When particles are suspended at air-water interfaces in the presence of a vertical magnetic field, dipoledipole repulsion competes with capillary attraction such that 2d ordered structures self-assemble. By adding a horizontal and oscillating magnetic field, periodic deformations of the assembly are induced. We show herein that pulsating particle arrangements start to swim at low Reynolds number. We identify the physical mechanisms and geometrical ingredients behind this cooperative locomotion. These physical mechanisms can be exploited to much smaller scales, offering the possibility to create artificial and versatile microscopic swimmers.

Droplets on fibers have been extensively studied in the recent years. Although the equilibrium shapes of simple droplets on fibers are well established, the situation becomes more complex for compound fluidic systems. Through experimental and numerical investigations, we show herein that compound droplets can be formed on fibers and that they adopt specific geometries. We focus on the various contact lines formed at the meeting of the different phases and we study their equilibrium state. It appears that, depending on the surface tensions, the triple contact lines can remain separate or merge together and form quadruple lines. The nature of the contact lines influences the behavior of the compound droplets on fibers. Indeed, both experimental and numerical results show that, during the detachment process, depending on whether the contact lines are triple or quadruple, the characteristic length is the inner droplet radius or the fiber radius.

Lately, curious structures have been erected in arid regions: they are large nets able to catch water from fog. Tiny droplets condense on the mesh and are collected on the bottom of it. This innovative technology is crucial to obtain drinkable water in these inhospitable areas. Many studies aim to understand the behavior of droplets trapped on this entanglement of fibers. However, the motion of a droplet sliding on a network of inclined fibers and encountering several crossings when going down remains an open question. Here, we look at the path chosen by such a drop and, especially, we analyze its behavior at the different nodes of the array. We show that droplets may change from one fiber to another one depending on the slope and the diameter of these fibers. We prove that we can force a droplet to follow a specific path simply by carefully designing the fiber mesh. These findings are expected to provide a very convenient way to manipulate small droplets in applications from microfluidics to fog harvesting.

Droplets spontaneously move when they are placed at the tip of a cone surface. Using three dimensionally printed structures, we experimentally explore a large panel of configurations regarding the aperture angle of the cone. We evidence a change of the droplet geometry while moving along the conical fiber. This transition is a switch of configuration from barrel to clamshell shape. The consequence is a change in the droplet dynamics. We estimate the position of this geometrical transition and we propose two models to describe the motion of the barrel and the clamshell droplets. While both shapes are driven by capillary forces, the dissipation is dependent on the geometrical configuration. For barrel shape droplets the main dissipation appears to be in the liquid wedge. For clamshell shape droplets the dissipation occurs mainly in the liquid film close to the conical fiber.

Recent works demonstrated that fiber arrays may constitue the basis of an open digital microfluidics. Various processes, such as droplet motion, fragmentation, trapping, release, mixing and encapsulation, may be achieved on fiber arrays. However, handling a large number of tiny droplets resulting from the mixing of several liquid components is still a challenge for developing microreactors, smart sensors or microemulsifying drugs. Here, we show that the manipulation of tiny droplets onto fiber networks allows for creating compound droplets with a high complexity level. Moreover, this cost-effective and flexible method may also be implemented with optical fibers in order to develop fluorescence-based biosensor.