Hello, The particle Reynolds number makes me confused and I hope someone can help me on this please! Normally (as I read in every books and papers) that when a bubble or drop rises in a fluid, the bubble/drop Reynolds number is calculated by: Re = ρUD/μ where U is particle velocity, D can be particle diameter, and ρ and μ are density and viscosity of continuous fluid my question is why don't use ρ and μ of bubble/drop? why use them of surrounding fluid? what is the physical meaning of this Re? Thanks in advance.