The self-assembly of helical ribbons is examined in a variety of multicomponent enantiomerically pure systems that contain a bile salt or a nonionic detergent, a phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol. In almost all systems, two different pitch types of helical ribbons are observed: high pitch, with a pitch angle of 54 ؎ 2°, and low pitch, with a pitch angle of 11 ؎ 2°. Although the majority of these helices are right-handed, a small proportion of left-handed helices is observed. Additionally, a third type of helical ribbon, with a pitch angle in the range 30-47°, is occasionally found. These experimental findings suggest that the helical ribbons are crystalline rather than liquid crystal in nature and also suggest that molecular chirality may not be the determining factor in helix formation. The large yields of helices produced will permit a systematic investigation of their individual kinetic evolution and their elastic moduli.Interest in molecular self-assembly of helical structures is driven by both technological and medical applications. Helices are often precursors in the growth of tubules (1-4), which can be used as a controlled release system for drug delivery in medicine and as templates for microelectronics and magnetic applications (5). The morphology of the tubules must be rationally optimized for each application. Therefore it is important to understand the role of the various constituent molecules in the formation of these structures.Helical ribbons have been observed in a variety of systems composed of chiral amphiphiles. Although the diameters and lengths of the helical structures varied from system to system, the pitch angle observed in all systems was either 45°(6-8) or Ϸ60°(9-12). In almost all cases, helical ribbons in enantiomerically pure systems were either right-or left-handed (6,8,13). Recently, however, Thomas et al. studied an enantiomerically pure phosphonate analogue of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine and related compounds, which self-assembled into a mixture of right-and left-handed helical ribbons (10, 11).A biologically important system in which helical ribbons form is model bile, consisting of a mixture of three types of chiral molecules in water: a bile salt, a phosphatidylcholine, and cholesterol (4, 14-21). Helical ribbons are metastable intermediates in the process of cholesterol crystallization in bile (2,19,20), which precedes cholesterol gallstone formation (4,(18)(19)(20)(21)(22)(23). In contrast to all other systems studied, two pitch types of helical ribbons are observed in bile: high pitch, with a pitch angle of 54 Ϯ 2°, and low pitch, with a pitch angle of 11 Ϯ 2°. To date, the production of these two helical pitch types has been thought to be a property unique to model biles. Indeed, previous work showed that all three components of model bile are required for helical ribbons to form. In the absence of the phosphatidylcholine, only needle-like crystals form (4, 21), whereas without the bile salt only...

Helical ribbons with pitch angles of either 11 degrees or 54 degrees 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 degrees and a helical domain with a pitch angle of 16.5 degrees. 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.

The transition metal chalcogenide Ni(S,Se)2 is one of the few highly correlated, Mott-Hubbard systems without a strong first-order structural distortion that normally cuts off the critical behavior at the metal-insulator transition. The zero-temperature (T) transition was tuned with pressure, and significant deviations were found near the quantum critical point from the usual T1/2 behavior of the conductivity characteristic of electron-electron interactions in the presence of disorder. The transport data for pressure and temperature below 1 kelvin could be collapsed onto a universal scaling curve.

Background Research illustrates that student motivations influence learning engagement, persistence, and achievement in powerful ways and that positive motivations are linked to deeper learning, critical thinking, pro-social behavior, and better performance. Most studies of learner motivation, however, are conducted outside of STEM and are focused at the contextual level, which may describe why students attend college or choose a degree program, but not why they engage in classroom activities. Furthermore, there is little research that meaningfully connects learner motivations with gender identity and course pedagogy. This study addresses these gaps by examining the interconnections among course pedagogy, gender, and situational-level motivations, which reveal why learners engage in different course activities and how engagement may vary over time. This detailed perspective on learner motivations is essential for instructors to gain insights into how their pedagogical and course design choices influence students’ motivational responses and to more effectively develop interventions that support positive forms of motivation among all students. Results Participants in the study are undergraduate students enrolled in 72 introductory-level STEM courses across 11 institutions, and the dataset includes over 5000 unique responses to the Situational Motivation Scale, a Self-Determination Theory-based instrument that was administered weekly in each course. Analysis reveals seven typical motivational response types, ranging from a highly control-oriented to a highly autonomous response. Most students express multiple types of motivation during an academic term in a course, illustrating the dynamic nature of motivations. Cluster distributions by gender and pedagogy indicate significant differences in lecture-based learning courses, with women reporting less self-determined motivations compared to men. Motivational response profiles of women and men are both more similar, and more positive overall, in courses that employ active learning. Conclusions These findings have important implications for practitioners. The concept of motivational co-expression encourages instructors to move toward a more nuanced appraisal of learner motivation. The stability analyses challenge embedded beliefs about the fixed nature of learner motivation. The gender analyses raise questions about how instructors may more effectively promote the positive motivations of all students through their course design decisions.

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Background Despite long-standing initiatives to improve gender equity across STEM

The purpose of this study is to determine how introductory Project-Based Learning (PjBL) courses affect the self-efficacy of first-year engineering students. Grounded theory is used to analyze twelve interviews with first-year students about their experiences in two PjBL courses, Engineering Design and Physics Laboratory. Data indicate that students' self-efficacy within each course is correlated with the extent to which their course goal perceptions align with those intended by faculty. In Engineering Design, students' recognition of the faculty's intended course goals corresponds to higher levels of self-efficacy. Conversely, in Physics Laboratory, students' low self-efficacy is correlated with a large gap between their perceived and faculty intended course goals. Analysis further reveals that this difference in course goal perceptions may stem from the variations in the courses' contingent scaffolding. Finally, our findings suggest that students' self-efficacy may be further supported by dynamic course scaffolding that allows for an increase in students' autonomy throughout a course.