RESUMEN
Controlling environmental effects in surface plasticity/fracture of metals is of interest for areas as diverse as manufacturing processes, product performance, and structural safety. The key to controlling these effects is understanding the effect of adsorbates on surface energy (γ) and surface stress (f). While γ has been well studied, the role of surface stress has received much less attention. We characterize surface stress induced in metals by adsorption of organic monolayers. Linear alkanoic acids of varying chain length (3-18) are deposited by molecular self-assembly onto one side of an aluminum cantilever, several centimeters in length. The surface stress is estimated from in situ measurement of the cantilever deflection. We find that the organic adsorbates induce large surface stress of -4 to +30N/m. Furthermore, we show that f may be tuned by varying adsorbate-molecule chain length. The stress data explain beneficial embrittlement of metal surfaces by organic adsorbates in cutting and comminution processes, and point to a critical role, hitherto ignored, for f in environment assisted cracking (EAC) phenomena. Our results suggest opportunities for utilizing controlled environment-assisted fracture as an aid-fracture as a friend-to enhance material removal processes, apart from using surface stress itself as an experimental probe to explore various manifestations of EAC.
RESUMEN
Environment-assisted fracture phenomena in metals are usually associated with surface energy reduction due to an adsorbed film. Here we demonstrate a unique embrittlement effect in Al that is instead mediated by surface stress, induced by an adsorbed organic monolayer. Atomistic simulations show that the adsorbate carbon-chain length lc controls the surface stress via van der Waals forces, being compressive for lc < 8 and tensile otherwise. For lc > 8, we demonstrate experimentally that the nanoscale film causes a ductile-to-brittle transition on the macroscale. Concomitant with this transition is a nearly 85% reduction in deformation forces. Additional simulations reveal that the microscopic mechanism for the embrittlement is via suppression of dislocation emission at incipient crack-tips. In addition to challenging long-held views on environment-assisted fracture, our findings pertaining to surface-stress induced embrittlement suggest profitable utility in manufacturing processes such as machining and comminution.
RESUMEN
Adsorbed films often influence mechanical behavior of surfaces, leading to well-known mechanochemical phenomena such as liquid metal embrittlement and environment-assisted cracking. Here, we demonstrate a mechanochemical phenomenon wherein adsorbed long-chain organic monolayers disrupt large-strain plastic deformation in metals. Using high-speed in situ imaging and post facto analysis, we show that the monolayers induce a ductile-to-brittle transition. Sinuous flow, characteristic of ductile metals, gives way to quasi-periodic fracture, typical of brittle materials, with 85% reduction in deformation forces. By independently varying surface energy and molecule chain length via molecular self-assembly, we argue that this "embrittlement" is driven by adsorbate-induced surface stress, as against surface energy reduction. Our observations, backed by modeling and molecular simulations, could provide a basis for explaining diverse mechanochemical phenomena in solids. The results also have implications for manufacturing processes such as machining and comminution, and wear.
RESUMEN
The familial variant of cardiac myxomas is an autosomally dominant disease. Here, we report a case of recurrent multiple cardiac myxomas wherein the patient, a 36-year-old woman who was referred for palpitations and nocturnal dyspnea, had a relevant familial history. Her mother had undergone extirpation of cardiac myxoma when she was about 50 years old. Echocardiography showed the presence of multiple cardiac myxomas. One of the cardiac myxomas nearly obstructed the left ventricular inflow; therefore, we extirpated the myxomas under cardiopulmonary bypass and cardioplegic arrest. The operation was uneventful and the postoperative clinical course was good. Pathological examination confirmed that the extirpated mass was a myxoma. Approximately 3 years after the first cardiac operation, we found a recurrent cardiac myxoma in the same patient. She then underwent a second operation and was discharged without any complications. Pathological examination also confirmed that the cardiac mass was a myxoma and that there was no malignancy.
RESUMEN
A 52-year-old woman presenting with shortness of breath and having no related past medical history was diagnosed with takotsubo cardiomyopathy. However, she revealed respiratory failure atypical with takotsubo cardiomyopathy. We diagnosed myasthenia gravis with myasthenic crisis by acetylcholine receptor-binding antibody titer with mediastinal tumor. Physical or emotional stress is well known to trigger the onset of takotsubo cardiomyopathy. Similarly, myasthenia crisis is also triggered by stress. Here, we report a case of simultaneous occurrence of takotsubo cardiomyopathy and myasthenia crisis.