$('.sub').each(function() {
    var $this = $(this);

    var ink, d, x, y;

    setInterval(function() {
        if($this.find(".ink").length === 0){
            $this.prepend("<span class='ink'></span>");
        }

        ink = $this.find(".ink");
        ink.removeClass("animate");

        if(!ink.height() && !ink.width()){
            d = Math.max($this.outerWidth(), $this.outerHeight());
            ink.css({height: d, width: d});
        }

        x = Math.round(Math.random()*ink.width() - ink.width()/2);
        y = Math.round(Math.random()*ink.height() - ink.height()/2);

        ink.css({top: y+'px', left: x+'px'}).addClass("animate");
    }, 3000)
});