Final Render
Alternate Solution to Create Cacao
Using Professor Fowler's suggestion, I tried using an attribute interpolate node to scatter point onto the surface then deforming it with the moving mesh. However, because the point number and point order changes throughout, the scattered points were randomly moved every frame.
To keep the point order the same, I wrapped a sphere around the model using a ray node, then scattered points onto this surface.
This method however, does not let the particle know which direction the liquid is moving in. The particles would droops downward but not flow out together with the chocolate. The particle behaviour is not preferable but this method fixes the popping from the previous version.
Particle Jumping
The same problem happens with the cacao where it would not stick to the deforming collider. It is also very unstable because of this.
2nd Solution to Chocolate Stretching
After consulting with the Professor, I was advised to simulate the outer and inner parts separately. This way, I can freeze the chocolate on the outside while still keeping the ones on the inside moving.
Possible Solution to Chocolate Stretching
Because the chocolate on the outside was not sticking to the collider and kept being stretched, I came up with a possible cheat method to group the particles on the moving piece and freeze it. Then using the same animation of the moving piece of cake to transform them together.
However, for this to work, I need to delete particles that are moving in the original sim. Using group from bounding box and a SOP solver, I was able to create a To Delete group that accumulates points every frame.
To Delete particles in red
The group did not work perfectly as it missed some points towards the end, resulting in random particles appearing in the liquid.
Exploring Sticky Methods
Because the chocolate is slipping down when the piece is animated, I am also exploring methods to increase the stickiness. In this method, the Normal value on the collider is being pre-processed then brought in to be connected to the velocity.
Reference method: Houdini: Fluid Adhesion Tutorial (youtube.com)
Viscosity Customization
Using the method from this tutorial, I customized the viscosity to get higher as the particle ages. I also multiplied it with a noise that is plugged into velocity so areas with higher velocity will have a lower viscosity.
Preliminary Simulation Set-Up
I used the shelf "Emit Particle Fluid" tool as a a starting point for my sim. I decreased particle separation to .004 and increased viscosity to 10 to create the thick chocolate consistency. The cake and its animation are added as colliders.
Primary Modeling
I used a torus and peaked every 12th edge loops, then subdivided the mesh. I splitted the portion of the cake that is going to be animated out and poly filled both pieces of the geometry.
Reference
Blew me away!!