Understanding Spray Coating Process: Visual Observation of Impingement of Multiple Droplets on a Substrate

doi:10.1007/s12204-018-1914-0

摘要/Abstract

摘要： Spray coating is a facile deposition process with numerous existing and emerging applications. However, spray coating is a stochastic process comprising impingement of many droplets which upon impact on a heated substrate may dry or solidify individually or coalesce first to form a thin liquid film and then dry to yield a thin solid film. There is very limited knowledge on how this process occurs; therefore in this work, high speed imaging is used to visualize the spray coating process. Two model solutions including food-dye with properties like those of water, and poly (3, 4-ethylenedioxythiophene) : poly (styrenesulfonate) (PEDOT:PSS), a polymeric solution, are sprayed onto permeable glossy paper and regular impermeable glass substrates. Substrates are kept at room temperature and 80 ?C elevated temperature. In some cases, a vertical ultrasonic vibration is imposed on the substrate to study its effect on the coating process. It is observed that the spray coating process is highly random and stochastic. A higher substrate temperature results in a better coating process in that a more uniform and defect-free coating forms. Imposed vibration in the case of glossy paper substrates results in better droplet spreading and more uniform coating. The results also show that under the conditions of these experiments, impinged droplets dry individually or as islands of multiple coalesced droplets to form a coating. In other words, at used spray flow rate and spray droplet size, a continuous thin liquid film does not form prior to drying even at room temperature. Further systematic studies and high magnification lenses are required to visualize and understand the details of the process.

关键词: spray coating, visual observation, droplet impact

Abstract: Spray coating is a facile deposition process with numerous existing and emerging applications. However, spray coating is a stochastic process comprising impingement of many droplets which upon impact on a heated substrate may dry or solidify individually or coalesce first to form a thin liquid film and then dry to yield a thin solid film. There is very limited knowledge on how this process occurs; therefore in this work, high speed imaging is used to visualize the spray coating process. Two model solutions including food-dye with properties like those of water, and poly (3, 4-ethylenedioxythiophene) : poly (styrenesulfonate) (PEDOT:PSS), a polymeric solution, are sprayed onto permeable glossy paper and regular impermeable glass substrates. Substrates are kept at room temperature and 80 ?C elevated temperature. In some cases, a vertical ultrasonic vibration is imposed on the substrate to study its effect on the coating process. It is observed that the spray coating process is highly random and stochastic. A higher substrate temperature results in a better coating process in that a more uniform and defect-free coating forms. Imposed vibration in the case of glossy paper substrates results in better droplet spreading and more uniform coating. The results also show that under the conditions of these experiments, impinged droplets dry individually or as islands of multiple coalesced droplets to form a coating. In other words, at used spray flow rate and spray droplet size, a continuous thin liquid film does not form prior to drying even at room temperature. Further systematic studies and high magnification lenses are required to visualize and understand the details of the process.

Key words: spray coating, visual observation, droplet impact

中图分类号:

TB 43

HUANG Jianchi (黄渐持), YUAN Zhihao (袁志豪), GAO Siyi (高思易),LIAO Jianshan (廖健杉), ESLA. Understanding Spray Coating Process: Visual Observation of Impingement of Multiple Droplets on a Substrate[J]. sa, 2018, 23(1): 97-105.

HUANG Jianchi (黄渐持), YUAN Zhihao (袁志豪), GAO Siyi (高思易),LIAO Jianshan (廖健杉), ESLAMIAN Morteza*. Understanding Spray Coating Process: Visual Observation of Impingement of Multiple Droplets on a Substrate[J]. Journal of Shanghai Jiao Tong University (Science), 2018, 23(1): 97-105.

参考文献

[1] ASHGRIZ N. Handbook of atomization and sprays:Theory and applications [M]. Berlin: Springer, 2011. [2] ESLAMIAN M. Spray-on thin film PV solar cells: Advances,potentials and challenges [J]. Coatings, 2014,4: 60-84. [3] ESLAMIAN M. Inorganic and organic solutionprocessedthin film devices [J]. Nano-Micro Letters,2017, 9: 3. [4] LI R, ASHGRIZ N, CHANDRA S. Droplet generationfrom pulsed micro-jets [J]. Experimental Thermal andFluid Science, 2008, 32(8): 1679-1686. [5] YARIN A L. Drop impact dynamics: Splashing,spreading, receding, bouncing [J]. Annual Review ofFluid Mechanics, 2006, 38: 159-192. [6] ROISMAN I V, RIOBOO R, TROPEA C. Normalimpact of a liquid drop on a dry surface: Model forspreading and receding [J]. Proceedings of the RoyalSociety A: Mathematical Physical and Engineering Sciences,2002, 458: 1411-1430. [7] ALIZADEH PAHLAVAN A, CUETO-FELGUEROSOL, MCKINLEY G H, et al. Thin films in partial wetting:Internal selection of contact-line dynamics [J].Physical Review Letters, 2015, 115: 034502. [8] BIRD J C, MANDRE S, STONE H A. Short-time dynamicsof partial wetting [J]. Physical Review Letters,2008, 100: 234501. [9] KOLINSKI J M, MAHADEVAN L, RUBINSTEIN SM. Drops can bounce from perfectly hydrophilic surfaces[J]. Europhysics Letters, 2014, 108: 24001. [10] IKEGAWA M, AZUMA H. Droplet behaviors on substratesin thin-film formation using ink-jet printing [J].JSME International Journal, Series B, 2014, 47(3):490-496. [11] PASANDIDEH-FARD M, QIAO Y M, CHANDRA S,et al. Capillary effects during droplet impact on a solidsurface [J]. Physics of Fluids, 1996, 8(3): 650-658. [12] BERNARDIN J D, STEBBINS S J, MUDAWAR I.Mapping of impact and heat transfer regimes of waterdrops impinging on a polished surface [J]. InternationalJournal of Heat and Mass Transfer, 1997, 40(2): 247-267. [13] RIOBOO R, MARENGO M, TROPEA C. Time evolutionof liquid drop impact onto solid, dry surfaces[J]. Experiments in Fluids, 2002, 33(1): 112-124. [14] ˇSIKALO ˇS, GANIC E N. Phenomena of dropletsurfaceinteractions [J]. Experimental Thermal andFluid Science, 2006, 31(2): 97-110. [15] SOLTANI-KORDSHULI F, ESLAMIAN M. Impactdynamics and deposition of pristine and graphenedopedPEDOT: PSS polymeric droplets on stationaryand vibrating substrates [J]. Experimental Thermaland Fluid Science, 2017, 89: 238-248. [16] ROISMAN I V, PRUNET-FOCH B, TROPEA C, etal. Multiple drop impact onto a dry solid substrate [J].Journal of Colloid and Interface Science, 2002, 256(2):396-410. [17] LI R, ASHGRIZ N, CHANDRA S, et al. Coalescence oftwo droplets impacting a solid surface [J]. Experimentsin Fluids, 2010, 48(6): 1025-1035. [18] DALILI A, CHANDRA S, MOSTAGHIMI J, et al.Formation of liquid sheets by deposition of droplets ona surface [J]. Journal of Colloid and Interface Science,2014, 418: 292-299. [19] WU J, HUANG J J, YAN W W. Lattice Boltzmanninvestigation of droplets impact behaviors onto a solidsubstrate [J]. Colloids and Surfaces A: Physicochemicaland Engineering Aspects, 2015, 484: 318-328. [20] RAMAN K A, JAIMAN R K, LEE T S, et al. LatticeBoltzmann study on the dynamics of successivedroplets on a solid surface [J]. Chemical EngineeringScience, 2016, 145: 181-195. [21] SAROJINI KG K, DHAR P, VARUGHESE A, et al.Coalescence dynamics of PEDOT : PSS droplets impactingat offset on substrates for inkjet printing [J].Langmuir, 2016, 32(23): 5838-5851. [22] ESLAMIAN M, SOLTANI-KORDSHULI F. Developmentof multiple-droplet drop-casting method forthe fabrication of coatings and thin solid films [J].Journal of Coatings Technology and Research, 2017.https://doi.org/10.1007/s11998-017-9975-9 (publishedonline). [23] LABERGUE A, GRADECK M, LEMOINE F. Experimentalinvestigation of spray impingement hydrodynamicon a hot surface at high flow rates using phaseDoppler analysis and infrared thermography [J]. InternationalJournal of Heat and Mass Transfer, 2016,100: 65-78. [24] KALANTARI D, TROPEA C. Spray impact onto flatand rigid walls: Empirical characterization and modelling[J]. International Journal of Multiphase Flow,2007, 33(5): 525-544. [25] YOON S S, DESJARDIN P E, PRESSER C, et al.Numerical modeling and experimental measurementsof water spray impact and transport over a cylinder[J]. International Journal of Multiphase Flow, 2006,32(1): 132-157. [26] ZABIHI F, ESLAMIAN M. Characteristics of thinfilms fabricated by spray coating on rough and permeablepaper substrates [J]. Journal of Coatings Technologyand Research, 2015, 12: 489-503. [27] ZABIHI F, ESLAMIAN M. Substrate vibrationassistedspray coating (SVASC): Significant improvementin nano-structure, uniformity, and conductivityof PEDOT: PSS thin films for organic solar cells [J].Journal of Coatings Technology and Research, 2015,12: 711-719. [28] CHEN Q, ZABIHI F, ESLAMIAN M. Improved functionalityof PEDOT:PSS thin films via graphenedoping, fabricated by ultrasonic substrate vibrationassistedspray coating [J]. Synthetic Metals, 2016, 222:309-317. [29] RAHIMZADEH A, ESLAMIAN M. Stability of thinliquid films subjected to ultrasonic vibration and characteristicsof the resulting thin solid films [J]. ChemicalEngineering Science, 2017, 158: 587-598. [30] ESLAMIAN M. Excitation by acoustic vibration as an effective tool for improving the characteristics of the solution-processed coatings and thin films [J]. Progressin Organic Coatings, 2017, 113: 60-73.