ParticleSystem.VelocityOverLifetimeModule.speedModifier

Description

Curve to control particle speed based on lifetime, without affecting the direction of the particles.

Additional resources: MinMaxCurve.

using UnityEngine;
using System.Collections;

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

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

        var velocityOverLifetime = ps.velocityOverLifetime;
        velocityOverLifetime.enabled = true;
        velocityOverLifetime.space = ParticleSystemSimulationSpace.World;

        AnimationCurve curve = new AnimationCurve();
        curve.AddKey(0.0f, 0.0f);
        curve.AddKey(1.0f, 1.0f);

        ParticleSystem.MinMaxCurve minMaxCurve = new ParticleSystem.MinMaxCurve(1.0f, curve);

        velocityOverLifetime.speedModifier = minMaxCurve;
    }

    void Update()
    {
        var velocityOverLifetime = ps.velocityOverLifetime;
        velocityOverLifetime.speedModifierMultiplier = hSliderValueSpeed;
    }

    void OnGUI()
    {
        GUI.Label(new Rect(25, 40, 100, 30), "Speed");

        hSliderValueSpeed = GUI.HorizontalSlider(new Rect(55, 45, 100, 30), hSliderValueSpeed, -50.0f, 50.0f);
    }
}