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The optimal droplet size is achieved by using a mechanically vibrating 3D printing head that leads to a frequency-following Rayleigh instability .
 process parameter: droplet size; concept or principle: mechanically vibrating; machine or equipment: 3D printing head; concept or principle: Rayleigh instability
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
 Entities: droplet size, mechanically vibrating, 3D printing head, Rayleigh instability 
Answer:
 process parameter: droplet size; concept or principle: mechanically vibrating; machine or equipment: 3D printing head; concept or principle: Rayleigh instability