Dynamics of collapse of free-surface bubbles: effects of gravity and viscosity

The rupture of the thin film at the top of a bubble floating at a liquid-gas interface leads to the axisymmetric collapse of the bubble cavity. We present scaling laws for such a cavity collapse, established from experiments conducted with bubbles spanning a wide range of Bond (${10^{-3}<Bo\leq1}$) and Ohnesorge numbers (${10^{-3}<Oh<10^{-1}}$), defined with the bubble radius $R$. The cavity collapse is a capillary-driven process, with a dependency on viscosity and gravity affecting, respectively, precursory capillary waves on the cavity boundary, and the static bubble shape. The collapse is characterised by tangential and normal velocities of the kink, formed by the intersection of the concave cavity opening after the top thin film rupture, with the convex bubble cavity boundary. The tangential velocity $U_t$ is constant during the collapse and is shown to be $U_t=4.5~U_c{\mathcal{W}}_R$, where $U_c$ is the capillary velocity and ${\mathcal{W}}_R(Oh,Bo)={(1-\sqrt{Oh {\mathscr{L}}} )^{-1/2}}$ is the wave resistance factor due to the precursory capillary waves, with $\mathscr{L}(Bo)$ being the path correction of the kink motion. The movement of the kink in the normal direction is part of the inward shrinkage of the whole cavity due to the sudden reduction of gas pressure inside the bubble cavity after the thin film rupture. This normal velocity is shown to scale as $U_c$ in the equatorial plane, while at the bottom of the cavity $\overline{U}_{nb}=U_c(Z_c/R)({\mathcal{W}_R}/ {\mathscr{L}})$, where $Z_c(Bo)$ is the static cavity depth. The total volume flux of cavity-filling, which is entirely contributed by this shrinking, scales as ${Q_T\simeq 2\pi R Z_c U_c}$; remains a constant throughout the collapse.Comment: 22 page

Characterizing the Metal–SAM Interface in Tunneling Junctions

his paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)TS/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/C≡CR), where R is an n-alkyl group. Values of J0 and β (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).Chemistry and Chemical Biolog

On the scaling of jetting from bubble collapse at a liquid surface

International audienceWe present scaling laws for the jet velocity resulting from bubble collapse at a liquid surface which bring out the effects of gravity and viscosity. The present experiments conducted in the range of Bond numbers and Ohnesorge numbers were motivated by the discrepancy between previous experimental results and numerical simulations. We show here that the actual dependence of on is determined by the gravity dependency of the bubble immersion (cavity) depth which has no power-law variation. The power-law variation of the jet Weber number, , suggested by Ghabache et al. (Phys. Fluids, vol. 26 (12), 2014, 121701) is only a good approximation in a limited range of values ( ). Viscosity enters the jet velocity scaling in two ways: (i) through damping of precursor capillary waves which merge at the bubble base and weaken the pressure impulse, and (ii) through direct viscous damping of the jet formation and dynamics. These damping processes are expressed by a dependence of the jet velocity on Ohnesorge number from which critical values of are obtained for capillary wave damping, the onset of jet weakening, the absence of jetting and the absence of jet breakup into droplets

Shape parameters of a floating bubble

International audienceFor a floating bubble, in the range of Bond numbers based on an equivalent spherical radius, 0 < Boe < 1, we present analytical expressions for various shape parameters of the bubble as functions of Boe. Expressions are obtained for the radius of the rim Rr, the radius of the thin film cap Rc, the height of the top of the cap from the rim hcap, the height of the rim above the free surface hr, and the depth of the bubble cavity from the free surface Zc. To obtain these expressions, we solve equations formulated in terms of these shape parameters for the meniscus outside the bubble, the force balance of the bubble, the pressure balance at the centre line of the bubble, and geometrical constraints, after neglecting the deformation of the bubble cavity for Boe < 1. The obtained expressions are shown to match well with our experimental measurements of the shape of the bubble. In addition to these expressions, we also present simpler approximations that can be used accurately as scaling laws for these shape parameters up to Boe < 0.5

Impact of Factors Influencing Cyber Threats on Autonomous Vehicles

Advanced Technologies are transforming the Automotive industry and the pace of innovation is accelerating at a breakneck speed. Autonomous Vehicles (AVs) incorporate many different systems and technologies and their increased computer functionality and connectivity lead to enormous cybersecurity risk. The aim of this research is to explore the significant factors that influence cyber threats on AVs and to examine their level of importance. Partial Least Squares path modeling was preferred for research studies for its flexible modeling and identifying key drivers. The data analysis was carried out using ADANCO 2.0. to develop and evaluate the structural model and the causal relationships between the variables. Correlation of in-vehicular network vulnerabilities with trust and the correlation between the “workload of the driverless system” with cyber-attacks and cyber threats to AVs are two relations but have not been touched upon in previous studies. In this research, a modified framework is proposed based on the Cyber Cycle and integrated model of Diamond Model of Intrusion Analysis with the Active Cyber Defense Cycle

The Origin of the Odd–Even Effect in the Tunneling Rates across EGaIn Junctions with Self-Assembled Monolayers (SAMs) of <i>n</i>‑Alkanethiolates

Odd–even effects in molecular junctions with self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates have been rarely observed. It is challenging to pinpoint the origin of odd–even effects and address the following question: are the odd–even effects an interface effect, caused by the intrinsic properties of the SAMs, or a combination of both? This paper describes the odd–even effects in SAM-based tunnel junctions of the form Ag^A‑TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions with a large range of molecular lengths (<i>n</i> = 2 to 18) that are characterized by both AC and DC methods along with a detailed statistical analysis of the data. This combination of techniques allowed us to separate interface effects from the contributions of the SAMs and to show that the odd–even effect observed in the value of <i>J</i> obtained by DC-methods are caused by the intrinsic properties of the SAMs. Impedance spectroscopy (an AC technique) allowed us to analyze the SAM resistance (<i>R</i>_SAM), SAM capacitance (<i>C</i>_SAM), and contact resistance, within the junctions separately. We found clear odd–even effects in the values of both <i>R</i>_SAM and <i>C</i>_SAM, but the odd–even effect in contact resistance is very weak (and not responsible for the observed odd–even effect in the current densities obtained by <i>J</i>(V) measurements). Therefore, the odd–even effects in Ag^A‑TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions are attributed to the properties of the SAMs and SAM–electrode interactions which both determine the shape of the tunneling barrier

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Tuning charge transport across junctions of ferrocene-containing polymer brushes on ITO by controlling the brush thickness and the tether lengths

This paper describes the electrical characteristics of junctions of ferrocene (Fc)-containing polymer brushes (PBs) grafted on indium tin oxide (ITO) bottom electrodes contacted with eutectic gallium indium (EGaIn) top electrodes. We studied the charge transport phenomena across these junctions as a function of the PB thickness as well as the length of the tether (i.e., an alkyl chain side linker) between the Fc units and the polymethyl methacrylate backbone. Junctions with PBs with a thickness of 20 nm do rectify. Temperature dependent charge transport measurements revealed that the charge transport mechanism is dominated by space charge limited conduction (SCLC).ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Published versio

Equivalent Circuits of a Self-Assembled Monolayer-Based Tunnel Junction Determined by Impedance Spectroscopy

The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule–electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates (S­(CH₂)_<i>n</i>−1CH₃ ≡ SC_<i>n</i> with <i>n</i> = 8, 10, 12, or 14) of the form Ag^TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn (a protective thin (∼0.7 nm) layer of GaO_<i>x</i> forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (<i>R</i>_S, which includes the SAM-electrode contact resistance), the capacitance of the SAM (<i>C</i>_SAM), and the resistance of the SAM (<i>R</i>_SAM). A plot of <i>R</i>_SAM vs <i>n</i>_C yielded a tunneling decay constant β of 1.03 ± 0.04 <i>n</i>_C^–1, which is similar to values determined by DC methods. The value of <i>C</i>_SAM is similar to previously reported values, and <i>R</i>_S (2.9–3.6 × 10^–2 Ω·cm²) is dominated by the SAM–top contact resistance (and not by the conductive layer of GaO_<i>x</i>) and independent of <i>n</i>_C. Using the values of <i>R</i>_SAM, we estimated the resistance per molecule <i>r</i> as a function of <i>n</i>_C, which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions