ParticleSystem.CollisionModule.maxCollisionShapes

Description

The maximum number of collision shapes Unity considers for particle collisions. It ignores excess shapes. Terrains take priority.

Additional resources: ParticleSystem.collision.

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(ParticleSystem))]
public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public bool maxToggle;

    void Start()
    {
        ps = GetComponent<ParticleSystem>();

        var main = ps.main;
        main.startColor = Color.red;

        var shape = ps.shape;
        shape.shapeType = ParticleSystemShapeType.Sphere;

        var collision = ps.collision;
        collision.enabled = true;
        collision.type = ParticleSystemCollisionType.World;
        collision.mode = ParticleSystemCollisionMode.Collision3D;
        collision.bounce = 0.0f;

        var collider1 = GameObject.CreatePrimitive(PrimitiveType.Sphere);
        collider1.transform.parent = ps.transform;
        collider1.transform.localPosition = new Vector3(0.0f, 0.0f, 13.0f);
        collider1.transform.localScale = new Vector3(20.0f, 20.0f, 20.0f);

        var collider2 = GameObject.CreatePrimitive(PrimitiveType.Sphere);
        collider2.transform.parent = ps.transform;
        collider2.transform.localPosition = new Vector3(0.0f, 0.0f, -13.0f);
        collider2.transform.localScale = new Vector3(20.0f, 20.0f, 20.0f);
    }

    void Update()
    {
        var collision = ps.collision;
        collision.maxCollisionShapes = maxToggle ? 2 : 1;
    }

    void OnGUI()
    {
        maxToggle = GUI.Toggle(new Rect(25, 40, 300, 30), maxToggle, "Toggle Max Collision Shapes (" + (maxToggle ? "2" : "1") + ")");
    }
}