# small bubbles in a dense liquid

Hi all,
I am using Multiphase-LBM code to estimate the surface tension using Laplace law. Following are the parameters i used in simulation :

NX = 80
NY = 80

tau = 1
G = 6 => rho_l = 2.65, rho_v = 0.07

initial radius of the bubble = 5 lu

Now the problem i am having is that the bubble is not stable for this value of radius, and it just evaporates in few time steps, on the other hand if i have drop in vapour it is stable. Physically it means (if i am correct) that in case of bubble in liquid the outside pressure is higher then inside pressure ( p = p(rho) ) and hence evaporates the bubble, on the other hand drop in vapor is stable for the same reason . Does any one using Multiphase LBM having the same problem ? if yes how can i make a stable bubble in liquid of size 5 or less lu ?

thanks

I think the problem is a finite-size effect of the simulation box. This is a well-known effect, but I am not an expert in the field. You should have a look at the literature. There are theoretical descriptions available. Interestingly, this effect is also expected in a finite physical system, i.e., your observation is not necessarily related to the discretization of LBM.

Hello,

I agree with Timm - it’s related to the discretization or initial effects. It’s really hard to establish proper initial density for the vapor phase. You never get what you expect from analytical formulas for the gas phase, not the liquid phase. I suggest two things:

1. Play with initial conditions - especially with the bubble density.
2. Try to increase the bubble size and to obtain the stable condition. As long as you control physical parameters, you can play with sizes. Once I obtained the stable bubble with R=10.

Cheers,
Alex