149 research outputs found

    Rotation of an immersed cylinder sliding near a thin elastic coating

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    It is known that an object translating parallel to a soft wall in a viscous fluid produces hydro- dynamic stresses that deform the wall, which, in turn, results in a lift force on the object. Recent experiments with cylinders sliding under gravity near a soft incline, which confirmed theoretical arguments for the lift force, also reported an unexplained steady-state rotation of the cylinders [Saintyves et al. PNAS 113(21), 2016]. Motivated by these observations, we show, in the lubrication limit, that an infinite cylinder that translates in a viscous fluid parallel to a soft wall at constant speed and separation distance must also rotate in order to remain free of torque. Using the Lorentz reciprocal theorem, we show analytically that for small deformations of the elastic layer, the angular velocity of the cylinder scales with the cube of the sliding speed. These predictions are confirmed numerically. We then apply the theory to the gravity-driven motion of a cylinder near a soft incline and find qualitative agreement with the experimental observations, namely that a softer elastic layer results in a greater angular speed of the cylinder.Comment: 16 pages, 4 figure

    Optical Manipulation of Liquids by Thermal Marangoni Flow along the Air-Water Interfaces of a Superhydrophobic Surface

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    [Image: see text] The control of liquid motion on the micrometer scale is important for many liquid transport and biomedical applications. An efficient way to trigger liquid motion is by introducing surface tension gradients on free liquid interfaces leading to the Marangoni effect. However, a pronounced Marangoni-driven flow generally only occurs at a liquid–air or liquid–liquid interface but not at solid–liquid interfaces. Using superhydrophobic surfaces, the liquid phase stays in the Cassie state (where liquid is only in contact with the tips of the rough surface structure and air is enclosed in the indentations of the roughness) and hence provides the necessary liquid–air interface to trigger evident Marangoni flows. We use light to asymmetrically heat this interface and thereby control liquid motion near superhydrophobic surfaces. By laser scanning confocal microscopy, we determine the velocity distribution evolving through optical excitation. We show that Marangoni flow can be induced optically at structured, air-entrapping superhydrophobic surfaces. Furthermore, by comparison with numerical modeling, we demonstrate that in addition to the Marangoni flow, buoyancy-driven flow occurs. This effect has so far been neglected in similar approaches and models of thermocapillary driven flow at superhydrophobic surfaces. Our work yields insight into the physics of Marangoni flow and can help in designing new contactless, light-driven liquid transport systems, e.g., for liquid pumping or in microfluidic devices

    Energy backtransfer and infrared photoresponse in erbium-doped silicon p-n diodes

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    Temperature-dependent measurements of the photoluminescence (PL) intensity, PL lifetime, and infrared photocurrent, were performed on an erbium-implanted silicon p - n junction in order to investigate the energy transfer processes between the silicon electronic system and the Er 4 f energy levels. The device features excellent light trapping properties due to a textured front surface and a highly reflective rear surface. The PL intensity and PL lifetime measurements show weak temperature quenching of the erbium intra-4 f transition at 1.535 mm for temperatures up to 150 K, attributed to Auger energy transfer to free carriers. For higher temperatures, much stronger quenching is observed, which is attributed to an energy backtransfer process, in which Er deexcites by generation of a bound exciton at an Er-related trap. Dissociation of this exciton leads to the generation of electron-hole pairs that can be collected as a photocurrent. In addition, nonradiative recombination takes place at the trap. It is shown for the first time that all temperature-dependent data for PL intensity, PL lifetime, and photocurrent can be described using a single model. By fitting all temperature-dependent data simultaneously, we are able to extract the numerical values of the parameters that determine the ~temperature-dependent! energy transfer rates in erbium-doped silicon. While the external quantum efficiency of the photocurrent generation process is small (1.831026) due to the small erbium absorption cross section and the low erbium concentration, the conversion of Er excitations into free e - h pairs occurs with an efficiency of 70% at room temperature

    Brain functional network integrity sustains cognitive function despite atrophy in presymptomatic genetic frontotemporal dementia

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    © 2020 The Authors. Alzheimer's & Dementia published by Wiley Periodicals, Inc. on behalf of Alzheimer's Association. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.Introduction: The presymptomatic phase of neurodegenerative disease can last many years, with sustained cognitive function despite progressive atrophy. We investigate this phenomenon in familial frontotemporal dementia (FTD). Methods: We studied 121 presymptomatic FTD mutation carriers and 134 family members without mutations, using multivariate data-driven approach to link cognitive performance with both structural and functional magnetic resonance imaging. Atrophy and brain network connectivity were compared between groups, in relation to the time from expected symptom onset. Results: There were group differences in brain structure and function, in the absence of differences in cognitive performance. Specifically, we identified behaviorally relevant structural and functional network differences. Structure-function relationships were similar in both groups, but coupling between functional connectivity and cognition was stronger for carriers than for non-carriers, and increased with proximity to the expected onset of disease. Discussion: Our findings suggest that the maintenance of functional network connectivity enables carriers to maintain cognitive performance.K.A.T. is supported by the British Academy Postdoctoral Fellowship (PF160048) and the Guarantors of Brain (101149). J.B.R. is supported by the Wellcome Trust (103838), the Medical Research Council (SUAG/051 G101400), and the Cambridge NIHR Biomedical Research Centre. R. S.‐V. is supported by the Instituto de Salud Carlos III and the JPND network PreFrontAls (01ED1512/AC14/0013) and the Fundació Marató de TV3 (20143810). M.M and E.F are supported by the UK Medical Research Council, the Italian Ministry of Health, and the Canadian Institutes of Health Research as part of a Centres of Excellence in Neurodegeneration grant, and also a Canadian Institutes of Health Research operating grant (MOP 327387) and funding from the Weston Brain Institute. J.D.R., D.C., and K.M.M. are supported by the NIHR Queen Square Dementia Biomedical Research Unit, the NIHR UCL/H Biomedical Research Centre, and the Leonard Wolfson Experimental Neurology Centre (LWENC) Clinical Research Facility. J.D.R. is supported by an MRC Clinician Scientist Fellowship (MR/M008525/1) and has received funding from the NIHR Rare Disease Translational Research Collaboration (BRC149/NS/MH), the MRC UK GENFI grant (MR/ M023664/1), and The Bluefield Project. F.T. is supported by the Italian Ministry of Health (Grant NET‐2011‐02346784). L.C.J. and J.V.S. are supported by the Association for Frontotemporal Dementias Research Grant 2009, ZonMw Memorabel project number 733050103 and 733050813, and the Bluefield project. R.G. is supported by Italian Ministry of Health, Ricerca Corrente. J.L. was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy within the framework of the Munich Cluster for Systems Neurology (EXC 2145; SyNergy ‐ ID 390857198). The Swedish contributors C.G., L.O., and C.A. were supported by grants from JPND Prefrontals Swedish Research Council (VR) 529‐2014‐7504, JPND GENFI‐PROX Swedish Research Council (VR) 2019‐02248, Swedish Research Council (VR) 2015‐ 02926, Swedish Research Council (VR) 2018‐02754, Swedish FTD Initiative‐Schorling Foundation, Swedish Brain Foundation, Swedish Alzheimer Foundation, Stockholm County Council ALF, Karolinska Institutet Doctoral Funding, and StratNeuro, Swedish Demensfonden, during the conduct of the study.info:eu-repo/semantics/publishedVersio
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