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Active droplets play important roles in microfluidics, robotics, and micro-electromechanical systems. As a special class of active droplets that are conductive, reactive, and of high surface tension, liquid metal droplets (LMDs) can be driven by electric-field-induced surface (Marangoni) flows to function as reconfigurable components in actuators, sensors, catalytic reactors, and antennas. Stimulating LMDs using an electric field induces concurrent electro-hydrodynamic flows and electrochemical surface oxidation (passivation). It is however difficult to decouple these two effects which brings complexity in controlling LMD motions. To address this challenge, pulse time modulation (PTM) signals are used. PTM enables controlled LMD displacement by propelling the droplets forward during the voltage-on phases and facilitating surface recovery from oxidation during the voltage-off phases. Counterintuitively, by taking such intermittent "rests", the LMDs effectively inhibit the unfavorable impact of oxidation, granting high motion controllability. Combining high-speed imaging, motion tracking, machine learning, and electrochemical analysis, the study reveals how electro-hydrodynamic flows and surface oxide formation/dissolution interplay to generate well-defined motion regimes. The study further develops a quasi-analytical model to describe droplet motions and designs a rotary LMD motor to showcase the versatility of the approach. This work provides the fundamental framework and viable strategy for designing innovative liquid metal-based systems. [ABSTRACT FROM AUTHOR]

Purpose: Bombarding high-Z material with x-ray radiation releases Auger electrons and Coster-Kronig electrons, along with deeper penetrating fluorescent x-rays and photoelectrons. The Auger and Coster-Kronig electron penetration distance is on the order of nanometers to micrometers in water or tissue, creating a large dose enhancement accompanied by a RBE greater than 1 at the cellular level. The authors' aim is to measure the gold nanofilm dose enhancement factor (DEF) at the cellular level with unlaminated radiochromic film via primary 50 kVp tungsten x-ray spectrum interaction, similar to an electronic brachytherapy spectrum. Methods: Unlaminated Gafchromic® EBT2 film and Monte Carlo modeling were combined to derive DEF models. Gold film of thickness 23.1± 4.3 nm and surface roughness of 1.2±0.2 nm was placed in contact with unlaminated radiochromic film in a downstream orientation and exposed to a 50 kVp tungsten bremsstrahlung, mean energy 19.2 keV. Film response correction factors were derived by Monte Carlo modeling of electron energy deposition in the film's active layer, and by measuring film energy dependence from 4.5 keV to 50 kVp. Results: The measured DEF within a 13.6 µm thick water layer was 0.29 with a mean dose of 94±9.4 cGy from Au emissions and 324±32.4 cGy from the 50 kVp primary beam. Monte Carlo derived correction factors allowed determination of Au contributed dose in shallower depths at 0.25 µm intervals. Maximum DEF of 18.31 was found in the first 0.25 µm water depth. Conclusions: Dose enhancement from Au nanofilm can be measured at the cellular level using unlaminated radiochromic film. Complementing the measured dose value with Monte Carlo calculations allows estimation of dose enhancement at depth increments within the cellular range. [ABSTRACT FROM AUTHOR]