Design and animation of pooling liquid depends on two main factors; the thickness of the liquid and the surface on which it sits. Thin liquid like water or milk will spread very far on a smooth surface. On the same surface, the same volume of a thicker liquid, like custard or ice cream sits higher and doesn't spread as far.
Pooling liquid may seem like an incidental, almost trivial concern in the advanced realm of liquid animation. But it contains a number of components that help add depth, even weight to a scene, not to mention there's a huge variety of pleasant and interesting designs you can create with spills and splats.
We briefly touched on pooling liquids in the Viscosity lesson (0803). I won't repeat the same lessons here, but if you need a refresher, head back there and watch the reference videos that I filmed with chocolate sauce and milk.
The type of pooling we're looking at happens when a quantity of liquid gathers or spreads on a surface. Whether or not that surface is completely flat doesn't really matter, as it follows the same general principles of cohesion and spread.
Key roles are played by viscosity and quantity, but the liquid resistance of the surface is also a major factor in how much a liquid spreads on a surface. Take for example a drop of water on canvas. If the canvas has had waterproofing treatment, the beads will sit high and round. Conversely, without any water repellent, liquid will seep into the fabric, drawn in and spread further through the fibres.
In the video below, I'm designing various pooling designs for liquids such as thick paint, blood and water on some kind of standard hard surface.
video 43_design
Animating this effect is generally a very simple job, as it's mostly a few well-designed key drawings, with many inbetweens. Inbetweening with digital tools makes this one of the simplest, quickest effects you'll ever need to do. Whether traditional or digital, if you're inbetweening this by hand, you'll find it to be very gradual work that requires patience and accuracy.
In the video demonstration below I'm animating thick liquid that spreads gracefully and gradually. You'd have a hard time motivating me to inbetween this effect by hand, as I can get decent and very fast results with morphing hints, as demonstrated in the video.
video 43_animation
The one main, obvious treatment for pooling liquid is the highlight, but as mentioned previously don't forget to consider reflections and shadows as well. You can create some really stunning effects if you apply your knowledge of liquid treatment to even the simplest of liquid effects.
For example, in the scene below the animated reflection is barely noticeable in the context of the fast paced shot, but it nonetheless adds a subtle level of realism that many would dismiss as unnecessary, or simply overlook. Undeniably though, it plays no small part in lifting the quality of the scene as a whole.
video 43_treatment1
For patterned surfaces like bricks, tiles or anything more complex, you could animate it with morphing and hints as seen in the animation video above, or you may consider using masking to help with the effect.
Pooling liquid is much more about design, than animation. Once you have a pleasant design, it's all about making it move gradually in a natural way. Even It only becomes a complex animation task when there are special conditions, such as splashing or smearing.
