Harnessing Viscoelasticity to Suppress Irreversibility Buildup in a Colloidal Stirling Engine.

Typically, the rate at which a heat engine can produce useful work is constrained by the buildup of irreversibility with increasing operating speed. Here, using a recently developed reservoir engineering technique, we designed and quantified the performance of a colloidal Stirling engine operating in a viscoelastic bath. While the bath acts like a viscous fluid in the quasistatic limit, and the engine's performance agrees with equilibrium predictions, on reducing the cycle time to the bath's structural relaxation time, the increasingly elastic response of the bath aids suppress the buildup of irreversibility. We show that the elastic energy stored during the isothermal compression step of the Stirling cycle facilitates quick equilibration in the isothermal expansion step. This results in equilibriumlike efficiencies even for cycle times shorter than the equilibration time of the colloidal particle.

Overcoming power-efficiency tradeoff in a micro heat engine by engineered system-bath interactions.

All real heat engines, be it conventional macro engines or colloidal and atomic micro engines, inevitably tradeoff efficiency in their pursuit to maximize power. This basic postulate of finite-time thermodynamics has been the bane of all engine design for over two centuries and all optimal protocols implemented hitherto could at best minimize only the loss in the efficiency. The absence of a protocol that allows engines to overcome this limitation has prompted theoretical studies to suggest universality of the postulate in both passive and active engines. Here, we experimentally overcome the power-efficiency tradeoff in a colloidal Stirling engine by selectively reducing relaxation times over only the isochoric processes using system bath interactions generated by electrophoretic noise. Our approach opens a window of cycle times where the tradeoff is reversed and enables the engine to surpass even their quasistatic efficiency. Our strategies finally cut loose engine design from fundamental restrictions and pave way for the development of more efficient and powerful engines and devices.

Ultrafast time-resolved carrier dynamics in tellurium nanowires using optical pump terahertz probe spectroscopy.

We report carrier relaxation dynamics in semiconducting tellurium nanowires (average diameter ∼ 10 nm) using ultrafast time-resolved terahertz spectroscopy. After photoexcitation using an 800 nm pump pulse, we observed an initial increase in the THz conductivity due to the absorption of THz radiation by photoexcited carriers. The time evolution of the differential conductivity (Δσ(τpp) = σpump on(τpp) - σpump off) shows a bi-exponential relaxation with the initial fast decay time scale of τ1 ∼ 25 ps followed by a longer relaxation time constant of τ2 ∼ 100 ps. Interestingly, the two time scales depend on the amount of the capping agent present on the surface of TeNWs, showing a faster relaxation of the photoexcited carriers as the percentage of capping decreases. This is physically interpreted as the surface state mediated relaxation mechanism of the photo-pumped carriers depending on the density of available surface states. A quantitative understanding is obtained using a coupled rate equation model taking into account the decay mechanisms determined from the surface mediated relaxation rate (DS) and direct recombination rate (DR) of the electron-hole pairs. Furthermore, the measured lattice temperature (TL) dependent dynamics, showing a faster relaxation at lower temperature, is understood using the same rate equation model, giving a power law dependence of the electron-hole recombination rate (DR) on TL as DR ∝ TL-1/2. This is explained by estimating DR using the van Roosbroeck-Shockley theory taking into account the density of states () of one-dimensional nanowires. Furthermore, to understand the measured frequency-dependent THz photoconductivity, we model Δσ(ω) using the Boltzmann transport equation taking into account the energy-dependent scattering rates showing the dominant role of short range (Γsr) and Coulomb scattering (ΓC) rates in the relaxation process, which further provides a measure of the charged and neutral impurity concentrations.

Multi-scale time-resolved electron diffraction: A case study in moiré materials.

Ultrafast-optical-pump - structural-probe measurements, including ultrafast electron and x-ray scattering, provide direct experimental access to the fundamental timescales of atomic motion, and are thus foundational techniques for studying matter out of equilibrium. High-performance detectors are needed in scattering experiments to obtain maximum scientific value from every probe particle. We deploy a hybrid pixel array direct electron detector to perform ultrafast electron diffraction experiments on a WSe2/MoSe2 2D heterobilayer, resolving the weak features of diffuse scattering and moiré superlattice structure without saturating the zero order peak. Enabled by the detector's high frame rate, we show that a chopping technique provides diffraction difference images with signal-to-noise at the shot noise limit. Finally, we demonstrate that a fast detector frame rate coupled with a high repetition rate probe can provide continuous time resolution from femtoseconds to seconds, enabling us to perform a scanning ultrafast electron diffraction experiment that maps thermal transport in WSe2/MoSe2 and resolves distinct diffusion mechanisms in space and time.

Intermediate-range order governs dynamics in dense colloidal liquids.

The conventional wisdom is that liquids are completely disordered and lack nontrivial structure beyond nearest-neighbor distances. Recent observations have upended this view and demonstrated that the microstructure in liquids is surprisingly rich and plays a critical role in numerous physical, biological, and industrial processes. However, approaches to uncover this structure are either system-specific or yield results that are not physically intuitive. Here, through single-particle resolved three-dimensional confocal microscope imaging and the use of a recently introduced four-point correlation function, we show that bidisperse colloidal liquids have a highly nontrivial structure comprising alternating layers with icosahedral and dodecahedral order, which extends well beyond nearest-neighbor distances and grows with supercooling. By quantifying the dynamics of the system on the particle level, we establish that it is this intermediate-range order, and not the short-range order, which has a one-to-one correlation with dynamical heterogeneities, a property directly related to the relaxation dynamics of glassy liquids. Our experimental findings provide a direct and much sought-after link between the structure and dynamics of liquids and pave the way for probing the consequences of this intermediate-range order in other liquid state processes.

Gender Differences in Self-reported Faculty Developmental Networks.

Scholars have long recognized gender variation in social relationship dynamics. However, how gender shapes developmental networking relationships for career advancement, particularly among university faculty members, is understudied. This area of research is important since women comprise an increasing proportion of faculty and yet report receiving less mentoring and lower career satisfaction, productivity, and advancement than their male counterparts. This cross-sectional study assessed gender differences in self-reported dimensions of faculty participants' developmental networks by collecting information on relationships with developers, who are people who have taken concerted action and offered professional and personal guidance to help participants advance in their careers over the past year. The investigators used egocentric network data from an electronically administered Mentoring Network Questionnaire collected from 159 faculty involved in a mentoring intervention during the pandemic. Faculty were from multiple Southwest and Mountain West institutions. Statistical analyses were performed using the Chi-squared test, Wilcoxon rank-sum test, and unadjusted multilevel regression. Female faculty chose developers of lower gender diversity than male faculty (p=0.01). Compared to male faculty, female faculty reported receiving more psychosocial support from individual developers (p=0.03). Female faculty members' developers were more often characterized as friends and less often described as sponsors and allies than male faculty, based on relative levels of career and psychosocial support that individual developers provided (p<0.001). No gender differences were found in other network characteristics. Female faculty build developmental networks that have different factors compared to male faculty. Greater levels of psychosocial support and fewer allies and sponsors for female faculty may have long-term implications for differential career advancement for women vs. men in academic careers. Strategies to enhance networking should address gender differences and include a structured framework for assessing network gaps.

Inequity Analysis in Faculty Recognition Awards at a School of Medicine.

As part of developmental networks, sponsors help provide recognition and visibility opportunities to their faculty protégés. Recognition awards given to the School of Medicine (SOM) faculty are an important mechanism for acknowledging what is valued in academic medicine. Beyond their impact on individual careers, awards help define the culture and climate of an organization. The literature suggests inequities in recognition awards for women and racial/ethnic underrepresented minority faculty. The study's purpose was to examine the characteristics of the awardees relative to the SOM faculty in a minority-serving institution in a minority-majority state. In this observational cross-sectional study, 47 SOM faculty were recognized between 2000-2023 as Regents' Lecturers (9), Regents' Professors (20), Community Engagement Awardees (5), and Gold-headed Cane Awardees (13). SOM sought nominations which a search committee competitively reviewed. Award recipients were characterized by their department, rank, academic track, degree, country of origin, sex, and race/ethnicity, and were compared to all SOM faculty. Male faculty were more likely than women faculty to receive an award (p=0.04). Faculty with tenure, Ph.D. degree, or Professor rank were more likely to receive an award than their counterparts (p<0.001, all analyses). Faculty in basic and diagnostic specialties were more likely to receive an award than medical or surgical specialties (p<0.001). Although rates of awards for racial/ethnic URM faculty were about half that of non-URM faculty, this difference did not reach statistical significance (p=0.14). In addition to demonstrating sex-related inequity in awards, recognized faculty are traditionally associated with the scholarship of discovery compared to other models of scholarship or clinical activity. Sponsors should promote women, physicians, and clinician educators for recognition awards to advance their academic careers. SOM leaders need to examine award criteria and processes to ensure recognition of the diversity of talents and achievements that are critical to the future of academic medicine.

Inter-Rater Reliability of the Mentor Behavioral Interaction Rubric.

An objective assessment of a mentor's behavioral skills is needed to assess the effectiveness of mentor training interventions in academic settings. The Mentor Behavioral Interaction (MBI) Rubric is a newly developed, content-valid, observational measure of a mentor's behavioral skill during single-episode interactions with a mentee. The purpose of this study was to assess the inter-rater reliability (IRR) of the MBI Rubric when used to assess video-recorded mentor-mentee interactions. Three of a pool of four faculty raters with expertise in mentor training synchronously rated 26 videos of mentor-mentee interactions using structured guidelines. The MBI Rubric includes six items (Part 1), each with ratings on a 3- or 4-point scale, and ten yes/no items (Part 2) that characterize the content of the interaction. After initial individual ratings were completed, the three raters met, reviewed disagreements, and reached decisions about final item scores by either consensus or majority vote. Mean total Part 1 scores ranged between 1.42-2.69. IRRs ranged from good (Part 1 IRR=0.67) to excellent (Part 2 IRR=0.83). No training effects were observed, with no decrease (i.e., showing less variability) in inter-rater standard deviations over time. Rater effects in initial individual scoring were observed, with a significant difference between one vs. the other three raters on Part 1 individual scores, with no effects for Part 2 scores. Raters tended to score lower on initial individual scores than the final score for both Part 1 and 2. The MBI Rubric is the first observational measure to assess single episodes of video-recorded mentor-mentee interactions and has demonstrated content validity, and now inter-rater reliability. It may be used in parallel with other instruments to measure the efficacy of mentor training. Limitations include possible ceiling effects, and resource-intensive administration in terms of rater expertise and time. Future work will assess the responsiveness of the Rubric to change in mentor skill and construct validity.

Mentoring Network Questionnaire Support Scales Reliable and Valid with University Faculty.

The Developmental Network Questionnaire (DNQ) is used in business to self-assess relationships with developers, or people who support one's career. The Mentoring Network Questionnaire (MNQ) is an online modification of the DNQ and includes two scales that rate developer's contributions to career or psychosocial help. The psychometrics of these scales for different populations are unreported. This study analyzed the construct validity and reliability of the two scales measuring support provided by developers of university faculty. Mentors and mentees (G=156) from multiple Southwestern and Mountain West universities rated 741 developers on the MNQ's five-item career- and psychosocial-support scales. Participants responded on a seven-point scale ranging from "never, not at all" to "to the maximum extent possible." Multilevel confirmatory factor analysis (MCFA) using Mplus and the multi-level reliability coefficient omega assessed construct validity and internal consistency reliability, respectively. Results supported the validity of two latent constructs of career- and psychosocial support, each measured by the established five-item scale: Comparative fit index (CFI)=.93, Tucker-Lewis Index (TLI)=.91, root mean square error of approximation (RMSEA)=.06, standardized root mean square residual (SRMR): W=.09, B=.10. The measurement model was improved when the "removes barriers" item was removed from the career-support scale (CFI=.96, TLI=.95, RMSEA=.05, SRMR: W=.06 B=.09. Factor loadings at both the within- and between-levels were strong and statistically significant. Reliability omegas ranged from .85 to .92. Career and psychosocial support provided to university faculty by developers in their networks may be validly and reliably measured at both the within- and between-levels by a modified four-item career support scale and the original five-item psychosocial support scale from the DNQ and the modified MNQ. Limitations include reduced statistical power due to small sample size and lack of testing at the university level. Future work will assess the responsiveness of these scales to measuring change over time in the amount of support provided.

Curriculum-based Faculty Training in Networking: Knowledge and Self-efficacy Outcomes.

Although the advantages of developmental networks are well-known, most faculty do not know how to participate in such networks actively. Additionally, institutions face challenges in teaching faculty the best practices of networking. This deficiency constitutes a critical gap in the literature, which may slow career advancement for faculty, particularly from underrepresented groups. The study's purpose was to examine the effectiveness of a curriculum-based faculty training in developmental networks, utilizing the Extension for Community Health Outcomes (ECHO) platform. In this pre-post study, 33 faculty members participated in the intervention utilizing eight modules involving four competencies. Each module followed a standard format, including a short didactic, two facilitated case study discussions based on real-life scenarios, and self-reading of selected literature. Outcomes included (i) change in knowledge scores obtained from two questions per module and (ii) self-efficacy scores measured on a scale of 0-100. Paired student's t-test and mixed model regression analyses were used. A significant increase in knowledge score was documented using mixed model regression for 4 of the eight modules (mean change score 0.4-0.8, p≤0.03 for all analyses). The proportion of faculty participants reporting correct knowledge items for all modules increased from 49.8% (pre) to 64.3% (post), which was statistically significant (p<0.001). Significant increases in paired self-efficacy scores were reported for each of the eight modules (mean change score 17-37, p<0.05 for all analyses). This study highlights the importance of curriculum-based training in networking. Participants showed a significant increase in pre-post networking self-efficacy and knowledge scores. Our ECHO-based curriculum, facilitator training, and manual enable easy implementation in other institutions, ensuring scalability and adaptability. Our analysis provides the evidence basis for examining the impact of a developmental network intervention in enhancing individual career networks.

Faculty Retention at a School of Medicine, 2010-2022.

Faculty retention at academic health centers is a concern with about one-fifth of physicians reporting intentions to leave. We studied factors affecting faculty at risk for attrition, defined as women, racial/ethnic underrepresented minorities (URM), and clinical faculty. Identification of factors predicting retention of at-risk faculty may help mentors and minority-serving institutions devise novel targeted retention strategies. Our study site was a minority-serving institution in a majority-minority state in the US Southwest where at-risk faculty constitute the majority group. Faculty characteristics and departure dates were extracted from an institutional database maintained by the University of New Mexico (UNM) School of Medicine (SOM) for 2,427 participants employed from July 2009 through June 2022. Annual attrition rates and relative risk (RR) of attrition were estimated by discrete-time hazard rate models assuming a Poisson distribution. The overall annual attrition rate was 11.5%, which projects to 50% attrition in 6.0 years. Time to 50% attrition was 4.6 years for assistant professors, 8.9 years for associate professors 7.2 years for full professors. Faculty with a PhD degree had lower attrition (7.2%, RR=0.69, 95% CI 0.60, 0.79) compared to faculty with an MD degree (10.5%) in adjusted analyses. Clinician educators had a higher attrition rate (8.9%) compared to tenure track (6.4%, RRtenure track=0.72, 95% CI 0.61, 0.85). Black faculty had a higher risk of attrition compared to White faculty (RR=1.56, 95% CI 1.09, 2.25), and non-Hispanic White faculty had a lower risk of attrition (RR=0.83, 95% CI 0.71, 0.98). Annual attrition rates increased over the study period with most of the increase before about 2016. We did not detect significant differences in attrition due to sex or URM status.

Why Faculty Leaders Leave a School of Medicine?

Faculty attrition at academic health centers (AHCs) is significant at about 11% nationally, with one in five physicians intending to leave, and replacement costs averaging $500,000 per physician. Attrition among AHC faculty leaders is inadequately studied. This study compares reasons to leave between exiting faculty leaders and faculty non-leaders at the University of New Mexico School of Medicine (UNM SOM). The SOM deans interview all exiting faculty using a structured exit survey. 329 faculty non-leaders and 58 faculty leaders left UNM SOM between July 2017 and June 2022. Distributions of each variable were analyzed for statistically significant differences between the two groups using Fisher's 2-sided exact test. Text comments by leaders were analyzed qualitatively for content using a team-based, iterative process. As compared to non-leaders, exiting faculty leaders were more likely to be professors (51.7% vs 16.7%, p<0.001), and hold tenure (32.8% vs. 12.2%, p=0.001). Faculty leaders were more likely than non-leaders to cite high-level leadership as a reason to leave (41.4% vs. 24.3% p=0.01) and better leadership as a critical issue in development and retention (51.7% vs. 36.8% p=0.04). Qualitative analyses of textual leader comments showed a similar distribution of themes as the quantitative variables when examining open text related to the survey questions related to reasons to leave and the most critical issues. In addition, when asked what would need to change for them to return, qualitative data showed open-ended responses by exiting faculty leaders were twice as frequent to include leadership comments than those by non-leaders (34.2% vs. 16.2%). Exiting faculty leaders disproportionately cite high-level leadership as a reason to leave. The mediatory factors for this association are not known. Investigations to determine the causes for the study findings, and data-driven intervention strategies to retain faculty leaders at SOMs are needed.

Navigating Barriers and Challenges to Achieving Critical Career Milestones Among Faculty Mentees.

For faculty members, job satisfaction, compensation, and career advancement hinge on achieving 'critical' career milestones (e.g., external funding, tenure). Faculty face unique barriers to achieving career milestones (Bagley et al., 2018), and therefore must employ tailored strategies to overcome challenges. The current project extends research on barriers to career milestones (Soller et al., 2022) to examine strategies faculty employ to overcome barriers in the pursuit of critical career milestones. Thirty-seven faculty members participated across eight US academic institutions, including 22 under-represented minorities in science (URM-S; women or racial/ ethnic minorities). Respondents identified critical career milestones they achieved or will pursue within the next 24 months and then discussed strategies used and suggestions for achieving milestones during semi-structured qualitative interviews. The research team conducted a thematic, qualitative, descriptive analysis of qualitative data using NVivo software in a systematic, interactive, team-based process. Four key strategies emerged for navigating barriers in the pursuit of critical career milestones: 1) Careful engagement of mentors and allies; 2) Collaborate and network; 3) Set boundaries and prioritize; and 4) Reflect on values and use personal strengths. Administrators should aim to remove structural barriers, particularly those that reduce equity (Davis et al., 2022). Identifying strategies that faculty employ to overcome challenges can enhance mentoring by helping mentors understand how mentees overcome unique challenges, particularly those that are not easily addressed through structural interventions.

Shear jamming and fragility in fractal suspensions under confinement.

Under applied stress, the viscosity of many dense particulate suspensions increases drastically, a response known as discontinuous shear-thickening (DST). In some cases, the applied stress can even transform the suspension into a solid-like shear jammed state. Although shear jamming (SJ) has been probed for dense suspensions with particles having well-defined shapes, such a phenomenon for fractal objects has not been explored. Here, using rheology and in situ optical imaging, we study the flow behaviour of ultra-dilute fractal suspensions of multi-walled carbon nanotubes (MWCNT) under confinement. We show a direct transition from flowing to SJ state without a precursory DST in fractal suspensions at an onset volume fraction, ϕ ∼ 0.5%, significantly lower than that of conventional dense suspensions (ϕ ∼ 55%). The ultra-low concentration enables us to demonstrate the fragility and associated contact dynamics of the SJ state, which remain experimentally unexplored in suspensions. Furthermore, using a generalized Wyart-Cates model, we propose a generic phase diagram for fractal suspensions that captures the possibility of SJ without prior DST over a wide range of shear stress and volume fractions.

Synergistic action in colloidal heat engines coupled by non-conservative flows.

Colloidal heat engines are model systems to analyze mechanisms of transduction of heat to work at the mesoscale. While engines developed hitherto were realized using conservative potentials and operated in isolation, biological micromotors - their real counterparts - seldom perform under such simplifications. Here, we examine thermodynamics beyond such idealizations by constructing a pair of engines from two colloidal microspheres in optical traps at close separation. We demonstrate that at such proximity, non-conservative scattering forces that were hitherto neglected affect the particle motion. Hydrodynamics generated while dissipating these are hindered by the microsphere in the adjacent trap and energy that was otherwise rejected into the medium gets reused. Thus, despite being in contact with the same reservoir, the particles are driven out of equilibrium and can exchange energy, allowing cooperative behavior. Leveraging this in a manner analogous to microswimmers and active Brownian particles that utilize such flows to enhance propulsion, we construct two colloidal engines in close proximity. To estimate thermodynamic quantities, we develop a minimal model that is appropriate in the asymptotic limit and is similar to active Brownian particles. While complete theoretical frameworks to understand such scenarios remain to be developed, results based on our model demonstrate the intuitive idea that a pair of Stirling engines at close proximity outperform those that are well separated. Although these results explore the simplest case of two Stirling engines, the concepts unraveled could aid in designing larger collections akin to biological systems.

Observation of two-step melting on a sphere.

Melting in two-dimensional flat space is typically two-step and via the hexatic phase. How melting proceeds on a curved surface, however, is not known. Topology mandates that crystalline particle assemblies on these surfaces harbor a finite density of defects, which itself can be ordered, like the icosahedral ordering of 5-coordinated disclination defects on a sphere. Thus, melting even on a sphere, the simplest closed surface, involves the loss of both crystalline and defect order. Probing the interplay of these two forms of order, however, requires a system in which melting can be performed in situ, and this has not been achieved hitherto. Here, by tuning interparticle interactions in situ, we report an observation of an intermediate hexatic phase during the melting of colloidal crystals on a sphere. Remarkably, we observed a precipitous drop in icosahedral defect order in the hexatic phase where the shear modulus is expected to vanish. Furthermore, unlike in flat space, where disorder can fundamentally alter the nature of the melting process, on the sphere, we observed the signature characteristics of ideal melting. Our findings have profound implications for understanding, for instance, the self-assembly and maturation dynamics of viral capsids and also phase transitions on curved surfaces.

Active nonreciprocal attraction between motile particles in an elastic medium.

We show from experiments and simulations on vibration-activated granular matter that self-propelled polar rods in an elastic medium on a substrate turn and move towards each other. We account for this effective attraction through a coarse-grained theory of a motile particle as a moving point-force density that creates elastic strains in the medium that reorient other particles. Our measurements confirm qualitatively the predicted features of the distortions created by the rods, including the |x|^{-1/2} tail of the trailing displacement field and nonreciprocal sensing and pursuit. A discrepancy between the magnitudes of displacements along and transverse to the direction of motion remains. Our theory should be of relevance to the interaction of motile cells in the extracellular matrix or in a supported layer of gel or tissue.

Structural origin of excitations in a colloidal glass-former.

Despite decades of intense research, whether the transformation of supercooled liquids into glass is a kinetic phenomenon or a thermodynamic phase transition remains unknown. Here, we analyzed optical microscopy experiments on 2D binary colloidal glass-forming liquids and investigated the structural links of a prominent kinetic theory of glass transition. We examined a possible structural origin for localized excitations, which are building blocks of the dynamical facilitation theory-a purely kinetic approach for the glass transition. To accomplish this, we utilize machine learning methods to identify a structural order parameter termed "softness" that has been found to be correlated with reorganization events in supercooled liquids. Both excitations and softness qualitatively capture the dynamical slowdown on approaching the glass transition and motivated us to explore spatial and temporal correlations between them. Our results show that excitations predominantly occur in regions with high softness and the appearance of these high softness regions precedes excitations, thus suggesting a causal connection between them. Thus, unifying dynamical and thermodynamical theories into a single structure-based framework may provide a route to understand the glass transition.

Motile Topological Defects Hinder Dynamical Arrest in Dense Liquids of Active Ellipsoids.

Recent numerical studies have identified the persistence time of active motion as a critical parameter governing glassy dynamics in dense active matter. Here we studied dynamics in liquids of granular active ellipsoids with tunable persistence and velocity. We show that increasing the persistence time at moderate supercooling is equivalent to increasing the strength of attraction in equilibrium liquids and results in reentrant dynamics not just in the translational degrees of freedom, as anticipated, but also in the orientational ones. However, at high densities, motile topological defects, unique to active liquids of elongated particles, hindered dynamical arrest. Most remarkably, for the highest activity, we observed intermittent dynamics due to the jamming-unjamming of these defects for the first time.