Make Salt Air: Put lime juice, sucrose esters, and salt in a bowl with 2 cups water. Whip with an immersion blender or hand mixer (with whisk attachment) on high speed until mixture bubbles up into a thick, sturdy yet airy froth, about 10 minutes. Not all of the mix will froth—some will stay liquid underneath.
Make margarita: Pour tequila, luxardo, lime juice, and agave into a swing-top or other container. For each drink, pour 1/8 of mix into a cocktail shaker, fill with ice, and shake vigorously to mix. Strain into a margarita glass. With a soup spoon, scoop up a spoonful or two of Salt Air and lay gently across surface of margarita, lifting spoon to form a peak if you like.
Make ahead: Salt Air, up to 1 week, chilled (re-whip Salt Air before using). Batched margarita, 2 days.
* Sucrose esters, a food-science ingredient, is a powerful emulsifier. An innocent-looking white powder, it’s made from sugar and the fatty acids from palm oil. Find at modernistpantry.com ($11.99 for a 50-gram packet).