Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult to measure experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) numerical simulation framework is utilised, previously developed in the general context of the OpenFOAM CFD Toolbox. Droplet impacts on metallic meshes are performed both experimentally and numerically with satisfactory degree of agreement. From the experimental investigation three main outcomes are observed—deposition, partial imbibition, and penetration. The penetration into suspended meshes leads to spectacular multiple jetting below the mesh. A higher amount of liquid penetration is linked to higher impact velocity, lower viscosity and larger pore size dimension. An estimation of the liquid penetration is given in order to evaluate the impregnation properties of the meshes. From the parametric analysis it is shown that liquid viscosity affects the adhesion characteristics of the drops significantly, whereas droplet break-up after the impact is mostly controlled by surface tension. Additionally, wettability characteristics are found to play an important role in both liquid penetration and droplet break-up below the mesh.
The interaction between drops and porous matter has important applications in many fields such as painting, paper coating, design of textiles, filtration and therapeutic delivery, the latter can include also reconstructive surgery processes. Since the phenomenon of droplet impact onto a porous surface is particularly complex, a first step consists in analysing impacts on 2D structures, such as metallic porous layers. The present paper shows the case of drop impacts onto metallic meshes attached to a solid substrate. The pores are squared and not planar, due to the woven structure of the meshes: the dynamics of the flow is particulary complex, but it resembles more realistic cases. In analysing the impact of droplets of water, acetone and a mixture of glycerol and water on meshes with different pore sizes, three main outcomes were observed for both test cases: deposition, partial imbibition and penetration. Higher velocity impacts lead to droplet splashing followed by deposition, partial imbibition and penetration. A higher amount of liquid penetration is linked to a higher velocity impact, lower viscosity and a larger dimension of the pore size. A map of the regimes is proposed introducing two dimensionless numbers M and , that are functions of the Weber and Reynolds numbers and pore and wire sizes. Previous papers have not considered the role of the wire diameter. The two numbers allow a clear separation of the outcomes and a practical use of the results. List of symbols d Droplet diameter [m] D p Pore equivalent diameter [m] D w Wire diameter [m] * () M Re 0.9 We 2.1 p c Capillary pressure [Pa] p d Dynamic pressure [Pa] Re Reynolds number with droplet diameter Re p Reynolds number with pore diameter v i Impact velocity [m/s] We Weber number with droplet diameter We p Weber number with pore diameter γ 1 + D w D pore µ Viscosity [Pa s] ρ Liquid density [kg/m 3 ] σ Surface tension [N/m]
The drop impact onto porous surfaces has important applications in many fields, such as painting, paper coating, drug delivery and cosmetic sprays. In most of these applications, the optimisation of the deposition process is carried out empirically, without a proper understanding of the physics and a theoretical modelling of the spreading and the imbibition phenomena. The purpose of this study is to analyse droplet impacts on metallic meshes to define a general modelling strategy of the impact regimen on particular 2D regular porous surfaces. The application of this structure is relevant in process like filtration but also in the medical field, considering for example reconstructive surgery. By analysing the impact of droplets of water, acetone and a mixture of glycerol and water, having a diameter and an impact velocity in a range of 1.5-3mm and 2-4m/s, respectively, on meshes with a pore size ranging between 25 and 400 µm, a regime map was built considering 6 different impact outcomes. The outcomes were characterised by a deposition of the droplet on the substrate, or a partial imbibition, or a total imbibition. By increasing the impact velocity, a splash region was defined, which is still characterised by a final deposition, a partial imbibition and a total imbibition. It is found that the most influencing parameters are closely linked to the liquid properties and the impact velocity, more specifically liquid surface tension plays a major role in defining the impact outcome. In the case of Acetone, the lower surface tension brings to an almost instantaneous total imbibition whereas the experiments conducted using water and glycerol solution, showed a major distribution of the deposition regimes with respect to the other outcomes, due to the effect of a higher viscosity. It was found that the geometrical characteristics of the mesh such as pore size and wire diameter, play an important role as well in defining the total imbibition outcome. Finally, the defined transition maps, shows that for a certain combination of physical properties and initial condition, the outcome of the droplet impact is predictable.
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