Quantifying flow and stress in ice mélange, the world's largest granular material.

Tidewater glacier fjords are often filled with a collection of calved icebergs, brash ice, and sea ice. For glaciers with high calving rates, this "mélange" of ice can be jam-packed, so that the flow of ice fragments is mostly determined by granular interactions. In the jammed state, ice mélange has been hypothesized to influence iceberg calving and capsize, dispersion and attenuation of ocean waves, injection of freshwater into fjords, and fjord circulation. However, detailed measurements of ice mélange are lacking due to difficulties in instrumenting remote, ice-choked fjords. Here we characterize the flow and associated stress in ice mélange, using a combination of terrestrial radar data, laboratory experiments, and numerical simulations. We find that, during periods of terminus quiescence, ice mélange experiences laminar flow over timescales of hours to days. The uniform flow fields are bounded by shear margins along fjord walls where force chains between granular icebergs terminate. In addition, the average force per unit width that is transmitted to the glacier terminus, which can exceed 107 N/m, increases exponentially with the mélange length-to-width ratio. These "buttressing" forces are sufficiently high to inhibit the initiation of large-scale calving events, supporting the notion that ice mélange can be viewed as a weak granular ice shelf that transmits stresses from fjord walls back to glacier termini.

Stable small bubble clusters in two-dimensional foams.

Key features of the mechanical response of amorphous particulate materials, such as foams, emulsions, and granular media, to applied stress are determined by the frequency and size of particle rearrangements that occur as the system transitions from one mechanically stable state to another. This work describes coordinated experimental and computational studies of bubble rafts, which are quasi-two dimensional systems of bubbles confined to the air-water interface. We focus on small mechanically stable clusters of four, five, six, and seven bubbles with two different sizes with diameter ratio σL/σS ≃ 1.4. Focusing on small bubble clusters, which can be viewed as subsystems of a larger system, allows us to investigate the full ensemble of clusters that form, measure the respective frequencies with which the clusters occur, and determine the form of the bubble-bubble interactions. We emphasize several important results. First, for clusters with N > 5 bubbles, we find using discrete element simulations that short-range attractive interactions between bubbles give rise to a larger ensemble of distinct mechanically stable clusters compared to that generated by long-range attractive interactions. The additional clusters in systems with short-range attractions possess larger gaps between pairs of neighboring bubbles on the periphery of the clusters. The ensemble of bubble clusters observed in experiments is similar to the ensemble of clusters with long-range attractive interactions. We also compare the frequency with which each cluster occurs in simulations and experiments. We find that the cluster frequencies are extremely sensitive to the protocol used to generate them and only weakly correlated to the energy of the clusters.

Correction: Stable small bubble clusters in two-dimensional foams.

Correction for 'Stable small bubble clusters in two-dimensional foams' by Kai Zhang et al., Soft Matter, 2017, DOI: .

Investigating the effect of particle size on pulmonary surfactant phase behavior.

We study the impact of the addition of particles of a range of sizes on the phase transition behavior of lung surfactant under compression. Charged particles ranging from micro- to nanoscale are deposited on lung surfactant films in a Langmuir trough. Surface area versus surface pressure isotherms and fluorescent microscope observations are utilized to determine changes in the phase transition behavior. We find that the deposition of particles close to 20 nm in diameter significantly impacts the coexistence of the liquid-condensed phase and liquid-expanded phase. This includes morphological changes of the liquid-condensed domains and the elimination of the squeeze-out phase in isotherms. Finally, a drastic increase of the domain fraction of the liquid-condensed phase can be observed for the deposition of 20-nm particles. As the particle size is increased, we observe a return to normal phase behavior. The net result is the observation of a critical particle size that may impact the functionality of the lung surfactant during respiration.

How syllabi relate to outcomes in higher education: A study of syllabi learner-centeredness and grade inequities in STEM.

Fostering equity in undergraduate science, technology, engineering, and mathematics (STEM) programs can be accomplished by incorporating learner-centered pedagogies, resulting in the closing of opportunity gaps (defined here as the difference in grades earned by minoritized and non-minoritized students). We assessed STEM courses that exhibit small and large opportunity gaps at a minority-serving, research-intensive university, and evaluated the degree to which their syllabi are learner-centered, according to a previously validated rubric. We specifically chose syllabi as they are often the first interaction students have with a course, establish expectations for course policies and practices, and serve as a proxy for the course environment. We found STEM courses with more learner-centered syllabi had smaller opportunity gaps. The syllabus rubric factor that most correlated with smaller gaps was Power and Control, which reflects Student's Role, Outside Resources, and Syllabus Focus. This work highlights the importance of course syllabi as a tool for instructors to create more inclusive classroom environments.

Measurement of monolayer viscosity using noncontact microrheology.

Microrheological studies of phospholipid monolayers, bilayers, and other Langmuir monolayer systems are traditionally performed by observing the thermal fluctuations of tracers attached to the membrane or interface. Measurements of this type obtain surface moduli that are orders of magnitude different from those obtained using macroscopic or active techniques. These large discrepancies can result from uncertainties in the tracer's coupling to the monolayer or the local disruption of the monolayer by the tracer. To avoid such problems, we perform a microrheological experiment with the tracer particle placed at a known depth beneath the monolayer; this avoids the issues mentioned at the cost of generating a weaker, purely hydrodynamic coupling between the tracer and the monolayer. We calculate the appropriate response functions for this submerged particle microrheology and demonstrate the technique on three model monolayer systems.

Particle size effects on collapse in monolayers.

We report on the impact of differently sized particles on the collapse of a Langmuir monolayer. We use an SDS-DODAB monolayer because it is known to collapse reversibly under compression and expansion cycles. Particles with diameters of 1 µm, 0.5 µm, 0.1 µm, and 20 nm are deposited on the SDS-DODAB monolayer. We find a critical particle size range of 0.1 to 0.5 µm that produces a transition from reversible to irreversible collapse. The nature of the collapse is determined through optical observations and surface pressure measurements. In addition, although 20 nm particles do not cause irreversible collapse in the monolayer, they significantly decrease the collapse pressure relative to the other systems. Therefore, we observe three distinct collapse behaviors-reversible, irreversible, and reversible at a reduced surface pressure.

Identifying systemic inequity in higher education and opportunities for improvement.

It is well established that there is a national problem surrounding the equitable participation in and completion of science, technology, engineering, and mathematics (STEM) higher education programs. Persons excluded because of their ethnicity or race (PEERs) experience lower course performance, major retention, sense of belonging, and degree completion. It is unclear though how pervasive these issues are across an institution, from the individual instructor, course, and discipline perspectives. Examining over six years of institutional data from a large-enrollment, research-intensive, minority-serving university, we present an analysis of racial opportunity gaps between PEERs and non-PEERs to identify the consistency of these issues. From this analysis, we find that there is considerable variability as to whether a given course section taught by a single instructor does or does not exhibit opportunity gaps, although encouragingly we did identify exemplar instructors, course-instructor pairs, courses, and departments that consistently had no significant gaps observed. We also identified significant variation across course-instructor pairs within a department, and found that certain STEM disciplines were much more likely to have courses that exhibited opportunity gaps relative to others. Across nearly all disciplines though, it is clear that these gaps are more pervasive in the lower division curriculum. This work highlights a means to identify the extent of inequity in STEM success across a university by leveraging institutional data. These findings also lay the groundwork for future studies that will enable the intentional design of STEM education reform by leveraging beneficial practices used by instructors and departments assigning equitable grades.

Tracking giant folds in a monolayer.

The collapse dynamics of giant folds in a catanionic monolayer at the air-water interface are examined. A monolayer of dioctadecyldimethylammonium bromide (DODAB) and sodium dodecyl sulfate (SDS) in a 1:1 ratio is the system of study that previously was found to fold upon compression in a Langmuir trough. Carboxylate-coated polystyrene beads (1 microm diameter) are deposited and bound to the monolayer. Displacement of the beads is measured with epifluorescence microscopy and particle image velocimetry, yielding a measurement of the velocity of the monolayer around the fold. Reversibility is confirmed by measuring the amount of monolayer material entering and leaving the fold. Material near folds are found to have a maximum relative velocity on the order of 0.1 mm/s, and fold depths are found to be on the order of 1 mm. The folds exhibit regular unfolding behavior, which can be explained qualitatively by a simple mechanical model.

Comparison of low-amplitude oscillatory shear in experimental and computational studies of model foams.

A fundamental difference between fluids and solids is their response to applied shear. Solids possess static shear moduli, while fluids do not. Complex fluids such as foams display an intermediate response to shear with nontrivial frequency-dependent shear moduli. In this paper, we conduct coordinated experiments and numerical simulations of model foams subjected to boundary-driven oscillatory planar shear. Our studies are performed on bubble rafts (experiments) and the bubble model (simulations) in two dimensions. We focus on the low-amplitude flow regime in which T1 events, i.e., bubble rearrangement events where originally touching bubbles switch nearest neighbors, do not occur, yet the system transitions from solid- to liquidlike behavior as the driving frequency is increased. In both simulations and experiments, we observe two distinct flow regimes. At low frequencies omega, the velocity profile of the bubbles increases linearly with distance from the stationary wall, and there is a nonzero total phase shift between the moving boundary and interior bubbles. In this frequency regime, the total phase shift scales as a power law Delta approximately omegan with n approximately 3. In contrast, for frequencies above a crossover frequency omega>omegap, the total phase shift Delta scales linearly with the driving frequency. At even higher frequencies above a characteristic frequency omeganl>omegap, the velocity profile changes from linear to nonlinear. We fully characterize this transition from solid- to liquidlike flow behavior in both the simulations and experiments and find qualitative and quantitative agreements for the characteristic frequencies.

Lateral stress relaxation and collapse in lipid monolayers.

Surfactants at air/water interfaces are often subjected to mechanical stresses as the interfaces they occupy are reduced in area. The most well characterized forms of stress relaxation in these systems are first order phase transitions from lower density to higher density phases. Here we study stress relaxation in lipid monolayers that occurs once chemical phase transitions have been exhausted. At these highly compressed states, the monolayer undergoes global mechanical relaxations termed collapse. By studying four different types of monolayers, we determine that collapse modes are most closely linked to in-plane rigidity. We characterize the rigidity of the monolayer by analyzing in-plane morphology on numerous length scales. More rigid monolayers collapse out-of-plane via a hard elastic mode similar to an elastic membrane, while softer monolayers relax in-plane by shearing.

Viscous shear banding in foam.

Shear banding is an important feature of flow in complex fluids. Essentially, shear bands refer to the coexistence of flowing and nonflowing regions in driven material. Understanding the possible sources of shear banding has important implications for a wide range of flow applications. In this regard, quasi-two-dimensional flow offers a unique opportunity to study competing factors that result in shear bands. One proposal for interpretation and analysis is the competition between intrinsic dissipation and an external source of dissipation. In this paper, we report on the experimental observation of the transition between different classes of shear bands that have been predicted to exist in cylindrical geometry as the result of this competition [R. J. Clancy, E. Janiaud, D. Weaire, and S. Hutzlet, Eur. J. Phys. E 21, 123 (2006)].

Rheology of two-dimensional F-actin networks associated with a lipid interface.

We report on the surface rheology of cross-linked F-actin networks associated with a lipid monolayer at the air-water interface of a Langmuir monolayer. The rheological measurements are made using a Couette cell. These data demonstrate that the network has a finite elastic modulus that grows as a function of the cross-linking concentration. We also note that under steady-state flow the system behaves as a power-law fluid in which the effective viscosity decreases with imposed shear.

Reversible plastic events in amorphous materials.

For crystalline materials, the microscopic origin of plasticity is well understood in terms of the dynamics of topological defects. For amorphous materials, the underlying structural disorder prevents such a description. Therefore identifying and characterizing the microscopic plastic events in amorphous materials remains an important challenge. We show direct evidence for the coexistence of reversible and irreversible plastic events (T1 events) at the microscopic scale in both experiments and simulations of two-dimensional foam. In the simulations, we also demonstrate a link between the reversibility of T1 events and pathways in the potential energy landscape of the system.

Combined macro- and microrheometer for use with Langmuir monolayers.

A Langmuir monolayer trough that is equipped for simultaneous microrheology and standard rheology measurements has been constructed. The central elements are the trough itself with a full range of optical tools accessing the air-water interface from below the trough and a portable knife-edge torsion pendulum that can access the interface from above. The ability to simultaneously measure the mechanical response of Langmuir monolayers on very different length scales is an important step for our understanding of the mechanical response of two-dimensional viscoelastic networks.

Aligning Practice to Policies: Changing the Culture to Recognize and Reward Teaching at Research Universities.

Recent calls for improvement in undergraduate education within STEM (science, technology, engineering, and mathematics) disciplines are hampered by the methods used to evaluate teaching effectiveness. Faculty members at research universities are commonly assessed and promoted mainly on the basis of research success. To improve the quality of undergraduate teaching across all disciplines, not only STEM fields, requires creating an environment wherein continuous improvement of teaching is valued, assessed, and rewarded at various stages of a faculty member's career. This requires consistent application of policies that reflect well-established best practices for evaluating teaching at the department, college, and university levels. Evidence shows most teaching evaluation practices do not reflect stated policies, even when the policies specifically espouse teaching as a value. Thus, alignment of practice to policy is a major barrier to establishing a culture in which teaching is valued. Situated in the context of current national efforts to improve undergraduate STEM education, including the Association of American Universities Undergraduate STEM Education Initiative, this essay discusses four guiding principles for aligning practice with stated priorities in formal policies: 1) enhancing the role of deans and chairs; 2) effectively using the hiring process; 3) improving communication; and 4) improving the understanding of teaching as a scholarly activity. In addition, three specific examples of efforts to improve the practice of evaluating teaching are presented as examples: 1) Three Bucket Model of merit review at the University of California, Irvine; (2) Evaluation of Teaching Rubric, University of Kansas; and (3) Teaching Quality Framework, University of Colorado, Boulder. These examples provide flexible criteria to holistically evaluate and improve the quality of teaching across the diverse institutions comprising modern higher education.

Characterization of anomalous flow and phase behavior in a Langmuir monolayer of 2-hydroxy-tetracosanoic acid.

Langmuir monolayers of long-chain fatty acids display a wide range of interesting flow and phase behaviors, but the correlation between these behaviors varies. In this paper, we report two interesting behaviors in a Langmuir monolayer of 2-hydroxy-tetracosanoic acid: phase coexistence between the L(2h) and L(2)' phases over a large pressure range and a peak in the viscosity and elastic modulus of the material as a function of pressure. The two behaviors are confirmed over a range of temperatures for which the location of the undistorted lattice appears to vary relative to the range of coexistence for the two phases. Evidence for a correlation between the phase behavior and the peak in the mechanical properties is presented.

Impact of noise on domain growth in electroconvection.

The growth and ordering of striped domains has recently received renewed attention due in part to experimental studies in diblock copolymers and electroconvection. One surprising result has been the relatively slow dynamics associated with the growth of striped domains. One potential source of the slow dynamics is the pinning of defects in the periodic potential of the stripes. Of interest is whether or not external noise will have a significant impact on the domain ordering, perhaps by reducing the pinning and increasing the rate of ordering. In contrast, we present experiments using electroconvection in which we show that a particular type of external noise decreases the rate of domain ordering.

Early time evolution of Freédericksz patterns generated from states of electroconvection.

We report on the early time ordering in a nematic liquid crystal subjected to a sudden change in an external ac electric field. We compare time evolution for two different initial states of electroconvection. Electroconvection is a highly driven state of a nematic liquid crystal involving convective motion of the fluid and periodic variations of the molecular alignment. By suddenly changing either the voltage or the frequency of the applied ac field, the system is brought to the same thermodynamic conditions. The time ordering of the system is characterized by the evolution of features of the power spectrum, including the average wave number, total power, and shape of the power spectrum. We observe that ordering of the system occurs faster after a sudden change in frequency than it does after a sudden change in voltage.

Flow transitions in two-dimensional foams.

For sufficiently slow rates of strain, flowing foam can exhibit inhomogeneous flows. The nature of these flows is an area of active study in both two-dimensional model foams and three dimensional foam. Recent work in three-dimensional foam has identified three distinct regimes of flow [S. Rodts, J. C. Baudez, and P. Coussot, Europhys. Lett. 69, 636 (2005)]. Two of these regimes are identified with continuum behavior (full flow and shear banding), and the third regime is identified as a discrete regime exhibiting extreme localization. In this paper, the discrete regime is studied in more detail using a model two-dimensional foam: a bubble raft. We characterize the behavior of the bubble raft subjected to a constant rate of strain as a function of time, system size, and applied rate of strain. We observe localized flow that is consistent with the coexistence of a power-law fluid with rigid-body rotation. As a function of applied rate of strain, there is a transition from a continuum description of the flow to discrete flow when the thickness of the flow region is approximately ten bubbles. This occurs at an applied rotation rate of approximately 0.07 s-1.