Perhaps a peristaltic pump[1], with the rollers spaced appropriately to achieve the minimum incremental amount?<p>1: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Peristaltic_pump" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Peristaltic_pump</a>