Air entrainment is one of the bottlenecks for many coating processes where one or several thin layers of liquid are deposited onto a continuous substrate. Air entrainment during the optical fiber coating process may lead to eccentrical or incomplete coatings that severely decline the strength and optical properties of fibers. The objective of the present work is to understand the mechanism of the air entrainment phenomenon within optical fiber coating process to finally improve the performance and productivity of the optical fiber. An experimental system was constructed to investigate the dynamic contact angle, the interfacial displacement depth, and the air entrainment phenomenon during the optical fiber coating process in which the fiber drawing velocity, the fiber entrance temperatu...[ Read more ]

Air entrainment is one of the bottlenecks for many coating processes where one or several thin layers of liquid are deposited onto a continuous substrate. Air entrainment during the optical fiber coating process may lead to eccentrical or incomplete coatings that severely decline the strength and optical properties of fibers. The objective of the present work is to understand the mechanism of the air entrainment phenomenon within optical fiber coating process to finally improve the performance and productivity of the optical fiber. An experimental system was constructed to investigate the dynamic contact angle, the interfacial displacement depth, and the air entrainment phenomenon during the optical fiber coating process in which the fiber drawing velocity, the fiber entrance temperature, and the coating material properties were varied. The 0.15 ±0.0001 mm diameter stainless steel AISI302 wire was used instead of the optical fiber mainly for the easier temperature controlling consideration. Glycerol/water and honey/water solutions with various viscosities and surface tensions were adopted as test liquids. Simultaneously, a numerical model to simulate the free surface flow of the optical fiber coating process as well as airflow upon the dynamic meniscus was built. The body program and free surface calculation method were both carefully validated with the literature or experimental data. The relations of the free surface shape, the dynamic contact angle, and the interfacial displacement depth with various parameters such as the fiber drawing velocity, coating material properties, and the coating cup geometry were then studied. The effect of various factors on the pressure field of the coating materials as well as the upper meniscus air, and the relation of pressure with the air entrainment phenomenon were also investigated. The results showed that the dynamic effects of air in the vicinity of the dynamic meniscus should not be disregarded at high velocity coating process such as optical fiber coating process. The pressure near the dynamic meniscus especially at the dynamic contact point in the coating flow was much smaller than the atmosphere pressure, while in the gas phase calculation the value was larger than the atmosphere one at the same position. These results indicated another explanation of the mechanism of air entrainment: the low pressure in the vicinity of the dynamic contact point in the liquid makes the gas easily dissolve into the coating materials, and the gas flow will even break the dynamic meniscus when the pressure difference between the gas and liquid phase is large enough.