EXPERIMENTAL STUDY ON VISCOUS NEWTONIAN DROPLET IMPACTS ON DRY OR PRE-WETTED MESHES
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摘要: 液滴撞击网面现象广泛存在于自然界和一系列应用中, 液滴撞网后会穿透破碎产生二次液滴或不破碎全部附着在网面上, 两种情况下都会残留液体在网面而形成预湿, 影响后续撞击结果, 但前人研究集中于低黏性液滴撞击干燥网面, 黏性牛顿流体液滴撞击干燥或预湿网面的演化与机理仍有待探索. 本文采用高速阴影成像技术, 研究了黏性液滴(甘油水溶液)撞击干燥和预湿网面形成液指和破碎的演化规律, 考虑了网面结构尺寸、液滴黏性及撞击前网面上预湿液膜厚度对撞击结果的影响. 实验结果表明, 液滴撞击干燥网面后形成液指的最大长度随网孔宽度降低、液滴黏性增加而减小; 液滴黏性增加、网孔宽度减小均会抑制液滴对干燥网面的完全穿透; 预湿液膜高度的增加抑制液滴对网面的完全穿透, 并使不完全穿透时形成液指的最大长度减小. 建立了考虑液滴黏性、网孔宽度和网面预湿的液滴撞击网面后不完全穿透时形成液指的最大长度预测模型, 以及出现完全穿透时的临界参数理论预测模型, 模型预测结果均与实验结果吻合良好.Abstract: Droplet impacts on meshes are ubiquitous both in nature and in a variety of applications. The impact may lead to liquid penetration through the mesh and formation of secondary droplets underneath the mesh, or spreading on the mesh and no occurrence of penetration. In either situation, liquid remains on the meshes and forms pre-wetted meshes after impacts, leading to different following impact outcomes compared with impacts on dry meshes. However, previous studies focused on impacts of low-viscosity droplets on dry meshes. The evolution and mechanism of viscous Newtonian droplets impacts on dry or pre-wetted meshes remain to be explored. In this paper, the liquid fingers and the fragmentation occurred underneath the mesh following a viscous droplet (aqueous glycerol solution) impacting a dry or pre-wetted mesh are investigated using high-speed shadow imaging technology, with special attention paid on the influence of mesh size, droplet viscosity and pre-wetted liquid film thickness on impact outcomes. It was observed that both a decrease of mesh size and an increase of droplet viscosity resulted in a decrease of maximum length of liquid finger and suppressed a complete penetration through a dry mesh, an increase of pre-wetted liquid film thickness suppressed a complete penetration and resulted in a decrease of maximum length of liquid finger. Considering the influence of mesh size, droplet viscosity, and pre-wetted liquid film thickness, theoretical models for predicting the maximum length of liquid finger when incomplete penetration occurs and for predicting the threshold parameters for complete penetration is proposed and is validated by comparisons with experimental results.
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Key words:
- viscous droplets /
- pre-wetted meshes /
- liquid fingers /
- secondary droplets
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表 1 甘油水液滴的物理属性(25°C)
Table 1. The physical properties of aqueous glycerol solutions at 25°C
ψ/wt% μ/(mPa·s) ρ/(kg·m−3) σ/(mN·m−1) 50 5.01 1113.41 68.05 64 11.54 1150.67 66.95 70 18.07 1167.41 66.85 75 27.73 1181.74 66.54 78 36.94 1190.50 66.16 80 45.37 1196.42 66.77 81.5 53.35 1200.90 65.43 83 63.16 1205.41 65.52 84 71.06 1208.43 66.20 85 80.16 1211.48 66.13 表 2 网面参数
Table 2. The mesh parameters
Nm ω/μm d/μm 60 250 160 80 200 140 100 150 125 120 125 90 180 80 56 表 3 甘油水溶液在网面上的静态接触角
Table 3. The equilibrium contact angles of aqueous glycerol solutions droplet on the mesh
Nm θeq/(°)
(μ = 5.01 mPa·s)θeq/(°)
(μ = 80.16 mPa·s)60 125.1 128.8 80 123.0 124.2 100 119.5 124.8 120 114.8 126.7 180 125.3 119.8 -
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