I suspect that mixing two or more disparate fluids is one of the first challenges Houdini newbies, like myself, set themselves. I blogged about an approach recently in Stripy Viscous Fluid Impacts in Houdini - I created a single point cloud to act as the fluid's initial location, with different colors depending on the position of each point.

 The slight problem with this approach is that because the fluid has a single surface, Houdini doesn't render any interior detail showing how different fluids interact. To simulate the effect in the video above, where drops of a blue liquid fall into a body of a red liquid, my new approach creates two separate fluid surfaces and that missing interior detail is visible.

 As well as looking better, this technique allows for totally different materials on each fluid. You could, for example, animate pouring mercury or oil into water.

Creating the liquid geometry source

The first step is to create the point cloud that is used to define the initial location of the fluid - the body of the liquid and the three drops. This is a geometry node with a tube for the liquid body and three spheres for the drops:

There are some important extra steps:
  • The two wrangle nodes set the density: my drips are more dense than the main liquid body, so I explicitly set the density of each in VEX with @density = 4; for the drips and 1 for the body.
  • The two group nodes assigns groups to the different liquids, DRIP and BODY, these will be used later when creating the surfaces.
  • The Point Separation in both Scatter nodes is set to 0.2 which will match the Particle Separation in the FLIP Object.
With the geometry in place, I used the FLIP Fluid from Object shelf tool to create a fluid from the geometry.

Building the glass

The drinking glass was created from two tubes with slightly different radii subtracted from each other with a Cookie node. A third, shorter tube acts as the base:


Containing the fluid

After building the glass, I went ahead and converted it to a rigid body hoping it would contain the fluid. However, no amount of fiddling would make its concave shape work nicely. 

 The solution was to create a rigid body from another tube (with end caps) and check "invert sign" in the static object node inside the DOP Network. 

Creating Multiple Fluid Surfaces

Now for the magic. The FLIP Fluid from Object shelf too creates a network to render the fluid surface. Using a Blast node (added just before the Fluid Compress), I delete all the points from one group (i.e. check Delete Non Selected and set the Group Type to points), then copy the entire network and change the group name in the Blast node to the other group. 

 These two fluid surface networks can be assigned their own materials. In my project, I simply copied the basic liquid and changed the colors so that I have red and blue liquids.

 The Object Merge node in the fluid interior network allows for multiple objects and I simply add the copied render node to that.

Gotchas

All well and good. However, since I slowed time down quite a lot, my fluid (especially the drips) was quite jittery. My first attempt to resolve this was to add smoothing to the Particle Fluid Surface. This went some way to helping, but fluid blobs would magically appear and vanish depending on smoothing.

 My solution is to increase the fluid substeps, up the Velocity Smoothing in FLIP Solver to 1, and check Limit Refinement in the Particle Fluid Nodes.

 @cgcris_com has also suggested turning off Rebuild SDF in the Particle Fluid Surface node. The video above hasn't implemented this suggestion, but I'll try that soon.
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It's been a fairly busy few months at my "proper" job, so my recreational Houdini tinkering has taken a bit of a back seat. However, when I saw my Swarm Chemistry hero, Hiroki Sayama tweeting a link to How a life-like system emerges from a simple particle motion law, I thought I'd dust off Houdini to see if I could implement this model in VEX.

The paper discusses a simple particle system, named Primordial Particle Systems (PPS), that leads to life-like structures through morphogenesis. Each particle in the system is defined by its position and heading and, with each step in the simulation, alters its heading based on the PPS rule and moves forward at a defined speed. The heading is updated based on the number of neighbors to the particle's left and right.
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This blog post discusses a technique for rendering SideFX Houdini FLIP fluids as sparse fields of wormlike particles (hence my slightly over-the-top Sparse Vermiform moniker) with their color based on the fluid system's gas pressure field.

The project begins with an oblate spheroid that is converted to a FLIP Fluid using the FLIP Fluid from Object shelf tool. The fluid object sits within a box that's converted to a static body with its volume inverted to act as a container.
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This post continues from my recent blog entry, Particle Advection by Reaction Diffusion in SideFX Houdini. In this project, I've done away with the VEX in the DOP network, and replaced it with a VDB Analysis node to create a vector field that represents the gradient in the reaction diffusion volume. This allows me to use a POP Advect by Volumes node in the DOP network rather than hand coding by own force wrangle.

After watching this excellent tutorial that discusses advecting particles by magnetic fields to create an animation of the sun, I was inspired to use the same technique to advect particles by fields that are a function of reaction diffusion systems. 

The source for my reaction diffusion vector field is a geometry node.
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I played with animating mitosis in Houdini last year (see Simulating Mitosis in Houdini), but the math wasn't quite right, so I thought I'd revisit my VEX to see if it could be improved. After some tinkering, the video above shows my latest (hopefully improved) results.
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This post describes a simple way to create a system comprising of a regularly surfaced fluid and a faux grain system. The video above contains three clips using the same basic technique: creating a single point source for the FLIP SOP initial data but using groups to render some as a fluid and some as individual tiny spheres - the grains. 

The first clip shows a granular sphere dropping into a fluid tank.
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Fibonacci spheres are created from a point set that follows a spiral path to form a sphere (you can see an example, with code at OpenProcessing).

This video contains five clips using SideFX Houdini's Grain Solver with an attached POP Wrangle that uses VEX to generate custom forces. Here's a quick rundown of the VEX I used for each clip (please forgive the use of a variable named oomph).

Clip One "Twin Peaks"

Here, I compare each grain's current angle to the scene's origin to the current time.

The Rayleigh-Taylor instability is the instability between two fluids of different densities. It can appear as "fingers" of a denser liquid dropping into a less dense liquid or as a mushroom cloud in an explosion.

The phenomenon "comes for free" in SideFX Houdini FLIP Fluids.
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Following on from my recent blog post, Mixing Fluids in Houdini, I wanted to simulate a toroidal eddy effect where the incoming drip takes the form of a torus and the fluid flows around the circumference of its minor radius. 

My first thought was to use a POP Axis Force, but that rotates particles around the circumference of the major radius. So, I took another approach: create lots of curves placed around a circle and use those as the geometry source for a POP Curve Force.
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