We consider the evolution of a thin viscous fluid sheet subject to thermocapillary effects. Using a lubrication approximation we find, for symmetric interfacial deflections, coupled evolution equations for the interfacial profile, the streamwise component of the fluid velocity and the temperature variation along the surface. Initial temperature profiles change the initial flow field through Marangoni-induced shear stresses. These changes then lead to preferred conditions for rupture prescribed by the initial temperature distribution. We show that the time to rupture may be minimized by varying the phase difference between the initial velocity profile and the initial temperature profile. For sufficiently large temperature differences, the phase difference between the initial velocity and temperature profiles determines the rupture location.

We used molecular genetic techniques and multipoint linkage analyses to locate the gene responsible for cutaneous malignant melanoma-dysplastic nevus. We evaluated 99 relatives and 26 spouses in six families with a predisposition to melanoma. Thirty-four family members had cutaneous malignant melanoma, and 31 of these 34 also had histologically confirmed dysplastic nevi. Twenty-four family members had dysplastic nevi alone. An analysis of the cosegregation of the cutaneous malignant melanoma–dysplastic nevus trait with 26 polymorphic DNA markers on the short arm of chromosome 1 demonstrated the presence of a gene for susceptibility to melanoma. The gene was located between an anonymous DNA marker (D1S47) and the gene locus for pronatrodilatin, a commonly used reference gene (PND), in chromosome band 1p36. The odds were greater than 260,000:1 in favor of linkage at this location.

To identify the chromosomal location of a gene responsible for familial hypertrophic cardiomyopathy, we used clinical and molecular genetic techniques to evaluate the members of a large kindred. Twenty surviving and 24 deceased family members had hypertrophic cardiomyopathy; 58 surviving members were unaffected. Genetic-linkage analyses were performed with polymorphic DNA loci dispersed throughout the entire genome, to identify a locus that was inherited with hypertrophic cardiomyopathy in family members. The significance of the linkage detected between the disease locus and polymorphic loci was assessed by calculating a lod score (the logarithm of the probability of observing coinheritance of two loci, assuming that they are genetically linked, divided by the probability of detecting coinheritance if they are unlinked). A DNA locus (D14S26), previously mapped to chromosome 14 and of unknown function, was found to be coinherited with the disease in this family. No instances of recombination were observed between the locus for familial hypertrophic cardiomyopathy and D14S26, yielding a lod score of +9.37 (θ= 0). These data indicate that in this kindred, the odds are greater than 2,000,000,000:1 that the gene responsible for familial hypertrophic cardiomyopathy is located on chromosome 14 (band q1).

Part of Focus on Micro- and Nanofluidics The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in chemical equilibrium with a surface whose double-layer voltage is of order the thermal voltage, kBT/e=25 mV. In nonlinear 'induced-charge' electrokinetic phenomena, such as ac electro-osmosis, several volts 100kBT/e are applied to the double layer, and the theory breaks down and cannot explain many observed features. We argue that, under such a large voltage, counterions 'condense' near the surface, even for dilute bulk solutions. Based on simple models, we predict that the double-layer capacitance decreases and the electro-osmotic mobility saturates at large voltages, due to steric repulsion and increased viscosity of the condensed layer, respectively. The former suffices to explain observed high-frequency flow reversal in ac electro-osmosis; the latter leads to a salt concentration dependence of induced-charge flows comparable to experiments, although a complete theory is still lacking.

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)] , was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

BACKGROUND Understanding more about student decisions to leave engineering may lead to higher retention. This study builds on the literature and focuses on the experiences of a cohort of students who aimed to complete their undergraduate work in 2007. PURPOSE (HYPOTHESIS) This paper presents the outcomes of the longitudinal administration of the Persistence in Engineering survey. The goal was to identify correlates of persistence in undergraduate engineering education and professional engineering practice. DESIGN/METHOD The survey was administered seven times over four years to a cohort of students who had expressed interest in studying engineering. At the end of the study, the participants were categorized as persisters or non-persisters. Repeated measures analysis of variance was used, in conjunction with other approaches, to test for differences between the groups. RESULTS Persisters and non-persisters did not differ significantly according to the majority of the constructs. Nevertheless, parental and high school mentor influences as a motivation to study engineering, as well as confidence in math and science skills, were identified as correlates of persistence. Intention to complete an engineering major was also a correlate of persistence; it appears to decline sharply at least two semesters prior to students leaving engineering. The findings also suggest that there might be differences among non-persisters when they are further grouped by when they leave engineering. CONCLUSIONS Facilitating higher levels of mentor involvement before college might increase student motivation to study engineering, and also constitute a mechanism for fostering confidence in math and science skills. Since the intention to complete an engineering degree decreases well before students act, there may be opportunities for institutions to develop targeted interventions for students, and help them make informed decisions.

Nanofluidic technology is gaining popularity for bioanalytical applications due to advances in both nanofabrication and design. One major obstacle in the widespread adoption of such technology for bioanalytical systems is efficient detection of samples due to the inherently low analyte concentrations present in such systems. This problem is exacerbated by the push for electronic detection, which requires an even higher sensor-local sample concentration than optical detection. This paper explores one of the most common preconcentration techniques, field-amplified sample stacking, in nanofluidic systems in efforts to alleviate this obstacle. Holding the ratio of background electrolyte concentrations constant, the parameters of channel height, strength of electric field, and concentration are varied. Although in micron scale systems, these parameters have little or no effect on the final concentration enhancement achieved, nanofluidic experiments show strong dependencies on each of these parameters. Further, nanofluidic systems demonstrate an increased concentration enhancement over what is predicted and realized in microscale counterparts. Accordingly, a depth-averaged theoretical model is developed that explains these observations and furthermore predicts a novel focusing mechanism that can explain the increased concentration enhancement achieved. Specifically, when the electric double layer is sufficient in size relative to the channel height, negatively charged analyte ions are repelled from negatively charged walls, and thus prefer to inhabit the centerline of the channels. The resulting induced pressure gradients formed due to the high and low electrical conductivity fluids in the channel force the ions to move at a slower velocity in the low-conductivity region, and a faster velocity in the high-conductivity region, leading to focusing. A simple single-channel model is capable of predicting key experimental observations, while a model that incorporates the details of the fluid inlet and outlet ports allows for more detailed comparisons between model and experiment.

We studied the energetics of hover-feeding Anna's hummingbirds, using three different simultaneous techniques: heat loss as estimated via thermal imaging, metabolic rate as measured at a feeder mask using flow-through respirometry, and aerodynamic power estimated from wingbeat kinematic data. These three methods yielded comparable estimates of power output at ambient air temperatures ranging from 18 degrees to 26 degrees C, whereas heat imbalance at higher air temperatures (up to 34 degrees C) suggested loss by mechanisms other than convection and radiation from the body, such as evaporative cooling and enthalpy rise associated with exhaled air and excreted water and convective heat loss from the patagia. Hummingbirds increased wingbeat frequency and decreased stroke amplitude as air temperature increased, but overall muscle efficiency was found to be approximately constant over the experimental range of air temperatures.

We study the existence of multiple equilibrium states in a simple fluid network using Newtonian fluids and laminar flow. We demonstrate theoretically the presence of hysteresis and bistability, and we confirm these predictions in an experiment using two miscible fluids of different viscosity—sucrose solution and water. Possible applications include blood flow, microfluidics, and other network flows governed by similar principles.

Single bubble sonoluminescence (SBSL) is the brief flash of light emitted from a single, stable, acoustically forced bubble. In experiments, the maximum pressure amplitude with which a bubble may be forced is limited by considerations of spherical stability. The traditional linear stability analysis predicts a threshold for SBSL at a much lower pressure amplitude than experimental observations. This work shows that if one constructs an accurate model of the radial dynamics, the traditional linear stability analysis predicts a boundary that is in excellent agreement with experimental data.

The critical fluctuations of CuGeO3 have been measured by synchrotron x-ray scattering, and two length scales are clearly observed. The ratio between the two length scales is found to be significantly different along the a axis, with the a axis along the surface normal direction. We believe that such a directional preference is a clear sign that random surface strains, especially those caused by dislocations, are the origin of the long length scale fluctuations.

We report a neutron-scattering study of the dynamic spin correlations in Rb2MnF4, a two-dimensional spin-5/2 antiferromagnet. By tuning an external magnetic field to the value for the spin-flop line, we reduce the effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal two-dimensional isotropic antiferromagnet. From the shape of the quasielastic peak as a function of temperature, we demonstrate dynamic scaling for this system and find a value for the dynamical exponent z. We compare these results to theoretical predictions for the dynamic behavior of the two-dimensional Heisenberg model, in which deviations from z=1 provide a measure of the corrections to scaling.

Helical ribbons with pitch angles of either 11° or 54° self-assemble in a wide variety of quaternary surfactant-phospholipid/fatty acid-sterol-water systems. By elastically deforming these helices, we examined their response to uniaxial forces. Under sufficient tension, a low pitch helix reversibly separates into a straight domain with a pitch angle of 90° and a helical domain with a pitch angle of 16.5°. Using a newly developed continuum elastic free energy model, we have shown that this phenomenon can be understood as a first order mechanical phase transition.

Escaping from predators often demands that animals rapidly negotiate complex environments. The smallest animals attain relatively fast speeds with high frequency leg cycling, wing flapping or body undulations, but absolute speeds are slow compared to larger animals. Instead, small animals benefit from the advantages of enhanced maneuverability in part due to scaling. Here, we report a novel behavior in small, legged runners that may facilitate their escape by disappearance from predators. We video recorded cockroaches and geckos rapidly running up an incline toward a ledge, digitized their motion and created a simple model to generalize the behavior. Both species ran rapidly at 12–15 body lengths-per-second toward the ledge without braking, dove off the ledge, attached their feet by claws like a grappling hook, and used a pendulum-like motion that can exceed one meter-per-second to swing around to an inverted position under the ledge, out of sight. We discovered geckos in Southeast Asia can execute this escape behavior in the field. Quantification of these acrobatic behaviors provides biological inspiration toward the design of small, highly mobile search-and-rescue robots that can assist us during natural and human-made disasters. We report the first steps toward this new capability in a small, hexapedal robot.

We consider general reaction dusion systems posed on rectangular lattices in two or more spatial dimensions. We show that travelling wave solutions to such systems that propagate in rational directions are nonlinearly stable under small perturbations. We employ recently developed techniques involving point-wise Green's functions estimates for functional erential equations of mixed type (MFDEs), allowing our results to be applied even in situations where comparison principles are not available.

Narrative identity is the internalized, evolving story of the self that each person crafts to provide his or her life with a sense of purpose and unity. A proliferation of empirical research studies focused on narrative identity have explored its relationship with psychological well-being. The present study is the first prospective, multiwave longitudinal investigation to examine short-term personality change via an emphasis on narrative identity as it relates to mental health. Forty-seven adults wrote rich personal narratives prior to beginning psychotherapy and after every session over 12 assessment points while concurrently completing a measure of mental health. Narratives were coded for the themes of agency and coherence, which capture the dual aims of narrative identity: purpose and unity. By applying in-depth thematic coding to the stories of participants, the present study produced 47 case studies of intraindividual personality development and mental health. By employing multilevel modeling with the entire set of nearly 600 narratives, the present study also identified robust trends of individual differences in narrative changes as they related to improvements in mental health. Results indicated that, across participants, the theme of agency, but not coherence, increased over the course of time. In addition, increases in agency were related to improvements in participants' mental health. Finally, lagged growth curve models revealed that changes in the theme of agency occurred prior to the associated improvements in mental health. This finding remained consistent across a variety of individual-difference variables including demographics, personality traits, and ego development.

We study the detailed locomotor mechanics of a small, lightweight robot (DynaRoACH, 10 cm, 25 g) which can move on a granular substrate of closely packed 3 mm diameter glass particles at speeds up to 50 cm/s (5 body length/s), approaching the performance of small, highperforming, desert-dwelling lizards. To reveal how the robot achieves this high performance, we used high speed imaging to capture kinematics, and developed a numerical multi-body simulation of the robot coupled to an experimentally validated discrete element method (DEM) simulation of the granular media. Average forward speeds measured in both experiment and simulation agreed well, and increased non-linearly with stride frequency, reflecting a change in the mode of propulsion. At low frequencies, the robot used a quasi-static “rotary walking” mode, in which the granular material yielded as the legs penetrated and then solidified once vertical force balance was achieved. At high frequencies, duty factor decreased below 0.5 and aerial phases occurred. The propulsion mechanism was qualitatively different: the robot ran rapidly by utilizing the speed-dependent fluid-like inertial response of the material. We also used our simulation tool to vary substrate parameters that were inconvenient to vary in experiment (e.g., granular particle friction) to test performance and reveal limits of stability of the robot. Using small robots as physical models, our study reveals a mechanism by which small animals can achieve high performance on granular substrates, which in return advances the design and control of small robots in deformable terrains.

The classical theory of electrokinetic phenomena is based on the mean-field approximation that the electric field acting on an individual ion is self-consistently determined by the local mean charge density. This paper considers situations, such as concentrated electrolytes, multivalent electrolytes, or solvent-free ionic liquids, where the mean-field approximation breaks down. A fourth-order modified Poisson equation is developed that captures the essential features in a simple continuum framework. The model is derived as a gradient approximation for nonlocal electrostatics of interacting effective charges, where the permittivity becomes a differential operator, scaled by a correlation length. The theory is able to capture subtle aspects of molecular simulations and allows for simple calculations of electrokinetic flows in correlated ionic fluids. Charge-density oscillations tend to reduce electro-osmotic flow and streaming current, and overscreening of surface charge can lead to flow reversal. These effects also help to explain the suppression of induced-charge electrokinetic phenomena at high salt concentrations.

The very high speed of collapse of cavitation bubbles is responsible for a number of phenomena of interest in science and engineering: Luminescence, sonochemistry, cavitation damage, ultrasonic cleaning, etc. Strongly forced bubbles may collapse with such violence that the relatively slow processes of diffusion of the heat of compression and of excess vapor to the bubble wall are obviated. This leads to an approximately adiabatic system with nearly constant mass during the final stages of extreme collapses, accompanied by the evolution of sharp thermal and compositional boundary layers on either side of the interface. It is shown that the boundary layers, which are involved in the determination of the interfacial temperature through the balance of sensible and latent heats, may profitably be described mathematically through integral equations. This complements well the boundary integral solution of the fluid dynamics, which has been the basis of much progress in the field.

We consider pulse formation dynamics in an actively mode-locked laser. We show that an amplitude-modulated laser is subject to large transient growth and we demonstrate that at threshold the transient growth is precisely the Petermann excess noise factor for a laser governed by a nonnormal operator. We also demonstrate an exact reduction from the governing PDEs to a low-dimensional system of ODEs for the parameters of an evolving pulse. A linearized version of these equations allows us to find analytical expressions for the transient growth below threshold. We also show that the nonlinear system collapses onto an appropriate fixed point, and thus in the absence of noise the ground-mode laser pulse is stable. We demonstrate numerically that, in the presence of a continuous noise source, however, the laser destabilizes and pulses are repeatedly created and annihilated.

Every classroom constructs its own culture through the interactions of all participants, students and instructors. This culture, often covert or invisible, has a direct impact on students’ opportunities to learn. Therefore, it is critical that instructors understand their classrooms’ interaction patterns and their effect on student learning. We suggest that discourse analysis may serve as a tool to enhance instructors’ understanding of their classrooms and to serve as an intervention particularly useful for junior faculty as they are beginning their teaching career. To this end, this paper (1) describes the theoretical foundation of discourse analysis and (2) demonstrates its application, effectiveness, and applicability in STEM classrooms, particularly at the introductory level, the time when students make their first steps in negotiating ‘academic literacies’.

Examining the perceptions of first-year undergraduates and their instructors can provide insight into these students’ experiences and shed light on the emerging issues of student attrition and lack of preparedness for the workforce. Students’ perceptions about introductory courses have been examined in previous work. On the other hand, as the high rate of university student dropouts has frequently been attributed to the poor quality of teaching in first-year undergraduate courses, this study aims to investigate the perceptions of faculty members instructing first-year undergraduates. Our analysis results in several emergent themes, which include (1) instructor’s beliefs about Project-Based Learning as a teaching practice, (2) instructor’s level of abstraction when talking about students, (3) instructor’s affect towards students, (4) value instructors place on one-on-one interactions with students, (5) instructors’ perceptions of their role in development of student motivation and interest toward their courses, (6) instructors’ perceived ability to impact students, (7) overall teaching goals, and (8) instructors’ motivation towards teaching. From analysis of these emergent themes, there appear to be two distinct instructor groups. These groups, which we will refer to as Personal Coaches and Group Ushers, are observed to have different attitudes and expressed behaviors towards teaching and their students. These findings are important as they shed light into one aspect of undergraduates’ experience, that of faculty support in students’ academic development. The implications of these findings have a profound effect on how we educate the next generation of our national workforce and particularly STEM professionals and we suggest further investigations in this direction. Understanding faculty perceptions is a key step to affect STEM educational reform.

We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.

In microfluidic applications it has been observed that flows with spatial gradients in electrical conductivity are unstable under the application of sufficiently strong electric fields. These electrohydrodynamic instabilities can drive a nonlinear flow despite the low Reynolds number. Such flows hold promise as a simple mechanism for mixing fluids. In this work, the effect of a time periodic electric field on the instability is explored. The case where an electric field is applied across a diffuse interface of two fluids with varying electrical conductivity is considered. Frequency-dependent behavior is found only in the regime where the instability growth rates are very slow and cannot outpace mixing by molecular diffusion. Improving mixing by modulation of the electric body force is not a viable strategy in this geometry.

We explore the stability of equilibrium solution(s) of a simple model of microvascular blood flow in a two-node network. The model takes the form of convection equations for red blood cell concentration, and contains two important rheological effects—the Fåhræus–Lindqvist effect, which governs viscosity of blood flow in a single vessel, and the plasma skimming effect, which describes the separation of red blood cells at diverging nodes. We show that stability is governed by a linear system of integral equations, and we study the roots of the associated characteristic equation in detail. We demonstrate using a combination of analytical and numerical techniques that it is the relative strength of the Fåhræus–Lindqvist effect and the plasma skimming effect which determines the existence of a set of network parameter values which lead to a Hopf bifurcation of the equilibrium solution. We confirm these predictions with direct numerical simulation and suggest several areas for future research and application.