You need to calculate the relevant Peclet number, Pe, given by uL/D. u is the Stokes' velocity. L is the channel height and D is the diffusion coefficient. When Pe is

Thank you so much for your answer sir. 

Normally in small channels (microchannels), we neglect the body force (gravity, buoyancy) and the only highlighted one is the surface force. Still, from literature, I can't able to find the exact definition for a microchannel. Some authors defined that 1mm ( hydraulic diameter) is the criteria, some others 200micrometer as the criteria. Some authors explaining using  Confinement number(Co) and Bond number (Bo).

Can it be defined in a single way? I mean a single criterion.

I think that it is arbitrary. Choose a definition that you think is rational, write: "following Someone I define microchannels as this" and no-one can argue with you.

Thank you for the reply sir, the confusion arise when I tried to avoid gravity by reducing channel size. I want the bubbles to be totally free from buoyancy force/gravity force. So that both liquid stream and bubble mixed and moving through the channel ( while flow boiling). I don't know what channel dimension range I need to start with to achieve the complete elimination of gravity.

Hi,

I would suggest you to calculate the Bond number (Bo) of the Bubble. Bond number (Bo) is defined as (difference of density of two phases*g*length scale2 )/surface tensionconsidering phase . Now, your length scale would be characteristics diameter of the bubble, surface tension would be of the bubble phase. Check if the Bond number (Bo) of the bubble is < 10^-3 then you can conclude that the dynamics of your bubbles are gravity-free. for more details, please follow the below-mentioned books;

1. The Motion of Bubbles and Drops in Reduced Gravity By R. Shankar Subramanian, R. Balasubramaniam.

2. Introduction to FLUID MECHANICS AND FLUID MACHINES (3rd Ed.) by Som, Biswas, and Chakraborty. (page-38-39).

However, reducing the channel dimension wouldn't help you much. Rather, it may create some other problems related to confinement effects. Moreover, practically, making mini or microchannel is a very challenging task subject to the manufacturing accuracy (dimensions, roughness etc.) and cost. So, I would suggest you to avoid this. I would be happy if you get some ideas from this discussion.

Thank you!