Experimental Correlation for Splashing Condition of Droplets on Solid Substrates

Droplet splashing behaviors of water-ethanol binary mixture liquids on roughened solid surfaces were experimentally observed in order to investigate the effects of surface tension, viscosity, and wettability/surface roughness on the splashing occurrence. The range of the droplet volumes was from 1.7 μL to 32.6 μL. The ranges of the surface tension and the viscosity were from 21.1 mN/m to 71.9 mN/m, and from 1 mPas to 2.91 mPas, respectively. The surface roughness range was from 0.03 μm to 1.25 μm for Ra. The present experimental data were evaluated on the basis of the existing models. Resulting from these experiments, a simple model using the Ohnesorge number evaluated by the capillary length was proposed and the accuracy of the predicted critical values such as the critical Weber and Reynolds numbers were discussed. The result indicated that the liquid properties and the quantification of the surface condition such as surface roughness are important factors for the prediction of the splashing behavior.

Feedforward control to suppress the leading edge bulge in photopolymerization-based additive manufacturing

Purpose This study aims to examine the feasibility of feedforward actuation of the recoater blade position to alleviate the resin surface non-uniformity while moving over deep-to-shallow transitions of submerged (already cured) geometric features. Design/methodology/approach A two-dimensional computational fluid dynamics (CFD) model has been used to determine optimized blade actuation protocols to minimize the resin surface non-uniformity. An experimental setup has been designed to validate the feasibility of the proposed protocol in practice. Findings A developed protocol for the blade height actuation is applied to a rectangular stair-like configuration of the underlying part geometry. The evaluation of the actuation protocol revealed the importance of two physical length scales, the capillary length and the size of the flow recirculation cell below in the liquid resin layer below the blade. They determine, together with the length scales defining the topography (horizontal extent and depth), the optimal blade trajectory. This protocol has also shown its efficiency for application to more complicated shapes (and, potentially, for any arbitrary geometry). Practical implications This study shows that incorporation of a feedforward control scheme in the recoating system might significantly reduce (by up to 80%) the surface unevenness. Moreover, this improvement of performances does not require major modifications of the existing architecture. Originality/value The results presented in this work demonstrate the benefits of the integration of the feedforward control to minimize the leading edge bulges over underlying part geometries in stereolithography.

Liquid Droplet Impact Over Hydrophobic Mesh Surfaces and Assessment of Weber Number Dependent Characteristics

Impacting droplets and droplet ejection from hydrophobic mesh surfaces have interest in biomedicine, heat transfer engineering, and self-cleaning of surfaces. The rate and the size of newborn droplets can vary depending on, the droplet fluid properties, Weber number, mesh geometry, and surface wetting states. In the present study, impacting water droplets onto hydrophobic mesh surface is investigated and impact properties including, spreading, rebounding, and droplet fluid penetration and ejection rates are examined. Droplet behavior is assessed using high recording facilities and predicted in line with the experiments. The findings reveal that the critical Weber number for droplet fluid penetrating/ejecting from mesh screen mainly depends on the droplet fluid capillary length, and hydrophobic mesh size. The contact time of impacting droplet over mesh surface reduces with increasing droplet Weber number, which opposes the case observed for impacting droplets over flat hydrophobic surfaces. The restitution coefficient attains lower values for impacting droplets over mesh surfaces than that of flat surfaces. The rate and diameter of the ejected droplet from the mesh increases as droplet Weber increases. At the onset of impact, streamline curvature is formed inside droplet fluid, which creates a stagnation zone with radially varying pressure at the droplet fluid mesh interface. This reduces the ejected droplet diameter from mesh cells as mesh cells are located away from the impacting vertical axis.

Ultrasonic Atomization: New Spray Characterization Approaches

In particle engineering, spray drying is an essential technique that depends on producing sprays, ideally made of equal-sized droplets. Ultrasonic sprays appear to be the best option to achieve it, and Faraday waves are the background mechanism of ultrasonic atomization. The characterization of sprays in this atomization strategy is commonly related to the relation between characteristic drop sizes and the capillary length produced by the forcing frequency of wavy patterns on thin liquid films. However, although this atomization approach is practical when the intended outcome is to produce sprays with droplets of the same size, drop sizes are diverse in real applications. Therefore, adequate characterization of drop size is paramount to establishing the relations between empirical approaches proposed in the literature and the outcome of ultrasonic atomization in actual operating conditions. In this sense, this work explores new approaches to spray characterization applied to ultrasonic sprays produced with different solvents. The first two introduced are the role of redundancy in drop size measurements to avoid resolution limitation in the measurement technique and compare using regular versus variable bin widths when building the histograms of drop size. Another spray characterization tool is the Drop Size Diversity to understand the limitations of characterizing ultrasonic sprays solely based on representative diameters or moments of drop size distributions. The results of ultrasonic spray characterization obtained emphasize: the lack of universality in the relation between a characteristic diameter and the capillary length associated with Faraday waves; the variability on drop size induced by both liquid properties and flow rate on the atomization outcome, namely, lower capillary lengths produce smaller droplets but less efficiently; the higher sensibility of the polydispersion and heterogeneity degrees in Drop Size Diversity when using variable bin widths to build the histograms of drop size; the higher drop size diversity for lower flow rates expressed by the presence of multiple clusters of droplets with similar characteristics leading to multimodal drop size distributions; and the gamma and log-normal mathematical probability functions are the ones that best describe the organization of drop size data in ultrasonic sprays.

Submicron drops from flapping bursting bubbles

Tiny water drops produced from bubble bursting play a critical role in forming clouds, scattering sunlight, and transporting pathogens from water to the air. Bubbles burst by nucleating a hole at their cap foot and may produce jets or film drops. The latter originate from the fragmentation of liquid ligaments formed by the centripetal destabilization of the opening hole rim. They constitute a major fraction of the aerosols produced from bubbles with cap radius of curvature (R) > ∼0.4 × capillary length (a). However, our present understanding of the corresponding mechanisms does not explain the production of most submicron film drops, which represent the main number fraction of sea spray aerosols. In this study, we report observations showing that bursting bubbles with R < ∼0.4a are actually mainly responsible for submicron film drop production, through a mechanism involving the flapping shear instability of the cap with the outer environment. With this proposed pathway, the complex relations between bubble size and number of drops produced per bubble can be better explained, providing a fundamental framework for understanding the production flux of aerosols and the transfer of substances mediated by bubble bursting through the air–water interface and the sensitivity of the process to the nature of the environment.

Estimation of the Hydrodynamic Parameters of Soils Subjected to Water Scarcity: Application of BEST and Introduction of a Specific Methodology

Propose a specific method (Junction Between Arya and Heitman and Haverkamp - JAHH), similar to BEST, to obtain the hydrodynamic parameters of soils in Pernambuco, Brazil. Sample: Department of Civil Engineering, Polytechnic School of Pernambuco – POLI, between March 2019 and February 2021. For this, BEST and JAHH were used to obtain the hydrodynamic characteristics of the collected materials, and the results of both methods were compared with simulations performed in Hydrus-1D. Sorptivity and Ks, acquired using both methods, presented differences reached 68.38% regarding Ks. The characteristic radius of the pores (λm) and capillary length (λc) obtained with BEST are not coherent, and this can be explained because during the evaluation of one sandy soil, λm values were the highest and λc were the lowest, when the opposite was expected. The use of JAHH to estimate soil parameters could generate more coherent estimates than BEST-slope, even though both of them have presented results of the same proportion, such as sorptivity and saturated hydraulic conductivity, for exemple. Therefore, the proposed method presented more pertinent results when compared to BEST regarding the studied soils.

The wetting properties of frosted glass

Frosted glass is a common, low cost material. Its roughness can be used to control how it is wet by water. In this paper, the wetting properties of silicone oil and water are investigated. For the oil, wetting is total since the oleophilic character of the glass is enhanced by its roughness. Due to the remarkable optical properties of frosted glass, the spreading of oil droplets on its surface was recorded over three months. Frosted glass is a parahydrophilic surface because of its large contact angle hysteresis (up to 80° ). The behaviour of oil and water droplets was compared on a long piece of inclined frosted glass. The trajectories (and the spreading) of the droplets were studied and phenomenological laws were deduced to describe the dependence of the droplet speed on the initial volume of the droplet and the angle of inclination. Such dependences of speed at long travel distances (100 times the capillary length) were deduced and rationalised with a simple model that takes into account the thickness of the wake. Moreover, we analysed the flow inside the wake of water droplets sliding on inclined frosted glass. Suggestions are given on how to exploit drainage of the water droplet wake and the high hysteresis of water within the framework of open microfluidics.

Spinneret geometry modulates the mechanical properties of man-made cellulose fibers

The production of cellulose-based textile fibers with high toughness is vital for extending the longevity and thus developing a sustainable textile industry by reducing the global burden of microplastics. This study presented strategies to improve fiber toughness by tuning spinneret geometries. Experimental studies were conducted by spinning with different spinneret geometries and measuring the mechanical and structural properties of the spun fibers. In addition, numerical simulation tools were used to better understand the effects of spinneret geometry. The altering parameters of the spinneret geometries were the capillary diameters D, the angle of the entry cone into the spinning capillary, and the ratio of capillary length to diameter L/D. The highest fiber toughness could be achieved at a capillary aspect ratio of 1 to 2. The obtained maximum fiber toughness was 93 MPa with a tensile strength of 60 cN/tex and a concomitant elongation of 16.5%. For these fiber properties, a 13 wt% solution of a high-purity pulp with higher viscosity in [DBNH][OAc] was spun into a 1.3 dtex fiber using a D100 spinneret with a capillary of 1:1 length/diameter and an entrance angle of 8°. It was noticeable that the microvoid orientations decreased almost linearly with increasing toughness of the fibers. The morphologies of the fibers were similar regardless of the spinneret geometries and the raw materials used in the spinning process. In summary, by modulating the spinneret geometries, Ioncell fibers obtained high toughness that have the potential to replace synthetic fibers.

Stereological Changes in Microvascular Parameters in Hippocampus of a Transgenic Rat Model of Alzheimer’s Disease

Background: Microcirculatory factors play an important role in amyloid-β (Aβ)-related neuropathology in Alzheimer’s disease (AD). Transgenic (Tg) rat models of mutant Aβ deposition can enhance our understanding of this microvascular pathology. Objective: Here we report stereology-based quantification and comparisons (between- and within-group) of microvessel length and number and associated parameters in hippocampal subregions in Tg model of AD in Fischer 344 rats and non-Tg littermates. Methods: Systematic-random samples of tissue sections were processed and laminin immunostained to visualize microvessels through the entire hippocampus in Tg and non-Tg rats. A computer-assisted stereology system was used to quantify microvessel parameters including total number, total length, and associated densities in dentate gyrus (DG) and cornu ammonis (CA) subregions. Results: Thin hair-like capillaries are common near Aβ plaques in hippocampal subregions of Tg rats. There are a 53% significant increase in average length per capillary across entire hippocampus (p≤0.04) in Tg compared to non-Tg rats; 49% reduction in capillary length in DG (p≤0.02); and, higher microvessel density in principal cell layers (p≤0.03). Furthermore, within-group comparisons confirm Tg but not non-Tg rats have significant increase in number density (p≤0.01) and potential diffusion distance (p≤0.04) of microvessels in principal cell layers of hippocampal subregions. Conclusion: We show the Tg deposition of human Aβ mutations in rats disrupts the wild-type microanatomy of hippocampal microvessels. Stereology-based microvascular parameters could promote the development of novel strategies for protection and the therapeutic management of AD.

Loss with ageing but preservation of frontal cortical capillary pericytes in post-stroke dementia, vascular dementia and Alzheimer’s disease

Cerebral pericytes are an integral component of the neurovascular unit, which governs the blood–brain barrier. There is paucity of knowledge on cortical pericytes across different dementias. We quantified cortical pericytes in capillaries in 124 post-mortem brains from subjects with post-stroke dementia (PSD), vascular dementia (VaD), Alzheimer’s disease (AD) and AD-VaD (Mixed) and, post-stroke non-demented (PSND) stroke survivors as well as normal ageing controls. Collagen 4 (COL4)-positive nucleated pericyte soma were identified as protrusions on capillaries of the frontal cortex. The COL4-positive somata or nodule-like cell bodies were also verified by platelet derived growth factor receptor-β (PDGFR-β) immunohistochemistry. The mean (± SEM) pericyte somata in frontal cortical capillaries in normal young controls (46–65 years of age) was estimated as 5.2 ± 0.2 per mm capillary length. This number was reduced by 45% in older controls (> 78 years) to 2.9 ± 0.1 per mm capillary length (P < 0.001). We further found that the numbers of pericyte cell bodies per COL4 mm2 area or per mm capillary length were not decreased but rather preserved or increased in PSD, AD and Mixed dementia groups compared to similar age older controls (P < 0.01). Consistent with this, we noted that capillary length densities identified by the endothelial marker glucose transporter 1 or COL4 were not different across the dementias compared to older controls. There was a negative correlation with age (P < 0.001) suggesting fewer pericyte somata in older age, although the % COL4 immunoreactive capillary area was increased in older controls compared to young controls. Using a proven reliable method to quantify COL4-positive nucleated pericytes, our observations demonstrate ageing related loss but mostly preserved pericytes in the frontal cortex of vascular and AD dementias. We suggest there is differential regulation of capillary pericytes in the frontal lobe between the cortex and white matter in ageing-related dementias.