本研究第一部分使用高速攝影機 (75k fps) 以俯角 50 拍攝純水液滴 (D0 = 2.37 mm) (於不同撞擊高度 H = 20 ~ 40 cm) 撞擊木材平板 (非洲黑檀，Rq = 0.51 ~ 3.95 m 與南美紫檀，Rq = 1.06 ~ 5.69 m ) 後 4 ms 內粗糙度對潤濕性的影響。實驗數據顯示，表面粗糙度會影響 (1) 液膜 (液滴撞擊固體平板後形成一沿著平板表面擴張之薄膜) 在擴張階段的流動性、(2) 撞擊液滴的完整性、(3) 撞擊液滴與木材平板間的摩擦力。例如：隨著表面粗糙度的增加，從液滴擴散因子 (D(t)/D0) 的減少，可以知道液膜在粗糙的表面上較難擴張而使其流動性降低、撞擊液滴液膜邊緣出現更多 finger 以及產生更多飛濺液滴，使其外觀相對於撞擊粗糙度小的木材表面較為不完整、液滴在較小粗糙度木材表面上達到最大潤濕直徑後會出現 pinning 的行為，而隨著粗糙度的增加，出現回縮的可能性增加，意味著液膜與木材表面之間的接觸減少，使毛細作用力足以克服摩擦力，進而使液滴出現回縮的行為。
第二部分使用高速攝影機 (30 ~ 6770 fps) 以側視 (side view) 紀錄在恆溫、恆濕 (t = 0 ~70 s，RH = 100%；t = 70 ~ 100 s，65%) 與零撞擊高度的情況下，以純水液滴撞擊木材平板 (非洲黑檀，Rq = 0.41 ~ 3.95 m) 之潤濕行為。實驗數據顯示，液滴撞擊木材平板後，在其擴張過程中似乎有兩個不同潤濕狀態的前進接觸角 (1) Cassie-Baxter (液體沒有潤濕木材表面凹陷的部分) 狀態與 (2) Wenzel (液體完全潤濕木材表面凹陷的部分) 狀態。液滴撞擊木材平板後，在其擴張過程中 (t = 20 ms) ，會出現趨近穩定的狀態 (接觸角趨於一定値而潤濕直徑緩慢增加)，但與較粗糙的表面相比，液滴在非洲黑檀 (Rq = 0..41 m) 木材平板上保持在此狀態的時間較長，之後液滴繼續擴張十幾秒後達到更穩定的狀態 (接觸角與潤濕直徑幾乎不變)。從上述接觸角與潤濕直徑的變化表明潤濕狀態可能由 Cassie-Baxter 狀態轉變為 Wenzel 狀態。透過計算液滴體積發現，在 t = 0 ~ 70 s 時由於木材平板孔隙度造成之吸水性於此研究之粗糙度範圍內，不受粗糙度影響。

In this study, the impingement of water drops (D0 = 2.37 mm) on Diospyros crassiflora (Gabon ebony) and Handroanthus spp. (Ipe) wood substrates, with root-mean-square roughness (Rq) ranging from 0.50 to 5.7 m, were investigated using a high-speed camera at impact heights ranging from 20 to 40 cm. In addition, the dynamic wettability of water drops on Diospyros crassiflora wood substrates with Rq = 0.41 – 3.95 m was examined to identify how surface roughness controlled its wettability as well. The results indicated that surface roughness controlled (i) the mobility of the liquid film, (ii) the integrity of the impinging drop, and (iii) the energy balance during the drop impact process. More specifically, the decreased mobility of the liquid film (at increased roughness) was evident from the more irregular lamella, prominent finger projections, and a decreased spreading. An increased roughness was characterized by the violent splashing of impinging water drops, localized rupturing of the liquid film, and highly fragmented splats. A water drop impinging on rougher wood surfaces generally exhibited a greater probability of exhibiting receding which implied, a reduced contact between the liquid film and the wood surface; subsequently resulting in a stronger influence of capillary force that drives the receding.
The results of the wettability studies showed that the wood surface roughness had a somewhat unique influence on its dynamic wettability. More specifically, an increase in roughness above 0.41 m corresponded to a suppressed spreading of a water drop during the very early stages of the sessile drop formation process. Regardless of the surface roughness, the water sessile drop was observed to remain in a somewhat metastable state (characterized by damped harmonic oscillations of the CA) once it was formed (at t = ~20 ms); after which, the water drop continued to spread for a couple of tens of seconds before reaching a more stable state. Note that, the aforementioned metastable state existed for a longer duration of time on the wood substrate with Rq = 0.41m when compared to the rougher substrates. Contrary to the findings in the literature, the changes in the CA and wetting diameter of a water drop could not be attributed to the porosity of the wood as the imbibition effect was observed to be negligible on the tested wood substrates. Subsequently, the observed changes in the CA and wetting diameter points towards a possible transition in the wetting regime from Cassie-Baxter to the Wenzel state.

1. M.Rein, “Phenomena of liquid drop impact on solid and liquid surfaces,” Fluid Dyn. Res.,12,61-93 (1993).
2. D.B. van Dam, C.L. Clerc, “Experimental study of the impact of an ink-jet printed droplet on a solid substrate,” Phys. Fluids, 16, 3403 (2004).
3. D.B. Smith, S.D. Askew, W.H. Morris, D.R. Shaw, M. Boyette, “Droplet size and leaf morphology effects on pesticide spray deposition, “ASAE 2000, 43,255-259 (2001).
4. X. Huang, P. Chen, M. Lan, X. Wang, G. Liao, “Experimental study of water drops with additive impact on wood surfaces,” Procedia Engineering, 62, 852-858 (2013).
5. T.Sasaki, H.Wada and T.Morikawa,” Suppression of micro-bubbles in photoresist coating by step Dynamic Coating” IEEE,329-332 (2001).
6. M. Morcillo, J. Simanacas, J. Bastidas, S. Feliu, C . Blanco and F. Camón “Comparison of laboratory tests and outdoor tests of paint coatings for atmospheric exposure.” In polymeric materials for corrosion control, american chemical society (1986).
7. D. Myers, “Surfaces, Interfaces, and Colloids: Principles and Applications”; Wiley-Vichy: New York (1999).
8. B.J. Palla, D.O. Shah, “Correlation of dispersion stability with surfactant concentration and abrasive particle size for chemical mechanical polishing (cmp) slurries,” J. Dispersion Sci. Technol, 21, 491. (2000).
9. E. Dickinson, “Hydrocolloids at interfaces and the influence on the properties of dispersed system”, Food Hydrocolloids, 17 (2003).
10. T.M. Pan, T.F. Lei, C.C. Chen, “Reliability models of data retention and read-disturb in 2-Bit Nitride storage flash memory cells,” IEEE Electron Device Letters, 21, 338. (2000).
11. G.E. Cossali, A. Coghe, M. Marengo, “The impact of a single drop on a wetted solid surface,” Exp. Fluids, 22, 463 (1997).
12. R. Rioboo, M. Marengo, C. Tropea, “Time evolution of liquid drop impact onto solid, dry surfaces,” Exp. Fluids, 33, 112 (2002).
13. L. Brian. Scheller and D.W. Bousfield, “Newtonian drop impact with a solid surface” AIChE J. 41, 1357 (1995).
14. M. Pasandideh-Fard, Y.M. Qiao, S. Chandra, J. Mostaghimi, “Capillary effects during droplet impact on a solid surface,” Phys. Fluids, 8, 650 (1996).
15. K.P. Gatne, M.A. Jog, R.M. Manglik, “Surfactant-induced modification of low weber number droplet impact dynamics,” Langmuir, 25, 8122 (2009).
16. M. Aytouna, D. Bartolo, G. Wegdam, D. Bonn, S. Rafai, “Impact dynamics of surfactant laden drops: Dynamic surface tension effects,” Exp. Fluids, 48, 49 (2010).
17. J.J. Cooper-White, R.C. Crooks, K. Chockalingam, D.V. Boger, “Dynamics of polymer - surfactant complexes: Elongational properties and drop impact behavior,” Ind. Eng. Chem. Res., 41, 6443 (2002).
18. S.D. Aziz, S. Chandra, “Impact, recoil and splashing of molten metal droplets,” Int. J. Heat Mass Tran., 43, 2841 (2000).
19. D.A. Gorham, “Anomalous behaviour of high velocity oblique liquid impact,” Wear, 41, 2 (1977).
20. D. Bartolo, F. Bouamrirene, É. Verneuil, A. Buguin, P. Silberzan, S. Moulinet, “Bouncing or sticky droplets: Impalement transitions on superhydrophobic micropatterned surfaces,” Europhys. Lett., 74, 299 (2006).
21. B.S. Kang, D.H. Lee, “ On the dynamic behavior of a liquid droplet impacting upon an inclined heated surface,” Exp. Fluids., 29, 380 (2000).
22. K. Range, F. Feuillebois, “Influence of surface roughness on liquid drop impact” J. Colloid Interface Sci., 203, 16-30 (1998).
23. S. Deivandren, K. Katagiri, T. Sato, H. Nishiyama, “Spreading behavior of an impacting drop on a structured rough surface, ” Phys. Fluids, 17, 100680 (2005).
24. R. Rioboo, M. Voué, A. Vaillant, J. De Coninck, “ Drop impact on porous superhydrophobic polymer surfaces,”Langmuir, 24, 14074 (2008).
25. R.E. Pepper, L. Courbin, “Splashing on elastic membranes: The importance of early-time dynamics,” Phys. Fluids, 20, 8 (2008).
26. R. Rioboo, M. Voué, H. Adão, J. Conti, A. Vaillant, D. Eveno, D. Coninck, “Drop impact on soft surfaces: Beyond the static contact angles,” Langmuir, 26, 4873 (2010).
27. R. Rioboo, C. Tropea, “Outcomes from a drop impact on solid surfaces,” Atom. Sprays, 11, 155 (2001).
28. F.T. Dodge, “The spreading of liquid droplets on solid surfaces,” J. Colloid Interface Sci., 121, 154 (1988).
29. H. Park, W.W. Carr, J. Zhu, J.F. Morris, “Single drop impaction on a solid surface,” AIChE J., 49, 2461 (2003).
30. S. Vafaeia, M.Z. Podowskia, “Analysis of the relationship between liquid droplet size and contact angle,” Adv. Colloid Interface Sci., 113, 133 (2005).
31. C. Ukiwe, D.Y. Kwok, “On the maximum spreading diameter of impacting droplets on well-prepared solid surfaces,” Langmuir, 21, 666 (2005).
32. Z. Levin, P.V. Hobbs, “Splashing of water drops on solid and wetted surfaces, Hydrodynamics and charge separation,” Phil Trans Roy Soc London Ser A. Math Phys Sci, 269, 555-585 (1971).
33. L. Xu, W.W. Zhang, S. R. Nagel, “Drop splashing on a dry smooth surface,” Phys. Rev. Lett., 94, 184505 (2005).
34. A.M. Worthington, “A study of splashes,” London: Longmans, Green., 129 (1908).
35. D.G.K. Aboud, A.M. Kietzig, “Splashing threshold of oblique droplet impacts on surfaces of various wettability,” Langmuir, 31, 10100-10111 (2015).
36. P.P. Chen, X.S. Wang, “Experimental study of water drop impact on wood surfaces,” J. Heat and Mass Transf.,54, 4143-4147 (2011).
37. C.D. Stow, M.G. Hadfield, “An experimental investigation of fluid flow resulting from the impact of a water drop with an unyielding dry surface,” Proc. R. Soc. London A, 373, 419-441 (1981).
38. C. Mundo, M. Sommerfeld, C. Tropea, “Droplet-wall collisions: experimental studies of the deformation and breakup process,” Int. J. Multipb. Flow, 21, 151-173 (1995).
39. A. Gupta, R. Kumar, ” Droplet impingement and breakup on a dry surface,” Comput. Fluids, 39, 1696-1703 (2010).
40. R.F. Allen, “The role of surface tension in splashing,” J. Colloid Interface Sci., 51, 350-351 (1975).
41. A. Georgescu, “Hydrodynamic stability theory”; Martinus Nijhoff Publishers:ROMANIA (1985).
42. C. Motzkus, E. Ge´hin, F. Gensdarmes, “Study of airborne particles produced by normal impact of millimetric droplets onto a liquid film,” Exp. Fluids, 45, 797-812 (2008) .
43. K.F. Löehr[, ‘‘Etalement et e´clatement de gouttes.’’ Ph.D. Thesis, Universite´Pierre et Marie Curie, Paris, France, (1990).
44. S.Y. Lin, K. Mckeigue, C. Maldarelli, “Diffusion-controlled surfactant adsorption studied by pendant drop digitization,” AIChE J., 36, 1785-1795 (1990).
45. A. Casandra, M.C. Chung, B.A. Noskov, S.Y. Lin*, “Adsorption kinetics of sodium dodecyl sulfate on perturbed air-water interfaces,” Colloid. Surf. A., 518, 241-248 (2017).
46. H. Gottwald, “ Ebenholzer and Persimmon. Die wirtschaftlich wichtigen Arten der Gattung Diospyros,” Holz-Zentralblatt, 110, 1025-1027 (1984).
47. L.Reinprecht, “Handbook of Wood Chemistry and Wood Composites,” CRC Press (2012)
48. C.Y. Chiang, T.Y. Yang, A. Casandra, S.Y. Lin, “A study of the splash phenomenon of water drops on wood – Emitted droplet velocity and kineric energy,” Exp. Therm. Fluid Sci., 88, 444-449 (2017).
49. P. Baas, “New Perspectives in Wood Anatomy,” Springer Netherlands, (1982).
50. N.Z. Mehdizadeh, S. Chandra, J. Mostaghimi, “Formation of fingers around the edges of a drop hitting a metal plate with high velocity,” J. Fluid Mech., 510, 353-373 (2004).
51. J.C. Beale, R.D. Reitz, “Modeling spray atomization with the Kelvin-Helmholtz/Rayleigh-Taylor hybrid model,” At. Sprays., 9, 623-650 (1999).
52. Y. Liu, P. Tan, L. Xu, “Kelvin-Helmholtz instability in an ultrathin air film cause drop splashing on smooth surface,” Proc. Natl. Acad. Sci. U. S. A., 112, 3280-3284 (2015).
53. L. Afferrante, G. Carbone, “Effect of drop volume and surface statics on the superhydrophobicity of randomly rough substrates,” J. Phys. Condens. Matter., 30, 045001 (2018).
54. J.B. Lee, N. Laan, K.G. DeBruin, G. Skantzaris, N. Shahidzadeh, D. Derome, J. Carmeliet, D. Bonn, “Universal rescaling of drop impact on smooth and rough surfaces,” J. Fluid Mech., 786, R41-R411 (2015).
55. D. Bartolo, F. Bouamrirene, É. Verneuil, A. Buguin, P. Silberzan, S. Moulinet, “Bouncing or sticky droplets: Impalement transitions on superhydrophobic micropatterned surface,” Europhys. Lett., 74, 299-305 (2006).
56. C. Amtonini, A. Amirfazli, M. Marengo, “Drop impact acd wettability: Drom hydrophilic to superhydrophobic surfaces,” Phys. Fluids, 24, 102104 (2012).
57. R. Rioboo, M. Marengo, C. Tropea, “Time evolution of liquid drop impact onto solid, dry surfaces,” Exp. Fluids, 33, 112-124 (2002).
58. C. Mundo, M. Sommerfeld, C. Tropea, “Droplet-wall collisions: Experimental studies of the deformation and breakup process,” Int. J. Multiph. Flow, 21, 151-173 (1995).
59. S.T. Thoroddsen, J. Sakakibara, “Evolution of the fingering pattern of an impacting drop,” Phys. Fluids, 10, 1359-1374 (1998).
60. J. Shen, X. Wang, “Substrate counts: Quantitative effects of surface roughness on fingering pattern and rim shape of an impacting drop,” Phys. Fluids, 32, 93313 (2020).
61. H. Zhang, X.Y. Wang, L.L. Zheng, X.Y. Jiang, “Studies of splat morphology and rapid solidification during thermal spraying,” Int. J. Heat Mass Transf, 44, 4579-4592 (2001).
62. G. Montavon, S. Sampath, C.C. Berndt, H. Herman, C. Coddet, “Effects of the spray angle on splat morphology during thermal spraying,” Surf. Coatings Technol., 91, 107-115 (1997).
63. N. Sanpo, J. Wang, A.S. M.Ang, C.C. Berndt, “Influence of the different organic chelating agents on the topography, physical properties and phase of SPPS-deposited spinel ferrite splats,” Appl. Surf. Sci., 284, 171-178 (2013).
64. B. Ugulino, R.E. Hernández, “Analysis of sanding parameters on surface properties and coating performance of red oak wood,” Wood Mater. Sci. Eng., 13,64-72 (2018).
65. G. Sinn, M. Gindl, A. Reiterer, S. Stanzl-Tschegg, “Changes in the surface properties of wood due to sanding,” Holzforschung, 58, 246-251 (2004).
66. J.C.F. Walker, “Basic wood chemistry and cell wall ultrastructure,” Springer Netherlands, 23-67 (2006).
67. C.Y. Chiang, A. Casandra, W. Suryaputra, S.Y. Lin, “Drop impingement of water and aqueous SDS solution on polycarbonates,” J. Ind. Eng. Chem., 49,189-197 (2017).
68. M. Plötze, P. Niemz, “Porosity and pore size distribution of different wood types as determined by mercury intrusion porosimetry,” Eur. J. Wood Wood Prod., 69, 649-657 (2011).
69. S.S. Yoon, R.A. Jepsen, S.C. James, J. Liu, G. Aguilar, “Are drop-impact phenomena described by Rayleigh-Taylor or Kelvin-Helmholtz theory?,” Dry. Technol., 27,316-321 (2009).