Electric dipole moment

The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, that is, a measure of the system's overall polarity. The SI unit for electric dipole moment is the coulomb-meter (C⋅m). The debye (D) is another unit of measurement used in atomic physics and chemistry. Theoretically, an electric dipole is defined by the first-order term of the multipole expansion; it consists of two equal and opposite charges that are infinitesimally close together, although real dipoles have separated charge. Elementary definition Often in physics the dimensions of a massive object can be ignored and can be treated as a pointlike object, i.e. a point particle. Point particles with electric charge are referred to as point charges. Two point charges, one with charge +q and the other one with charge −q separated by a distance d, constitute an electric dipole (a simple case of an electric multipole). For this case, the elec

Electric dipole moments of highly excited molecular vibrational states

Patrice Theulé

In this work, new spectroscopic techniques have been developed to measure electric dipole moments of highly excited rovibrational states of small polyatomic molecules in the gas phase. These techniques make use of lasers arid of microwave synthesizers. They enable one to measure the change on a molecular system caused by applying an external electric field, which is called Stark effect and from this, extract the dipole moment. The first technique, called microwave Stark spectroscopy, makes use of microwave-optical double resonance combined to either laser induced fluorescence or vibrational predissociation detection. The second approach, called Stark induced quantum beat spectroscopy, relies on the time evolution of a coherently prepared molecular wavepacket in an electric field, using either electronic photodissociation or laser induced fluorescence for detection. These techniques have been applied to H2C0, HOCl, HDO, and H20 for whom the electric dipole moment have been measured for several highly excited rovibrational states of the ground electronic state. Using these experimental measurements, the dependence of the dipole moment vector, both in orientation and in magnitude, on the vibrational excitation is discussed. Moreover the experimental data are used to test ab initio calculations potential energy and dipole moment surfaces and to establish critical benchmarks for future improvements. Due to the rarity of dipole moment data for highly excited vibrational states and their central role in transition intensities, intermolecular forces and collisions, these measurements are of special importance for chemical and energy transfer processes in atmospheric sciences, combustion studies, planetology, or more generally in the whole quantitative spectroscopy field, where transitions intensities are at least as important as line frequency positions.

EPFL2003

Inflight Microwave Drying Process of Micro-droplets for High Resolution 3D printing

Kwanghoon Choi

A new Additive-Manufacturing (AM) or 3D printing concept is proposed to improve the printing resolution for metal additive manufacturing in the frame of the SFA-AM project, Powder Focusing for Beam-Induced Laser 3D Printing. The project aims to transport small powder particles (largest diameter < 10 um) at a high throughput to a spot smaller than 20 µm. To realize the objective, a metal ion included liquid is suggested as a source medium similar to the inkjet process, and the droplet is employed as a container for the metal ion transport. The key challenges of achieving this goal are 1) droplet size control and 2) efficient microwave heating that can dry the liquid portion of the dynamic droplet at a high-speed (3 m/s). The most demanding part is the development of a miniaturized, very concentrated field density microwave resonator where the droplet is passing through. This follows from the extremely short interaction time between the microwave and the ejected droplets (research: 0.5 ms) which results from the requirement for a small system size for its integration. The primary medium examined for the experiment is water due to 1) a high dielectric loss proportional to the drying efficiency and 2) avoiding explosion risks at the laser sintering stage compared to organic solutions. Various droplet generation conditions were investigated to determine the optimum conditions for the custom droplet generator, in particular, two key parameters were focused on: frequency and flow rate. The droplets obtained were analyzed with respect to size, gap distance, and linearity affecting the uniformity of the AM process. Moreover, Navier-Stokes-based fluid dynamics simulations enabled understanding of droplets behaviors, especially break-up formation. Eventually, the optimal condition for producing the smallest droplets with a diameter of 100 µm is determined to be 15 kHz and 0.5 ml/min. Such droplets loaded with metallic particles could result in smaller than 20 um of remaining metal powders aggregate after the drying process.A new TEM (Transverse ElectroMagnetic) resonator has been developed for drying and sensing applications. A full-wave electromagnetic simulation software, CST Microwave Studio, supported the working mode of the device. Firstly, it is used for the dielectric characterizations of different water-ethanol mixtures contained in a micro capillary. The sensing relies on electric field perturbation due to the inserted sample, and is measured based on the change of both resonance frequency and Q-factor. These sample-dependent variations were analyzed to estimate the complex permittivity using different methods such as the Perturbation Method (PM), Least-Square Model (LSM), and Log-Linear Model (LLM). This work demonstrates that the proposed microwave module is capable of sensing nano-liter volume ranges even though the samples only partially influence the sensing area (error < 13 % in permittivity). In microwave heating, the developed resonator achieved a temperature increase of 28 K in the microwave exposal time of 0.5 ms when the microwave power of 45 W was applied. Finally, numerical models have been proposed to estimate the maximum temperature of the initial droplet before cooling starts attributable to heat and mass transfer. This model well corresponds to the microwave electromagnetic simulation resulting in similar temperature deviations. It helps to determine the net heating efficiency of the microwave device.

EPFL2022

Inflight Microwave Drying Process of Micro-droplets for High Resolution 3D printing

Kwanghoon Choi

EPFL2022