Controlling the distance of highly confined droplets in a capillary by interfacial tension for merging on-demand.

Droplet microfluidics is a powerful technology that finds many applications in chemistry and biomedicine. Among different configurations, droplets confined in a capillary (or plugs) present a number of advantages: they allow positional identification and simplify the integration of complex multi-steps protocols. However, these protocols rely on the control of droplet speed, which is affected by a complex and still debated interplay of various physico-chemical parameters like droplet length, viscosity ratio between droplets and carrier fluid, flow rate and interfacial tension. We present here a systematic investigation of the droplet speed as a function of their length and interfacial tension, and propose a novel, simple and robust methodology to control the relative distance between consecutive droplets flowing in microfluidic channels through the addition of surfactants either into the dispersed and/or into the continuous phases. As a proof of concept application, we present the possibility to accurately trigger in space and time the merging of two confined droplets flowing in a uniform cross-section circular capillary. This approach is further validated by monitoring a conventional enzymatic reaction used to quantify the concentration of H2O2 in a biological sample, showing its potentialities in both continuous and stopped assay methods.

Dynamics of Ferrofluid Drops on Magnetically Patterned Surfaces.

The motion of liquid drops on solid surfaces is attracting a lot of attention because of its fundamental implications and wide technological applications. In this article, we present a comprehensive experimental study of the interaction between gravity-driven ferrofluid drops on very slippery oil-impregnated surfaces and a patterned magnetic field. The drop speed can be accurately tuned by the magnetic interaction, and more interestingly, drops are found to undergo a stick-slip motion whose contrast and phase can be easily tuned by changing either the strength of the magnetic field or the ferrofluid concentration. This motion is the result of the periodic modulation of the external magnetic field and can be accurately analyzed because the intrinsic pinning due to chemical defects is negligible on oil-impregnated surfaces.

Highly sticky surfaces made by electrospun polymer nanofibers.

We report on a comprehensive study of the unique adhesive properties of mats of polymethylmethacrylate (PMMA) nanofibers produced by electrospinning. Fibers are deposited on glass, with varying of the diameter and the relative orientation of the polymer filaments (random vs. aligned configuration). While no significant variation is observed in the static contact angle (∼130°) of deposited water drops upon changing the average fiber diameter up to the micrometer scale, fibers are found to exhibit unequalled water adhesion. Placed vertically, they can hold up water drops as large as 60 µL, more than twice the values typically obtained with hairy surfaces prepared by different methods. For aligned fibers with anisotropic wetting behavior, the maximum volume measured in the direction perpendicular to the fibers goes up to 90 µL. This work suggests new routes to tailor the wetting behavior on extended areas by nanofiber coatings, with possible applications in adsorbing and catalytic surfaces, microfluidic devices, and filtration technologies.

Observing angular deviations in light-beam reflection via weak measurements.

An optical analog of the quantum weak measurement scheme proved to be very useful for the observation of optical beam shifts. Here we adapt the weak value amplification method to the observation of the angular Goos-Hänchen shift. We observe this effect in the case of external air-dielectric reflection, the more fundamental case in which it occurs. We show that weak measurements allow for a faithful amplification of the effect at any angle of incidence, even at Brewster's angle of incidence.

Influence of graphene coating on the adsorption and tribology of Xe on Au(1 1 1) substrate.

The adsorption and tribological properties of graphene have received increasing attention for the further development of graphene-based coatings in applications. In this work, we performed first principles calculations with the inclusion of the nonlocal van der Waals correction to study the effect of graphene coating on the adsorption geometries, sliding frictions and electronic properties of Xe monolayer on the Au(1 1 1) substrate. The calculated activation energies indicate that Xe becomes movable on pure Au(1 1 1) surface at a temperature of around 30 K, whereas its motion can be activated only at a high temperature of ~50 K on graphene and on graphene-coated Au(1 1 1) substrates, in good agreement with recent experimental measurements by quartz crystal microbalance technique.

Observation of the Imbert-Fedorov effect via weak value amplification.

Weak measurements have recently allowed for the observation of the spin-Hall effect of light in reflection or transmission, which is a spin-dependent light beam shift orthogonal to the plane of incidence. We report here the observation of the Imbert-Fedorov (IF) shift via a weak value amplification scheme. The IF effect does not depend on the spin of the incident photon only, but it has richer polarization dependence. We prove that weak measurements allow for a complete experimental characterization of the polarization properties of this tiny optical effect.

Sliding drops across alternating hydrophobic and hydrophilic stripes.

We perform a joint numerical and experimental study to systematically characterize the motion of 30 µl drops of pure water and of ethanol in water solutions, sliding over a periodic array of alternating hydrophobic and hydrophilic stripes with a large wettability contrast and a typical width of hundreds of microns. The fraction of the hydrophobic areas has been varied from about 20% to 80%. The effects of the heterogeneous patterning can be described by a renormalized value of the critical Bond number, i.e., the critical dimensionless force needed to depin the drop before it starts to move. Close to the critical Bond number we observe a jerky motion characterized by an evident stick-slip dynamics. As a result, dissipation is strongly localized in time, and the mean velocity of the drops can easily decrease by an order of magnitude compared to the sliding on the homogeneous surface. Lattice Boltzmann numerical simulations are crucial for disclosing to what extent the sliding dynamics can be deduced from the computed balance of capillary, viscous, and body forces by varying the Bond number, the surface composition, and the liquid viscosity. Beyond the critical Bond number, we characterize both experimentally and numerically the dissipation inside the droplet by studying the relation between the average velocity and the applied volume forces.

Tuning drop motion by chemical patterning of surfaces.

We report the results of extensive experimental studies of the sliding of water drops on chemically heterogeneous surfaces formed by square and triangular hydrophobic domains printed on glass surfaces and arranged in various symmetric patterns. Overall, the critical Bond number, that is, the critical dimensionless force needed to depin the drop, is found to be strongly affected by the shape and the spatial arrangement of the domains. Soon after the droplet begins to move, stick-slip motion is observed on all surfaces, although it is less pronounced than that on striped surfaces. On the triangular patterns, anisotropic behavior is found with drops sliding down faster when the tips of the glass hydrophilic triangles are pointing in the down-plane direction. Away from the critical Bond number, the dynamic regime depends mainly on the static contact angle and weakly on the actual surface pattern. Lattice Boltzmann numerical simulations are performed to validate the experimental results and test the importance of the viscous ratio between the droplet phase and the outer phase.

Stick-slip sliding of water drops on chemically heterogeneous surfaces.

We present a comprehensive study of water drops sliding down chemically heterogeneous surfaces formed by a periodic pattern of alternating hydrophobic and hydrophilic stripes. Drops are found to undergo a stick-slip motion whose average speed is an order of magnitude smaller than that measured on a homogeneous surface having the same static contact angle. This motion is the result of the periodic deformations of the drop interface when crossing the stripes. Numerical simulations confirm this view and are used to elucidate the principles underlying the experimental observations.

Weak measurement of the Goos-Hänchen shift.

It is well known from quantum mechanics that weak measurements offer a means of amplifying and detecting very small phenomena. We present here the experimental observation of the Goos-Hänchen shift via a weak measurement approach.

Suspension of water droplets on individual pillars.

We report results of extensive experimental and numerical studies on the suspension of water drops deposited on cylindrical pillars having circular and square cross sections and different wettabilities. In the case of circular pillars, the drop contact line is pinned to the whole edge contour until the drop collapses due to the action of gravity. In contrast, on square pillars, the drops are suspended on the four corners and spilling along the vertical walls is observed. We have also studied the ability of the two geometries to sustain drops and found that if we compare pillars with the same characteristic size, the square is more efficient in pinning large volumes, while if we normalize the volumes to pillar areas, the opposite is true.

Nanofriction of neon films on superconducting lead.

With a quartz crystal microbalance technique we have studied the nanofriction of neon monolayers deposited on a lead surface at a temperature around 7 K. Unlike heavier adsorbates, Ne is found to systematically slide at such low temperatures without any evidence of pinning. The crossing of the Pb superconducting-metal transition is not accompanied by any change in dissipation, suggesting that the electronic contribution to friction is negligible for this system.

Anisotropy of water droplets on single rectangular posts.

We report results of extensive experimental and numerical studies of the anisotropy of water drops deposited on single rectangular posts of mesoscopic size sculpted on different materials. Drops of different volume deposited on the top face of the posts assume an elongated shape along the post direction. Systematic investigations show that while the angle measured along the direction parallel to the post does not change, the one measured across them increases monotonically with the drop volume. The difference in these two angles is found to be proportional to the contact line eccentricity even for very elongated drops, regardless of the post size and material. Results obtained with the lattice Boltzmann method are consistent with these observations and indicate useful trends on the evolution of the drop shape with the system main parameters. We argue that drops deposited on single posts having a very sharp profile represent an ideal model system to investigate anisotropic wetting.

Structural depinning of Ne monolayers on Pb at T < 6/5 K.

We have studied the nanofriction of Ne monolayers with a quartz-crystal microbalance technique at temperatures below 6.5 K and in ultrahigh-vacuum conditions. Very homogeneous and smooth lead electrodes have been physically deposited on a quartz blank at 150 K and then annealed at room temperatures. With such a Pb-plated quartz-crystal microbalance, we have observed a pronounced depinning transition separating a low-coverage region, where the film is nearly locked to the oscillating electrode, from a high-coverage region characterized by slippage at the solid-fluid boundary. Such a behavior has been found to be very reproducible. These data are suggestive of a structural depinning of the solid Ne film when it becomes incommensurate with the lead substrate, in agreement with the results of an extensive molecular-dynamics study.

Crossover effects in the wetting of adsorbed films in linear wedges.

We have measured the growth of liquid Ar adsorbed on arrays of linear wedges structured in different ways. In the most regular patterns, a clear crossover from a planarlike to a geometry-dependent growth behavior is observed. This crossover is found to depend on the characteristic wedge size and its position, in the case of a regular pattern, agrees well with theoretical predictions. Near liquid-vapor bulk coexistence, the film mass is observed to diverge as a power law of the chemical potential difference from saturation with an exponent in very good agreement with the value of -2 expected for a linear wedge. This exponent is not affected by the opening angles of the wedges. The form of the next-to-leading order singular term in the asymptotic divergence of the mass has also been investigated. The experimentally determined value of the exponent is consistent with the expected theoretical result of -4/3.

Complete wetting on a linear wedge.

We have measured the growth of liquid films of Ar adsorbed on well defined arrays of microscopic linear wedges sculpted on thin Si wafers and on a stainless steel disk. On these patterns, a clear cross-over from a planarlike to a geometry dependent growth behavior is observed. This crossover is found to depend on the characteristic wedge size. Near liquid-vapor bulk coexistence, the film mass is observed to diverge as a power law of the chemical potential difference from saturation with an exponent in very good agreement with the value of -2 expected for a linear wedge. This exponent is not affected by the opening angles of the wedges. All these findings are in accordance with a recent scaling theory.

Depinning of atomically thin Kr films on gold.

We have employed the quartz-crystal microbalance technique to measure the sliding friction of krypton films physisorbed on gold. By slowly increasing the amplitude of the substrate oscillations, we have observed a sharp transition from a film locked to the substrate to a sliding one. This transition is characterized by hysteresis both in dissipation and inertial mass as the amplitude is decreased. Finally, the dependence of this transition on film coverage has been studied in some detail.