Eben Freeman, the mixologist at New York City's Tailor, creates a quick foam--a staple of any molecular mixologist--by simply shaking cucumber juice until it froths, then uses it to top his riff on a Pimm's Cup.
Puree the cucumber in a blender. Strain the puree through a fine sieve, pressing to extract as much juice as possible. Transfer the juice to a jar with a tight-fitting lid.
In a cocktail shaker, combine the gin and Pimm's. Add ice and shake for 20 seconds. Strain the drink into a tall glass over ice and top with the soda. Vigorously shake the cucumber juice for 30 seconds. Spoon some of the foam onto the drink and serve right away.