ParticleSystem.MainModule.randomizeRotationDirection

Description

Cause some particles to spin in the opposite direction.

Set between 0 and 1, where higher values will cause a higher proportion of particles to spin in the opposite direction.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public float hSliderValue = 0.0F;

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

        var rot = ps.rotationOverLifetime;
        rot.enabled = true;
        rot.zMultiplier = 90.0f * Mathf.Deg2Rad;

        var psr = GetComponent<ParticleSystemRenderer>();
        psr.material = new Material(Shader.Find("Sprites/Default"));    // this material renders a square billboard, so we can see the rotation
    }

    void Update()
    {
        var main = ps.main;
        main.randomizeRotationDirection = hSliderValue;
    }

    void OnGUI()
    {
        hSliderValue = GUI.HorizontalSlider(new Rect(25, 25, 100, 30), hSliderValue, 0.0F, 1.0F);
    }
}