Droplet-Impact Driven Formation of Ultralow Volume Liquid Marbles with Enhanced Mechanical Stability and Sensing Ability.

Liquid marbles (LMs), droplets encapsulated with micro/nanoparticles, have attracted significant attention owing to their potential applications in various fields, ranging from microbioreactors to sensors. The volume of the LMs is a key parameter determining their mechanical stability and gas sensing ability. It is ideal to work with small volumes because of their better mechanical stability and gas sensing power compared to the larger LMs. Though many methods exist for producing LMs in the volume range above 2 μL, no reliable method exists to prepare fully coated submicroliter LMs with tunable volume. The situation becomes even more difficult when one attempts to produce tiny Janus Liquid Marbles (JLMs). This paper presents a simple, single-step, and efficient strategy for obtaining both the pristine LMs and JLMs in the volume range 200 nL to 18 μL. The core idea relies on the impact of a liquid drop on a particle bed at a Weber number of ∼55 to produce two daughter droplets and to convert these droplets into LMs/JLMs. The whole process takes only a few tens of milliseconds (∼50 ms). We have rendered the experimental schemes so that both the JLMs and pristine LMs can be produced in a single step, with control over their volume. The mechanical stability analysis of the prepared marbles indicates that 200 nL is 5 times more stable than 10 μL of LMs. The 0.72 μL LMs prepared with a 0.5 v/v % phenolphthalein indicator solution showed 3 times faster response time to ammonia gas sensing than 10 μL of LMs. The results presented in this work open up a new route for the rapid and reliable production of both multilayered LMs and JLMs with tunable volume in a wide range (200 nL to 18 μL).