ParticleSystem.MainModule.startRotationX

Description

The initial rotation of particles around the x-axis when emitted.

Note that you should specify the value in radians. Additional resources: MinMaxCurve.

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(ParticleSystem))]
public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public float hSliderValue = 1.0F;

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

        var main = ps.main;
        main.startRotation3D = true;

        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.startRotationX = hSliderValue;
    }

    void OnGUI()
    {
        hSliderValue = GUI.HorizontalSlider(new Rect(25, 45, 100, 30), hSliderValue, 0.0F, 360.0F * Mathf.Deg2Rad);
    }
}