Dynamics of a Liquid Droplet on a Granular Bed of Microstructured Particles: From Lens Formation to Marble Effect

We explore the dynamics and morphologies of a droplet permeation on a loosely bound granular bed composed of microporous or micropatterned particles. Naturally abundant particles such as Cycas revoluta (sago), Papaver somniferum (poppy), and Sinapis alba (mustard) have been employed to prepare homogeneous and heterogeneous porous-granular beds. The experiments uncovered that a highly porous bed of mustard particles could stabilize a static water lens by pinning the three-phase contact line of the droplet on the micropatterned particle surface. Interestingly, a water-lens could transform into a “liquid marble” when the bed was composed of smaller poppy particles with a micropatterned surface. Simulations uncovered that an upward convective current due to the deformation and recoil of the droplet together with the vertical component of the capillary force near the pinned contact line helped the poppy particles to climb on the water-lens to form the liquid marbles. A bed of microporous sago particles were also found to show a phenomenon similar to the marble effect in which the particles remained partially or fully embedded inside the drop-surface. Simulations uncovered that an air current issuing out of the porous bed due to droplet permeation enforced microporous particles dislodging from the bed. Heterogeneous porous-granular beds composed of poppy-crushed sago particles showed the two different types of marble effects occurring simultaneously from different sides of the same droplet. The study on the kinetics of the drop permeation showed that the rate of permeation was much slower when the drop passed through the bed–air interface than when the droplet was already inside the bed. Addition of surfactant to the water droplet resulted in a faster permeation rate, which also ensured a smaller lifetime for the lens and marble formation. In comparison, an increase in viscosity could kinetically stabilize the lens and delay the formation of the marble. Concisely, the study uncovered a multitude of interesting dynamics of droplets over granular beds composed of particles with microporous and micropatterned surfaces