public float randomDirectionAmount { get; set; }

This accepts values from 0 to 1, where 0 causes the particles to retain their default direction and 1 causes a completely random direction. Values in between 0 and 1 blend the default shape direction with a random direction. This means that at 0.5, you get 50% of the default shape and 50% of a randomized direction. Additional Resources: ParticleSystem.ShapeModule.shapeType.