Some meanings of the preposition à require the definite article, others don’t.

• café au lait = coffee containing milk
• visite à l’américaine = American-style visit
• larmes aux yeux = tears at the eyes

vs.

• verre à vin = glass for wine
• bar à vins = bar for wine

Generally speaking, if you’d translate as for (i.e. if à expresses a purpose, a destination), then there is no article. Another case where there is no article is if à expresses a method — note that the distinction between method and ingredients (which do take an article) can occasionally be subtle. Objects made of distinguishable parts generally fall under the method case — the ingredients case is mostly for mixtures.

• bateau à vapeur = boat that operates with steam
• bateau à voile = boat that operates with a sail, boat one of whose parts is a sail
• verre à pied = glass one of whose parts is a stem

Related:

• verre de vin = glass of wine, glass containing wine
• planche de bois = plank entirely made of wood