While wet sand is both ubiquitous and literally child’s play, simulating the underlying interaction of water and sand certainly is not. In fact, this type of visual effect is rarely seen in feature movie productions, typically because it is considered too complex and difficult to achieve with existing particle-based simulation techniques. This is surprising given that water simulations, and to a lesser extent also sand simulations, are routinely undertaken in VFX. It is the complex nature of the changing material behaviors resulting from the mixing of sand and water that complicates simulation. However, we argue that this is precisely what makes the animation of wet sand so visually intriguing and hence desirable to master. Most practitioners in the VFX industry would agree that animation of water is a mature area of research, as evident by its abundance and the fact that several excellent commercial solutions exist today, c.f. Houdini, Maya, and RealFlow just to mention a few. Conversely, animation of sand is considered challenging by most, and is still a subject of significant research. Although the Material Point Method (MPM) [Sulsky et al. 1995] has been demonstrated to produce encouraging results for sand simulations, commercial solutions tend to use simpler particle based techniques, like Position Based Dynamics (PBD), which is easier to implement, but less well founded in continuum theory. However, mixtures of water and sand present a completely new set of simulation challenges for which, to the best of our knowledge, there is still no established best practice in VFX, let alone off-the-shelf commercial solutions. While there are many successful examples of wet sand animation, they tend to be based on art-directed constrained simulations or even procedural particle systems. Given the success of simulating sand with MPM it is natural to explore a similar approach for wet sand. Such a unified MPM description of the mixing of water and sand is exactly what motivated our work.

via SIGGRAPH 2017