6531
Despite previous experimental and theoretical studies devoted to this subject , direct observations of the dynamic behavior of gas bubbles and elucidation of how they form surface pore defects have not yet been achieved .
 experimental, theoretical, dynamic, gas, surface, pore, defects
Please list all entity words in the text that fit the category. Output format is "word1, 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} 
Answer:
 experimental, theoretical, dynamic, gas, surface, pore, defects