ParticleSystem.TextureSheetAnimationModule.speedRange

Description

Specify how particle speeds are mapped to the animation frames.

If a particle is travelling slower than the minimum speed, the first frame will be used. If a particle is travelling faster than the maximum speed, then the final frame will be used. For all other speeds, frames are chosen based on how far between the minimum and maximum value the speed is.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;

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

        var textureSheetAnimation = ps.textureSheetAnimation;
        textureSheetAnimation.enabled = true;
        textureSheetAnimation.speedRange = new Vector2(0.9f, 5.0f);
    }
}