# Monitoring the Capillary Jet Breakage by Vibration Using a Fast-Video Camera
###### Abstract  
The production of beads by simple extrusion dropwise of an alginate solution in a calcium bath is a simple method. It may be done at room temperature without any toxic compound. However, simple extrusion drop by drop from a needle may result in large capsules and a low flow rate. The solution must be extruded as a jet to get a smaller size and higher flow rate, which breaks into droplets either by vibration or a cutting tool. The present contribution reports jet breakage observations into droplets under vibration by extruding an alginate solution varying some parameters during the study. The droplet formation was observed using a high-speed camera, and images were analyzed. The size, length before breakage, and droplet velocity were obtained by examining 50 droplets, and experiments were repeated three times. The high-speed camera allowed us to observe more precisely the capillary jet breakage. The study showed the importance of selecting a well-designed vibrating system, presented data while varying nozzle size, frequency, and flow rate to get optimum breakage keeping across all the same alginate solution. Further experiments would be interesting, modify the extruded solution concentration and composition, and find a precise criterion to identify optimum conditions.  
simple extrusion; jet breakage; process optimization; vibrating system; electro-dripping; encapsulation +
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Initially, these methods were developed based on a simple extrusion drop by drop from a needle. It results in large capsules (2-3 mm) and a low flow rate (100 mL/h). Then, to reduce the size, different technologies were studied, such as electro-dripping, which is extrusion by applying an electrostatic potential between the needle and the gelation bath [7]. To get a smaller size and higher flow rate, the solution must be extruded as a jet and the jet itself broken into droplets either by a vibration [8] or a cutting tool [9]. Equipment for the first option has been developed by several companies (Buchi, Switzerland; Nisco, Switzerland; Brace, Germany), readily available commercially.  
The jet breakage by applying a vibration was proposed by [PERSON] in 1833 [10] and interpreted by [PERSON] in 1879 [11]. A liquid jet breaks in droplets with a mean diameter defined by the breakage frequency, mainly jet diameter, and linear speed. If all vibrations are eliminated, and the optimum frequency is applied, they would be monodispersed droplets. In fact, a size dispersion of 5% is generally observed.  
Even though the obtention of alginate beads by simple extrusion is one of the most popular encapsulation techniques used, while often used to get small, monodispersed alginate beads, the nozzle resonance technology is not fully understood and not used optimally. Especially, operators assume that the size could be controlled by simply varying the frequency.  
Many articles develop the [PERSON]'s theory, taking into account the rheology of the solution, its surface tension, and the formation of satellite droplets [12; 13; 14]. However, these models are difficult to apply for daily work. Alginate forms a shear-thinning solution but is not viscoelastic [8]. The solution generally has relatively low viscosity (<200 mPa\(\cdot\)s). This contribution therefore wishes to verify that a simpler model developed for Newtonian solutions could be applied to hydrogel solutions like alginate solution.  
Most data relating to the size and the operating conditions are based on the alginate beads and not directly the droplets. The present contribution reports direct observations of the alginate solution droplet formation with the assistance of fast recording cameras. It evaluates the impact of several parameters, such as frequency, type of membrane, nozzle size, and vibration amplitude on droplet size. Besides, the study compares and verifies those results with mathematical theories to better understand process optimizing and the importance of selecting a well-designed vibrating system.