The density different is the density of your sample liquid when you measure the surface tension of a liquid or contact angle between a liquid drop and a solid surface (under air). For example, when you conduct the measurement between water and air.
When you measure the interfacial tension between two immiscible liquids, the density difference is the density difference between these two liquids. When you measure the contact angle between a liquid drop and solid surface (under another liquid), the density difference is also the density difference between these two liquids.
1. For the surface tension measurement, you always need to correctly input the value.
2. For the contact angle measurement, you can leave it empty if you use polynomial method. And you to need to correctly input the value if you want to use the Yonge-Laplace method.
3. For measuring the titling angle, you don’t need to input the value.
4. For the energy: you can leave it empty if you use polynomial method.