You don’t get “free opponent interactions”, so you can’t do it for free. It would take an action.
A free object interaction would let you interact with just the sand for free — for example, to take it out of your pocket and hide it in your fist. Once you involve an opponent though you need to use an action of some kind.
The type of action would probably be an improvised action. See Improvising an Action and Contests in Combat in the PHB, on pages 193 and page 195 respectively, for information on handling actions that don’t fit into the existing list of action types. As those say, exactly how it works will be up to your DM. (Personally, I call this Dex vs. Dex and it won’t last long, so make it count!)