Decoupling of rotation and translation at the colloidal glass transition.

In dense particle systems, the coupling of rotation and translation motion becomes intricate. Here, we report the results of confocal fluorescence microscopy where simultaneous recording of translational and rotational particle trajectories from a bidisperse colloidal dispersion is achieved by spiking the samples with rotational probe particles. The latter consist of colloidal particles containing two fluorescently labeled cores suited for tracking the particle's orientation. A comparison of the experimental data with event driven Brownian simulations gives insights into the system's structure and dynamics close to the glass transition and sheds new light onto the translation-rotation coupling. The data show that with increasing volume fractions, translational dynamics slows down drastically, whereas rotational dynamics changes very little. We find convincing agreement between simulation and experiments, even though the simulations neglect far-field hydrodynamic interactions. An additional analysis of the glass transition following mode coupling theory works well for the structural dynamics but indicates a decoupling of the diffusion of the smaller particle species. Shear stress correlations do not decorrelate in the simulated glass states and are not affected by rotational motion.

Seeing Yourself as a Scientist: Increasing Science Identity Using Professional Development Modules Designed for Undergraduate Students.

As educators, we should not assume that students are progressing toward intended STEM careers simply because they have persisted and received a STEM degree. In addition to learning biology content and scientific skills, students need guidance in making optimal career choices. In this study, we present seven career development modules designed specifically to motivate students to consider their successes as scientists and to consider applying their biological knowledge and scientific skills to a range of biology careers. These modules highlight the value and the utility of a biology degree and are, therefore, designed to increase students' self-confidence as well as their science and biology identities. The career development modules presented here are easy to implement and, in our experience, encourage engagement and class discussions. Our analyses confirm that these modules collectively increase student science and biology identities, two predictors for entry into STEM careers.

Student Association of Lecture Content with the Five Core Concepts of Biology: Novel Results from an Introductory Biology Course.

Challenges in integration of concepts persist among undergraduate biology students. The 5 core concepts (5CCs) of biology presented in Vision and Change provide a comprehensive, concept-based description of the knowledge of biology, summarized in five main biological scales and five overarching principles that dictate natural biological phenomena and processes. The goal of this study was to collect information on students' interpretations of three introductory biology topics, (i) aquaporins, (ii) aerobic respiration, and (iii) DNA transcription, while associating their knowledge of these topics with the 5CCs. During three separate exam review sessions, students of a conventional lecture-based introductory biology class were asked to provide short responses of how each of the 5CCs related to the given class topic. An inductive coding analysis of student responses was performed to reveal the main connections students made between each of the three topics and the 5CCs. We found that for some core concepts it was easier for students to draw connections to a simple topic, such as aquaporins, while for other core concepts it was easier to draw connections to a multistep phenomenon, such as aerobic respiration. Although student connections were simple associations between a CC and a class topic, exploratory studies such as this one can be an important step toward designing teaching practices that are aligned with Vision and Change recommendations and could advance student conceptual understanding and integration of biological knowledge.

The Effect of Structure on the Memory for Procedural Text.

The present study examined how the structure of procedural texts affected recall of those texts. Past research has found that procedural text is comprehended best when readers expend a moderate amount of effort in processing it; the amount of effort may depend on the structure of the procedural text. Sixty-three participants read six procedural texts describing how to construct simple machines. One group of participants read texts that contained a diagram of the object, whereas the other group read texts with no diagram. Two types of texts were presented: Narrative and list-like procedural texts. Results showed that rereading increased recall of the list-like text, but had little effect for the narrative text. The elaboration hypothesis explains the recall differences after a single reading, but it is still unclear why the list-like texts were recalled better than the narrative texts after a second reading.

Preparation and Tracking of Oblate Core-Shell Polymethyl-Methacrylate Ellipsoids.

Although single-particle level studies on prolate ellipsoidal colloids are relatively abundant, similar studies on oblate ellipsoids are rare because suitable model systems are scarcely available. Here, we present the preparation of monodisperse hard core-shell oblate ellipsoids that can be imaged and tracked in 3D with confocal laser scanning microscopy. Using a thermomechanical squeezing method, we transform spherical core-shell polymethyl-methacrylate (PMMA) particles into oblate ellipsoids. We show how the shape polydispersity as well as the aspect ratio of the obtained oblate ellipsoids can be controlled. In addition, we discuss how the core-shell geometry limits the range of aspect ratios because of the different viscoelastic properties of the cross-linked PMMA core and linear PMMA shell. We further demonstrate imaging of the core-shell oblate dispersions on a single-particle level in real space and time and the tracking of position and orientation using our recently developed tracking algorithm for anisotropic core-shell colloids. Our results thus provide the tools for the future investigation of the behavior of oblate ellipsoids, especially in dense suspensions.

Formation of nematic order in 3D systems of hard colloidal ellipsoids.

Suspensions of hard ellipsoidal particles exhibit complex phase behavior as shown by theoretical predictions and simulations of phase diagrams. Here, we report quantitative confocal microscopy experiments of hard prolate colloidal ellipsoids with different aspect ratio a/b. We studied different volume fractions φ of ellipsoids in density and refractive index matched suspensions. Large 3D sample volumes were investigated and the positions as well as the orientations of all ellipsoids were extracted by image analysis routines. By evaluating the translational and orientational order in the system we determined the presence of isotropic and nematic phases. For ellipsoids with a/b = 2.0 we found that isotropic phases form at all φ, while ellipsoids with a/b = 7.0 formed nematic phases at high φ, as expected from theory and simulations. For a/b = 3.5 and a/b = 4.1, however, we observed the absence of long-range orientational order even at φ where nematic phases are expected. We show that local orientational order formed with the emergence of nematic precursors for a/b = 3.5 and short-ranged nematic domains for a/b = 4.1. Our results provide novel insight into the phase behavior and orientational order of ellipsoids with different aspect ratios.