Keyvan Piroird

Drops impacting inclined fibers

Mats of fibers are often used to capture liquid drops, such as in filters or in fogs nets. It is desired to optimize the efficiency of capture, in particular in the limit of drops larger than the fibers, for which filters remain highly permeable. Here we show that the efficiency of capture is dramatically increased by tilting the fibers: then, the velocity V* below which a drop is fully captured is made much larger; moreover, the tilt maximizes the liquid volume left on the fiber when the impact velocity exceeds V*.

Capillary Flow of Oil in a Single Foam Microchannel

When using appropriate surfactants, oil and aqueous foam can be intimately mixed without the foam being destroyed. In this Letter, we show that a foam, initially free of oil, can draw an oil drop under the action of capillary forces and stretch it through the aqueous network. We focus on the suction of oil by a single horizontal foam channel, known as a Plateau border. In such confined channels, imbibition dynamics are governed by a balance between capillarity and viscosity. Yet, the scaling law for our system differs from that of classical imbibition in porous media such as aqueous foam. This is due to the particular geometry of the liquid channels: Plateau borders filled with foaming solution are always concave whereas they can be convex or flat when filled with oil. Finally, the oil slug, confined in the Plateau border, fragments into droplets following a film breakup.

Detergency in a tube

We study the motion of a drop of oil inside a capillary tube induced by a gradient of surfactant concentration. We first show that the wetting condition selects whether the drop moves towards or away from the surfactant. We then focus on the non-wetting case, for which the drop eventually escapes from the tube, and explore the different dynamics of propulsion. On a long time scale, we observe two regimes: either the drop reaches a constant velocity, or it moves intermittently, by successive jumps and stops. As a paradoxical consequence, viscous drops can be as mobile or even faster than inviscid ones.

Reshaping and capturing Leidenfrost drops with a magnet

Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at room temperature. In this article, we first study the deformation of oxygen drops in a magnetic field and show that it can be described via an effective capillary length, which includes the magnetic force. In a second part, we describe how these ultra-mobile drops passing above a magnet significantly slow down and can even be trapped. The critical velocity below which a drop is captured is determined from the deformation induced by the field.

Capillary imbibition of aqueous foams by miscible and nonmiscible liquids.

When put in contact with a large liquid drop, dry foams wick owing to surface-tension-driven flows until reaching equilibrium. This work is devoted to the dynamics of this imbibition process. We consider imbibition of both wetting or nonwetting liquid, by putting the dry foam into contact either with the foaming solution that constitutes the foam or with organic oils. Indeed, with the appropriate choice of surfactants, oil spontaneously invades the liquid network of the foam without damaging it. Our experiments show an early-time dynamics in t(1/2) followed by a late-time dynamics in t(1/4). These features, which differ from theoretical works predicting a t(1/3) dynamics, are rationalized considering the influence of the initial liquid fraction of the foam in the driving capillary force and the impact of gravity through the capillary-gravity equilibrium.