Borosilicate nozzles manufactured by reproducible fire shaping

Borosilicate glass nozzles are used by researchers in many fields. They are mainly manufactured by two methods: (i) heating and drawing (or pulling), and (ii) fire shaping (or fire polishing). The latter produces the nozzle by heating one capillary end with a flame, and nozzles are typically much shorter than those obtained by pulling, what may be necessary for some applications. In this work, we built an experimental setup and we evaluated the performance and reproducibility of two different approaches to fire shaping. The first approach heats the tip of a horizontal capillary on the side of the flame. The resulting nozzles are very short and have very good symmetry at the neck. However, there is a small but significant misalignment angle between the nozzle neck and the capillary axes that may be unacceptable for its use in some experimental setups. The second fire shaping approach heats a vertical capillary at the top of the flame. The resulting nozzles show very good symmetry and a very low, and acceptable, misalignment angle between the nozzle neck and the capillary axes. As the phenomena involved in the process are complex, the diameter variability for the same experimental conditions (heating position and time) is high. Results show that using the nozzle shortening to control the neck diameter may be an alternative to produce nozzles of a desired size. However, the shape reproducibility is very good, as nozzles of the same diameter have the same shape, even when they have been produced in different conditions.