Q&A: bub­bles

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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 dis­tor­tions

cause by the curved sur­faces.

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