Time-resolved in situ nanoparticle size evolution during magnetron sputtering onto liquids.

Despite extensive research since 1996, there are still open questions regarding the primary location of the nucleation process, the growth mechanism of the nanoparticles (NPs), and the influence of the liquid properties on the ultimate size of the NPs for the magnetron sputtering of metals onto liquids. Hence, for the first time to the authors' knowledge, the particle size evolution is in situ and in real-time examined during and after the sputtering of the silver atoms onto silicone oil, i.e., Sputtering onto Liquids (SoL) process. The particle size distribution (PSD) is measured via the Light Extinction Spectroscopy (LES) technique, and the deposition rate and stirring speed effects on the PSDs are analyzed. Based on De Brouckere mean diameters, the size evolution of silver nanoparticles (Ag NPs) over time is monitored. Ag NPs bigger than 20 nm are detected, and the PSDs are shown to be poly-disperse, which is also supported by the ex situ TEM measurements and in situ time-resolved absorption spectra. Moreover, it is shown that aggregation and growth of Ag NPs occur both at the plasma-liquid interface and inside the silicone oil during and after the magnetron sputtering. Despite the same amount of deposited silver, the growth kinetics of Ag NPs in silicone oil vary at different deposition rates. In particular, at higher deposition rates, larger NPs are formed. Stirring is seen to help disaggregate the particle lumps. Faster stirring does not substantially influence the final size but promotes the formation of smaller NPs (<20 nm). Also, low colloidal stability of Ag NPs in silicone oil is observed.

Role of Nanoparticles in Nanofluid Droplet Impact on Solid Surfaces.

Splashing of a liquid droplet onto a substrate, while ubiquitous, sits at the intersection of several key fluid mechanical regions. Typically, this problem is often simplified to the transition between spreading and splashing, even for splashing on complex surfaces. Recently, there has been increased interest in using not just pure liquids but also nanofluids in applications such as spray cooling. While the addition of a few percent of nanoparticles to a Newtonian fluid does not change its apparent viscosity, the influence of the nanoparticles on the splashing transition is pronounced. We often view splashing in terms of fluid mechanics where a simple material is subjected to a complex flow and the fluid can be simply characterized by a Newtonian viscosity. For nanofluids, we have an apparently simple material in a complex flow, but the results show that the impact of the particles is nontrivial. This implies that we must now combine some of the insights we obtain from studying the rheological properties of nanosuspensions with this already complex problem.

Liquid dynamics sloshing in cylindrical containers: A 3D free-surface reconstruction dataset.

The present dataset provides experimental measurements of the 3D liquid/gas interface shape during lateral water sloshing in a partially filled cylindrical container. The measurement technique used to acquire the data is the Reference Image Topography [1] based on a Synthetic Schlieren Free-Surface Reconstruction method [2]. A modified version of the processing algorithm has been used. This one transforms the coordinate system from Cartesian to polar so that the computational domain only includes the area where the fluid is present. Moreover, it uses the conservation of the fluid volume into the investigated area that permits to obtain the absolute height. This allows overcoming the strong limitation of the RIT method regarding the inability to detect changes of the mean surface height, at the condition that the complete and only liquid domain recorded in the images is used in the inversion algorithm. The complete details of the post-processing of the images is reported in the paper associated to this DIB [3]. The dataset includes the external excitation history and maps of the liquid/gas interface acquired during the experiment. These data are considered fundamental for the validation of CFD simulations of sloshing and of simplified theoretical models. A set of 12 test cases are reported in this DIB. A part of these test cases refers to steady state sloshing and a part to sloshing damping. In the last cases, also the 3D map of damping coefficients, calculated using the logarithmic decrement method, is provided.

Metachronal patterns in artificial cilia for low Reynolds number fluid propulsion.

Cilia are hair-like organelles, present in arrays that collectively beat to generate flow. Given their small size and consequent low Reynolds numbers, asymmetric motions are necessary to create a net flow. Here, we developed an array of six soft robotic cilia, which are individually addressable, to both mimic nature's symmetry-breaking mechanisms and control asymmetries to study their influence on fluid propulsion. Our experimental tests are corroborated with fluid dynamics simulations, where we find a good agreement between both and show how the kymographs of the flow are related to the phase shift of the metachronal waves. Compared to synchronous beating, we report a 50% increase of net flow speed when cilia move in an antiplectic wave with phase shift of -π/3 and a decrease for symplectic waves. Furthermore, we observe the formation of traveling vortices in the direction of the wave when metachrony is applied.

Real-time on-machine observations close to interelectrode gap in a tool-based hybrid laser-electrochemical micromachining process.

A tool-based hybrid laser-electrochemical micromachining process involves concurrent application of two process energies i.e. electrochemical and laser in the same machining zone by means of a hybrid tool which serves as an ECM tool as well as a multimode waveguide. It is a relatively novel process finding applications in defect-free machining of difficult-to-cut materials without affecting their microstructure. In order to understand the physical phenomena occurring during this process, in-situ observations are required. Therefore, in this work, a real time observation was carried out of a novel tool-based hybrid laser electrochemical micromachining process. A combination of high-speed imaging and Large Scale Particle Image Velocimetry (LSPIV) was used to visualize the tool-based hybrid laser-ECM process in real time. It also allowed to carry out experimental investigations on the by-products and bubble generation which have a direct effect on process performance in terms of accuracy and efficiency. The real-time on-machine observations are unique of its kind and they will facilitate the understanding of underlying mechanisms governing this hybrid laser-electrochemical micromachining process. This will ultimately help in improving the quality of parts manufactured. This research is also a step forward towards making these physics-based hybrid processes deterministic by employing high-speed imaging in a closed loop control.

Unraveling Particle Formation: From Single Droplet Drying to Spray Drying and Electrospraying.

Spray drying and electrospraying are well-established drying processes that already have proven their value in the pharmaceutical field. However, there is currently still a lack of knowledge on the fundamentals of the particle formation process, thereby hampering fast and cost-effective particle engineering. To get a better understanding of how functional particles are formed with respect to process and formulation parameters, it is indispensable to offer a comprehensive overview of critical aspects of the droplet drying and particle formation process. This review therefore closely relates single droplet drying to pharmaceutical applications. Although excellent reviews exist of the different aspects, there is, to the best of our knowledge, no single review that describes all steps that one should consider when trying to engineer a certain type of particle morphology. The findings presented in this article have strengthened the predictive value of single droplet drying for pharmaceutical drying applications like spray drying and electrospraying. Continuous follow-up of the particle formation process in single droplet drying experiments hence allows optimization of manufacturing processes and particle engineering approaches and acceleration of process development.

Feasibility of using glory and speckle patterns for sizing spherical and irregular particles.

A backscattered laser light technique for sizing spherical and irregular particles is investigated in this paper. Two different interference patterns (glory and speckle), appearing in the backscatter region when a single droplet is illuminated with a laser light source, were recorded by a CCD camera. A theoretical model, based on a geometrical optics approximation, has been first developed to retrieve particle size from the analysis of these patterns and then applied to liquid and frozen water droplets with sizes ranging from 1 to 2 mm. The satisfactory agreement obtained between the theoretical model and the experimental data encourage further development of this technique.

Assessment of refractive index gradients by standard rainbow thermometry.

Up to now the application of rainbow thermometry has been limited to particle systems possessing a uniform refractive index. This is mostly due to the absence of an appropriate data inversion algorithm that takes into account the presence of a refractive index gradient. In this paper, for the first time to our knowledge, exploiting a generalization of the Airy theory, a data inversion algorithm for a single droplet, presenting a parabolic refractive index gradient, is proposed. This data inversion algorithm is used to compute the diameter and the refractive index at the core and at the surface of a simulated burning droplet. The results are compared to the analytical solutions showing a satisfactory agreement.

Generalization of the rainbow Airy theory to nonuniform spheres.

Rainbow techniques permit measurement of refractive indices, and hence the temperatures of liquid droplets through determination of the absolute angular position of a rainbow interference image in space. The Airy theory, which is commonly used to explain the rainbow effect, permits the determination of a unique refractive-index value, even in the presence of nonuniformities in the droplet. An extension of this theory to spheres that exhibit internal refractive-index gradients is proposed. The case of burning droplets is considered as an example of such spheres, and the results obtained are successfully compared with those presented in the literature.

Global rainbow thermometry: improvements in the data inversion algorithm and validation technique in liquid-liquid suspension.

Improvements on the validation of a nonintrusive laser-based measurement technique are presented. This new technique, called global rainbow thermometry (GRT), is capable of determining the temperature and the size distributions of liquid droplets dispersed in a liquid or gaseous bulk. We propose a new data inversion algorithm that takes into account the whole rainbow pattern. Experimental validation of the GRT technique is performed for a liquid-liquid suspension. We performed the validation by comparing the measurements obtained with the GRT technique for the mean droplet temperature and size with the results obtained with alternative techniques.