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- ItemPseudo-chemotaxis of active Brownian particles competing for food(San Francisco, California, US : PLOS, 2020) Merlitz, Holger; Vuijk, Hidde D.; Wittmann, René; Sharma, Abhinav; Sommer, Jens-UweActive Brownian particles (ABPs) are physical models for motility in simple life forms and easily studied in simulations. An open question is to what extent an increase of activity by a gradient of fuel, or food in living systems, results in an evolutionary advantage of actively moving systems such as ABPs over non-motile systems, which rely on thermal diffusion only. It is an established fact that within confined systems in a stationary state, the activity of ABPs generates density profiles that are enhanced in regions of low activity, which is thus referred to as ‘anti-chemotaxis’. This would suggest that a rather complex sensoric subsystem and information processing is a precondition to recognize and navigate towards a food source. We demonstrate in this work that in non-stationary setups, for instance as a result of short bursts of fuel/food, ABPs do in fact exhibit chemotactic behavior. In direct competition with inactive, but otherwise identical Brownian particles (BPs), the ABPs are shown to fetch a larger amount of food. We discuss this result based on simple physical arguments. From the biological perspective, the ability of primitive entities to move in direct response to the available amount of external energy would, even in absence of any sensoric devices, encompass an evolutionary advantage.
- ItemNon-synchronization of lattice and carrier temperatures in light-emitting diodes([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Zhang, Jihong; Shih, Tienmo; Lu, Yijun; Merlitz, Holger; Chang, Richard Ru-Gin; Chen, ZhongPulse implementation or switching-off (PISO) of electrical currents has become a common operation in junction-temperature (Tj) measurements for semiconductor devices since 2004. Here we have experimentally discovered a substantial discrepancy between Tj values with and without, PISO (e.g., 36.8 °C versus 76.5 °C above the ambient temperature at 25.0 °C). Our research indicates that methods associated with PISO are flawed due to non-synchronization of lattice temperatures and carrier temperatures in transient states. To scrutinize this discrepancy, we propose a lattice-inertia thermal anchoring mechanism that (1) explains the cause of this discrepancy, (2) helps to develop a remedy to eliminate this discrepancy by identifying three transient phases, (3) has been applied to establishing an original, accurate and noninvasive technique for light-emitting diodes to measure Tj in the absence of PISO. Our finding may pave the foundation for LED communities to further establish reliable junction-temperature measurements based on the identified mechanism.
- ItemMaximal rectification ratios for idealized bi-segment thermal rectifiers([London] : Macmillan Publishers Limited, part of Springer Nature, 2015) Shih, Tien-Mo; Gao, Zhaojing; Guo, Ziquan; Merlitz, Holger; Pagni, Patrick J.; Chen, ZhongThermal rectifiers whose forward heat fluxes are greater than reverse counterparts have been extensively studied. Here we have discovered, idealized and derived the ultimate limit of such rectification ratios, which are partially validated by numerical simulations, experiments and micro-scale Hamiltonian-oscillator analyses. For rectifiers whose thermal conductivities (κ) are linear with the temperature, this limit is simply a numerical value of 3. For those whose conductivities are nonlinear with temperatures, the maxima equal κmax/κmin, where two extremes denote values of the solid segment materials that can be possibly found or fabricated within a reasonable temperature range. Recommendations for manufacturing high-ratio rectifiers are also given with examples. Under idealized assumptions, these proposed rectification limits cannot be defied by any bi-segment thermal rectifiers.