var game = new Phaser.Game(800, 600, Phaser.CANVAS, 'phaser-example', { preload: preload, create: create, update: update });

var leftEmitter;
var rightEmitter;

function preload() {

    game.load.image('ball1', 'assets/sprites/red_ball.png');
    game.load.image('ball2', 'assets/sprites/blue_ball.png');

}

function create() {

    leftEmitter = game.add.emitter(50, game.world.centerY - 200);
    leftEmitter.bounce.setTo(0.5, 0.5);
    leftEmitter.setXSpeed(100, 200);
    leftEmitter.setYSpeed(-50, 50);
    leftEmitter.makeParticles('ball1', 0, 250, 1, true);

    rightEmitter = game.add.emitter(game.world.width - 50, game.world.centerY - 200);
    rightEmitter.bounce.setTo(0.5, 0.5);
    rightEmitter.setXSpeed(-100, -200);
    rightEmitter.setYSpeed(-50, 50);
    rightEmitter.makeParticles('ball2', 0, 250, 1, true);

    // explode, lifespan, frequency, quantity
    leftEmitter.start(false, 5000, 20);
    rightEmitter.start(false, 5000, 20);

}

function update() {
    game.physics.collide(leftEmitter, rightEmitter);
}