RS80 humidity data. Analyses of these uncertainties are presented in the paper.

Extensional rheometry has only recently been developed into a commercially available tool with the introduction of the capillary breakup extensional rheometer (CaBER). CaBER is currently being used to measure the transient extensional viscosity evolution of mid to low-viscosity viscoelastic fluids. The elegance of capillary breakup extensional experiments lies in the simplicity of the procedure. An initial step-stretch is applied to generate a fluid filament. What follows is a self-driven uniaxial extensional flow in which surface tension is balanced by the extensional stresses resulting from the capillary thinning of the liquid bridge. In this paper, we describe the results from a series of experiments in which the step-stretch parameters of final length, and the extension rate of the stretch were varied and their effects on the measured extensional viscosity and extensional relaxation time were recorded. To focus on the parameter effects, well-characterized surfactant wormlike micelle solutions, polymer solutions, and immiscible polymer blends were used to include a range of characteristic relaxation times and morphologies. Our experimental results demonstrate a strong dependence of extensional rheology on step-stretch conditions for both wormlike micelle solutions and immiscible polymer blends. Both the extensional viscosity and extensional relaxation time of the wormlike micelle solutions were found to decrease with increasing extension rate and strain of the step-stretch. For the case of the immiscible polymer blends, fast step-stretches were found to result in droplet deformation and an overshoot in the extensional viscosity which increased with increasing strain rates. Conversely, the polymer solutions tested were found to be insensitive to step-stretch parameters. In addition, numerical simulations were performed using the appropriate constitutive models to assist in both the interpretation of the CaBER results and the optimization of the experimental protocol. From our results, it is clear that any rheological results obtained using the CaBER technique must be properly considered in the context of the stretch parameters and the effects that preconditioning has on viscoelastic fluids.

An associative polymer–surfactant system has been used to observe the effects of chain conformation in the entry flow through a microfabricated planar 16:1:16 contraction–expansion geometry. The well-studied system of the flexible polymer poly(ethylene oxide) (PEO) and anionic surfactant sodium dodecyl sulfate (SDS) was used. Dilute polymer solutions with increasing SDS concentration were characterized in steady and dynamic shear, as well as capillary breakup extensional rheology. Based on this characterization, the primary quantitative difference is an increase in zero-shear viscosity as a result of the PEO chain expansion brought on by association of SDS surfactant micelles. However, these quantitatively similar solutions were observed to exhibit much more qualitatively different flow patterns via fluorescent streak imaging in the entry flow. In contrast to previous work on PEO solutions, the PEO–SDS systems were observed to transition to a steady viscoelastic flow regime characterized by stable lip vortices at much lower elasticity and Weissenberg numbers. The resulting insight gained regarding the utility of microfluidic flows in elucidating effects of subtle conformational changes further illustrates the potential for using microfabricated devices as rheometric tools for measuring the properties of dilute and weakly viscoelastic fluids.

A filament stretching extensional rheometer and capillary breakup extensional rheometer are used to measure the extensional rheology of a series of wormlike micelle solutions experiencing a uniaxial elongational flow. The experiments are performed using a series of wormlike micelle solutions of both cetylpyridinium chloride and sodium salicylate (NaSal) in an aqueous sodium chloride solution and cetyltrimethylammonium bromide and NaSal in de-ionized water. The linear viscoelasticity of all the wormlike micelle solutions is well described by a Maxwell model with just one or two relaxation times while the steady shear measurements all demonstrate characteristics of shear banding at large shear rates. In transient homogeneous uniaxial extension imposed by a filament stretching rheometer, each of the wormlike micelle solutions demonstrate significant strain hardening. At large extension rates, the wormlike micelle solution filaments are all found to fail through a dramatic rupture near the axial midplane at a constant stress independent of imposed extension rate. The result is an extensional viscosity that decays linearly with increasing extension rate. This filament failure likely stems from the local scission of individual wormlike micelle chains. For the more concentrated solutions, as the imposed extension rate is reduced, a critical extension rate is found below which the filament does not rupture, but instead elastocapillary pinch off is recovered and the elastic tensile stresses achieved in the fluid filament grow far beyond the value observed at rupture. This dramatic upturn in the elastic tensile stress and the extensional viscosity at low extension rates is not intuitively expected and is most likely a result of structural changes to the entangled wormlike micelle solution. Strain hardening is also observed in capillary breakup rheometry experiments, however, when the results of filament stretching and capillary breakup rheometry measurements at nominally the same extension rate are superimposed, the results do not agree; the extensional viscosity measurements from filament stretching are in some instances more than an order of magnitude larger. This result calls into question the use of capillary breakup rheometry for quantitatively measuring the extensional viscosity of wormlike micelle solutions.

A series of experiments were performed to further investigate the phenomenon of shear-banding in surfactant solutions. Many surfactant solutions, through their unique amphiphilic chemistry, form long wormlike micelle structures which behave like living polymers. These wormlike micelles have interesting viscoelastic properties and have been the subject of a number of recent studies. These water-based surfactant systems are widely used in many commercial and industrial applications; however, many aspects of their complex flow behavior are still not fully understood. In this study, a Couette cell was designed to allow for high-resolution optical access in a simple shear flow of a surfactant system comprised of cetylpyridinium chloride and sodium salicylate in aqueous sodium chloride. Beyond a critical stress, this system is found to enter a non-linear regime in which there is a plateau in the shear stress with increasing shear rate. Within this plateau, the fluid forms distinct bands of varying shear rate. The goal of this study was to obtain high spatial and temporal resolution particle-image velocimetry and flow-induced birefringence results in both steady and transient-startup flows. As a consequence of the high resolution, steady PIV results suggest the existence of multiple-shear bands. In the transient PIV experiments, we observe a propagating damped elastic wave, as well as fluctuations in the shear-band evolution on timescales of less than one relaxation time. Pointwise FIB gap profiles show a diffuse birefringent region prior to the onset of shear-banding in the velocity profiles. These results provide insight on the flow behavior, as well as a full set of experimental data which will drive development of constitutive models capable of predicting shear-banding.

The sharkskin surface instability is commonly observed in the extrusion of polymer melts. We present a series of experiments in which a specifically designed rectangular slit die with insulated and independently heated sides and is used to induce precise temperature gradients across a flowing polyethylene melt. Our previous experiments demonstrated that the character of the surface distortions produced by the sharkskin instability was a function of the die wall temperature and therefore the extrudate had viscoelastic properties at the surface. In this paper, we explore the role of temperature and viscoelastic property gradients near the capillary wall. The amplitude of the sharkskin instability is quantified and plotted against apparent shear and extension rates. Analysis of the data demonstrates that the amplitude and frequency of the instability is independent of bulk temperature and temperature gradient and is dependent only on wall temperature. The data are normalized using a dimensionless Weissenberg number based on the extension rate to collapse the data collected over all temperatures and gradients onto a single master curve. We conclude with an example of a rectangular extrudate exhibiting varying surface roughness due to differential die heating and discuss the implications of our observations on the sharkskin surface instability mechanism and on commercial applications.

Addressing insurgency requires the same application of operational art as utilized in conventional warfare planning. Counterinsurgency strategy will be driven by the nature of the insurgent movement, with campaigns constructed to use tactics appropriately so that key facets of the insurgent campaign are neutralized. It is especially important to determine whether terror is used as a tactic (a method of action) by an insurgency or as a stand-alone strategy (a logic of action) by a challenger divorced from a mass base. Insurgencies, in turn, will normally emphasize strategically either winning allegiance of the target population or using violence as a substitute for other methods. Each of these approaches requires the weighting of the appropriate campaign elements of the counterinsurgency strategy. Sri Lanka, having faced both approaches, is an especially useful case study.

This letter reports experimental observations of the flow kinematics and stability of thin fluid sheets produced by impinging obliquely aligned laminar jets of a series of viscoelastic worm-like micelle solutions. As the velocity of the impinging jets is increased, the sheets of viscoelastic fluid grow larger and eventually become unstable. High speed imaging reveals a transition to a striking new flow structure resembling fluid webs, previously unobserved for Newtonian or non-Newtonian fluids. These newly discovered flow structures are complex and comprised of highly interconnected filaments created by the growth of multiple internal failures within the fluid sheet. Increasing viscoelasticity of the test fluid was found to stabilize the sheets and the fluid webs while increasing the drop size produced by their eventual breakup under capillary stresses.

The extrusion of polymer melts is often rate limited by the onset of an elastic surface instability known as sharkskin. Appearance of these surface distortions is generally unacceptable for commercial applications. The desire to forestall the onset of sharkskin to higher output rates has motivated a considerable amount of research to characterize the nature of the instability. In this manuscript, we will present a series of detailed experiments using a custom fabricated extruder and die. By incorporating thermal breaks and precise localized temperature control of the die and barrel, predetermined temperature gradients could be induced across the extrudate. Polymers are typically very poor thermal conductors, and therefore the effects of heating or cooling from a boundary can be designed to only affect the properties of the extrudate very close to the die wall. We will present data correlating the amplitude and frequency of the sharkskin instability to the bulk and die surface temperature as well as the shear rates. The result is a quantitative processing map that characterizes the instability and demonstrates that by modifying the rheology of the polymeric fluid very near the die exit corner, it is possible to suppress or control the sharkskin instability through isolated die heating or cooling. By reformulating our data into Weissenberg and Deborah numbers using the relaxation time evaluated at the wall temperature, we demonstrate that the sharkskin surface instability is dependent only on flow kinematics and viscometric properties of the fluid very near the die wall, a result of the stress singularity present at the die exit, and independent of bulk fluid properties. This technique could conceivably increase the profitability of extrusion processes and be extended to develop precisely-controlled sharkskin for designing specific functionality into extruded surfaces.

A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (similar to1-5 m s(-1)) ascent frequently creates an environment favorable to the initiation of convection, quite close (within similar to 10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12-13 May 1985. The analysis presented here reveals that the mesoscale structure of the nocturnal dryline prior to the formation of deep convection is a gently sloping, slow-moving interface. The movement of the dryline at night was related to the evolution of the low-level jet within the moist air. Wavelike structures and evidence for vertical mixing were observed in the moist air as low Richardson numbers occurred below the height of the jet. The previously discussed strong ascent is largely lacking in the present nocturnal case so that the circulations inherent to an undisturbed dryline at night are far less favorable for the initiation of deep convection than in the afternoon and early evening.In the present case, severe convection developed as a weak cold front approached and merged with the nocturnal dryline and the environment rapidly destabilized. Between soundings taken 2.5 h apart, the convective available potential energy increased from 524 to 3417 J kg(-1) and the absolute value of the convective inhibition decreased from 412 to 9 J kg(-1). The vertical shear of the horizontal wind also dramatically increased with time, so that the bulk Richardson number was within values normally associated with supercell convection. The timescale of the changes in stability and in the moisture field (similar to1-2.5 h) has implications for the type of observing network needed to nowcast severe convection and for assessing the performance of research and operational numerical models.

(1999). Walton's Boethius: From manuscript to print. English Studies: Vol. 80, No. 5, pp. 398-407.