Microrollers flow uphill as granular media Wilson-Whitford, Samuel R; Gao, Jinghui; Roffin, Maria Chiara ...
Nature communications,
09/2023, Letnik:
14, Številka:
1
Journal Article
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Abstract
Pour sand into a container and only the grains near the top surface move. The collective motion associated with the translational and rotational energy of the grains in a thin flowing layer ...is quickly dissipated as friction through multibody interactions. Alternatively, consider what will happen to a bed of particles if one applies a torque to each individual particle. In this paper, we demonstrate an experimental system where torque is applied at the constituent level through a rotating magnetic field in a dense bed of microrollers. The net result is the grains roll uphill, forming a heap with a negative angle of repose. Two different regimes have been identified related to the degree of mobility or fluidisation of the particles in the bulk. Velocimetry of the near surface flowing layer reveals the collective motion of these responsive particles scales in a similar way to flowing bulk granular flows. A simple granular model that includes cohesion accurately predicts the apparent negative coefficient of friction. In contrast to the response of active or responsive particles that mimic thermodynamic principles, this system results in macroscopic collective behavior that has the kinematics of a purely dissipative granular system.
Essays on Hayek Machlup, Fritz
1976, 20131016, 2003, 2013-10-16
eBook
Varying according to the scope of Hayek's contributions, the papers in this volume include among others:* An affirmation of the "relevance" of Hayek's work* A survey of his contribution to knowledge* ...An appraisal of Hayek's innovative work on the methodology of the social sciences* A discussion of Hayek's achievements as scholar and mentorThe contributors are: Fritz Machlup, Geroge Roche, Arthur Shenfield, Max Hartwell, William Buckley, Gottfried Dietze, Shirley Letwin.
National Review has always published letters from readers. In 1965 the magazine decided that certain letters merited different treatment, and William F. Buckley, the editor, began a column called ...Notes & Asides" in which he personally replied to the most notable and outrageous correspondence. Culled from four decades of the column, Cancel Your Own Goddam Subscription includes exchanges with such well-known figures as Ronald Reagan, Richard Nixon, John Kenneth Galbraith, A.M. Rosenthal, Auberon Waugh, Arthur Schlesinger Jr., and many others. There are also hilarious exchanges with ordinary readers, as well as letters from Buckley to various organizations and government agencies. Combative, brilliant, and uproariously funny, Cancel Your Own Goddam Subscription represents Buckley at his mischievous best.
Flying High Buckley Jr., William F
2009, 2009-04-08
eBook
In Flying High, William F. Buckley Jr. offers his lyrical remembrance of a singular era in American politics, and a tribute to the modern Conservative movement's first presidential standard-bearer, ...Barry Goldwater. Goldwater was in many ways the perfect candidate: self-reliant, unpretentious, unshakably honest, and dashingly handsome. And although he lost the election, he electrified millions of voters with his integrity and a sense of decencyqualities that made him a natural spokesman for Conservative ideals and an inspiration for decades to come. In an era when Republicans are looking for a leader, Flying High is a reminder of how real political visionaries inspire devotion.
OBJECTIVE: Deficits in static postural control related to chronic ankle instability (CAI) and fatigue have been investigated separately, but little evidence links these factors to performance of ...dynamic postural control. Our purpose was to investigate the effects of fatigue and CAI on performance measures of a dynamic postural-control task, the Star Excursion Balance Test. DESIGN AND SETTING: For each of the 3 designated reaching directions, 4 separate 5 (condition) x 2 (time) x 2 (side) analyses of variance with a between factor of group (CAI, healthy) were calculated for normalized reach distance and maximal ankle-dorsiflexion, knee-flexion, and hip-flexion angles. All data were collected in the Athletic Training Research Laboratory. SUBJECTS: Thirty subjects (16 healthy, 14 CAI) participated. MEASUREMENTS: All subjects completed 5 testing sessions, during which sagittal-plane kinematics and reaching distances were recorded while they performed 3 reaching directions (anterior, medial, and posterior) of the Star Excursion Balance Test, with the same stance leg before and after different fatiguing conditions. The procedure was repeated for both legs during each session. RESULTS: The involved side of the CAI subjects displayed significantly smaller reach distance values and knee-flexion angles for all 3 reaching directions compared with the uninjured side and the healthy group. The effects of fatigue amplified this trend. CONCLUSIONS: Chronic ankle instability and fatigue disrupted dynamic postural control, most notably by altering control of sagittal-plane joint angles proximal to the ankle.
Ankle sprains represent a common musculoskeletal injury that clinicians are tasked with preventing and treating. Because of the prevalence of this injury, ankle braces have been designed to ...prophylactically protect the joint and reduce the incidence of repetitive sprains. Although an abundance of literature exists focusing on the efficacy of braces in preventing ankle sprains in young, healthy, and physically active populations, there is a scarcity of evidence specific to the impact of these apparatuses on functional performance; therefore, the purpose of this critically appraised topic (CAT) is to investigate the effects of ankle braces on functional performance measures in such individuals. The outcomes of this CAT will assist sport rehabilitation specialists with informed clinical decision making in managing young, healthy, and physically active populations using ankle braces. Do ankle braces hinder functional performance measures when compared with an unbraced condition in a young, healthy, and physically active population? A minimum of level II evidence research studies were surveyed for this CAT. For this CAT, 1 randomized controlled trial and 3 prospective cohort studies were selected. One study found a statistically significant main effect of increased agility run times while participants wore ankle braces. Another study demonstrated a statistically significant decrease in vertical jump height and ankle range of motion while wearing braces. No other statistically significant findings were reported among studies comparing unbraced with braced conditions. Current data indicate that young, healthy, and physically active individuals may experience varied performance effects when executing specific functional performance tasks while wearing ankle braces. In general, bracing does not appear to significantly impair performance on most functional tasks; however, decrements were noted to increases in agility run time and decreases in vertical jump height. Subsequent analysis indicated that a brace may result in decreased ankle plantarflexion, dorsiflexion, eversion, and inversion range of motion, which may underpin noted performance deficits.
The Dynamic Leap Balance Test (DLBT) is a new dynamic balance task that requires serial changes in base of support with alternating limb support and recovery of dynamic stability, as compared with ...the Y modification of the Star Excursion Balance Test (Y-SEBT), which assesses dynamic stability over an unchanging base of support.
To assess the dynamic balance performance in 2 different types of dynamic balance tasks, the DLBT and the SEBT, in subjects with unilateral chronic ankle instability (CAI) when compared with matched controls. The authors hypothesized that the DLBT score would significantly differ between the CAI involved and uninvolved limbs (contralateral and healthy matched) and demonstrate a modest (r = .50) association with the SEBT scores.
Case-control.
Controlled laboratory.
A total of 36 physically active adults, 18 with history of unilateral CAI and 18 without history of ankle injury, were enrolled in the study. CAI subjects were identified using the Identification of Functional Ankle Instability questionnaire.
The DLBT and the SEBT were performed in a randomized order on a randomly selected limb in CAI and healthy subjects.
Time taken to complete the DLBT and the reach distances performed on the SEBT were compared between the CAI and the healthy subjects.
There were no statistically significant differences (P < .05) in SEBT reach distances between groups. The DLBT time was greater (P < .01) for unstable ankles compared with the stable ankle. The authors found no correlation (P > .05) between DLBT time and any of the SEBT reach distances suggesting that the DLBT provides unique information in the assessment of patients with CAI.
The DLBT challenges the ability to maintain postural control in CAI subjects differently than the SEBT. There is a need of more dynamic balance assessment tools that are functional and clinically relevant.
Purpose To profile the standing flexion angle and its association with subjective outcomes in ipsilateral hamstring tendon autograft anterior cruciate ligament–reconstructed patients. A secondary aim ...was to describe prone position–measured hamstring strength as a predictor of flexion angle. Methods Fifteen women (mean age, 20.47 ± 1.96 years; mean height, 1.69 ± 0.08 m; mean weight, 68.51 ± 12.64 kg; mean Tegner score, 6.80 ± 1.52), at a mean of 25.93 ± 11.25 months after surgery, were matched to 15 healthy participants by sex and approximate age, height, mass, and activity level (mean age, 20.93 ± 1.22 years; mean height, 1.65 ± 0.06 m; mean weight, 66.52 ± 10.69 kg; mean Tegner score, 6.13 ± 1.06). The independent variable was leg condition (involved, uninvolved, or matched). Dependent variables included goniometric flexion angle, Knee Injury and Osteoarthritis Outcome Score (KOOS) subscale scores, and absolute isokinetic hamstring strength. We used 1-tailed paired and 2-sample t tests to analyze side and group differences, respectively. Corresponding effect sizes ( d ) were also quantified. Linear regression assessed relations between flexion angle and the KOOS, as well as strength and flexion angle. P < .05 denoted statistical significance. Results The involved leg showed a significantly lesser flexion angle (112.9° ± 8.1°) compared with the uninvolved leg (116.1° ± 8.4°, P = .024) and matched leg (117.1° ± 4.5°, P = .044), with corresponding weak side ( d = 0.380) and strong group ( d = 0.958) effect sizes. Significant associations existed between flexion angle and subjective outcomes ( r 2 = 60.3% and P = .001 for KOOS pain subscale, r 2 = 37.8% and P = .015 for KOOS subscale for function in activities of daily living, and r 2 = 39.2% and P = .012 for KOOS subscale for function in sports and recreation) for the involved leg. Hamstring strength was not a significant predictor of flexion angle for all legs ( P > .05). Conclusions Our results support the hypotheses that standing flexion angle insufficiencies exist for the involved leg, superior subjective outcomes are associated with greater flexion capacity, and hamstring strength at deep knee angles does not predict the standing flexion angle. Level of Evidence Level IV, therapeutic study, case series.
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