A semi-experimental procedure for the estimation of permeability of microfluidic pore network.

Microfluidic porous media systems are used for various applications ranging from chemical molecule detection to enhanced oil recovery studies. Absolute permeability data of the microfluidic porous media are important for those applications. However, it is a significant challenge to measure the permeability due to the difficulty in accurately measuring the ultra-low pressure drop across the pore network. This article presents a semi-experimental procedure to estimate the permeability of a microfluidic pore network. The total pressure drop across the porous media chip (ΔPchip) at a given flow rate of a single-phase liquid was obtained from the difference in the inlet pressures at the microfluidic pump with and without the pore network chip connected. The pressure drops in the inlet (ΔPinlet channel) and outlet (ΔPoutlet channel) channels of the pore network are estimated using the hydraulic resistance equation for Poiseuille flow in a wide rectangular cross section. Then the pressure drop across the pore network of the chip (ΔPpore network) is obtained by subtracting (ΔPinlet channel + ΔPoutlet channel) from ΔPchip. Subsequently the permeability of the pore network is calculated using the Darcy's law. •The proposed method is applicable for both homogenous and heterogeneous pore networks.•This method does not require a differential pressure sensor across the microfluidic chip.•This method eliminates the possibility of gas entrapment that can affect the permeability measurement.

Jumping drops on hydrophobic surfaces, controlling energy transfer by timed electric actuation.

Aqueous sessile drops are launched from a super-hydrophobic surface by electric actuation in an electrowetting configuration with a voltage pulse of variable duration. We show that the jump height, i.e. the amount of energy that is transferred from surface energy to the translational degree of freedom, depends not only on the applied voltage but also in a periodic manner on the duration of the actuation pulse. Specifically, we find that the jump height for a pulse of optimized duration is almost twice as high as the one obtained upon turning off the voltage after equilibration of the drop under electrowetting. Representing the drop by a simple oscillator, we establish a relation between the eigenfrequency of the drop and the optimum actuation time required for most efficient energy conversion. From a general perspective, our experiments illustrate a generic concept how timed actuation in combination with inertia can enhance the flexibility and efficiency of drop manipulation operations.

Trapping of drops by wetting defects.

Controlling the motion of drops on solid surfaces is crucial in many natural phenomena and technological processes including the collection and removal of rain drops, cleaning technology and heat exchangers. Topographic and chemical heterogeneities on solid surfaces give rise to pinning forces that can capture and steer drops in desired directions. Here we determine general physical conditions required for capturing sliding drops on an inclined plane that is equipped with electrically tunable wetting defects. By mapping the drop dynamics on the one-dimensional motion of a point mass, we demonstrate that the trapping process is controlled by two dimensionless parameters, the trapping strength measured in units of the driving force and the ratio between a viscous and an inertial time scale. Complementary experiments involving superhydrophobic surfaces with wetting defects demonstrate the general applicability of the concept. Moreover, we show that electrically tunable defects can be used to guide sliding drops along actively switchable tracks-with potential applications in microfluidics.