Q&A: bubbles

I’m having trouble depicting bubbles in a liquid. Any ideas?

Bruno Shearer, Canada

Answer

Mark replies

When painting materials, you should always consider their physical attributes. Bubbles usually appear in water or other liquids when heat is applied and the liquid starts to evaporate. It turns into gas because of the energy of the heat. We can see these bubbles, because the refraction index of the gas bubble is different to that of the surrounding substance’s. Essentially, we’re seeing the reflective qualities of the liquid inside the bubble.

Painted bubbles can go from a stylised sphere, to highly realistic with lots of reflections. However, it’s more important to capture the randomness and other physical behaviours of the bubbles as they form. Bubbles occur around the source of heat, which is why I paint more of them at the base of the tank. They then migrate towards the surface, because they’re lighter than the liquid most of the times. Multiple bubbles usually merge on the way to the surface, so it’s always better to paint a few larger bubbles some distance from the heat source.

To give a bit of extra realism to the image I also play with distortion and the reflective qualities of the outer shell of the glass tube. In case of a tube the reflections are always going to be sharper and more distorted around the edges, before slowly fading away towards the centre, where we can more easily see through it. The tube also slightly distorts objects behind it, because the angle of the light changes when it strikes a different material.

When painting a bubbling liquid in a glass tube you have to focus on the lights, the material’s reflective qualities and the slight distortionscause by the curved surfaces.