We investigated the electrostatic behavior of ferroelectric liquid droplets exposed to the pyroelectric field of a lithium niobate ferroelectric crystal substrate. The ferroelectric liquid is a nematic liquid crystal, in which almost complete polar ordering of the molecular dipoles generates an internal macroscopic polarization locally collinear to the mean molecular long axis. Upon entering the ferroelectric phase by reducing the temperature from the nematic phase, the liquid crystal droplets become electromechanically unstable and disintegrate by the explosive emission of fluid jets. These jets are mostly interfacial, spreading out on the substrate surface, and exhibit fractal branching out into smaller streams to eventually disrupt, forming secondary droplets. We understand this behavior as a manifestation of the Rayleigh instability of electrically charged fluid droplets, expected when the electrostatic repulsion exceeds the surface tension of the fluid. In this case, the charges are due to the bulk polarization of the ferroelectric fluid, which couples to the pyroelectric polarization of the underlying lithium niobate substrate through its fringing field and solid–fluid interface coupling. Since the ejection of fluid does not neutralize the droplet surfaces, they can undergo multiple explosive events as the temperature decreases.

The generation of adhesive regions on a z-cut lithium niobate crystal without an additional voltage supply is demonstrated. We show that the origin of the attractive force in the respective solvent is electrophoresis, which can selectively trap charged particles in illuminated regions. Using digital holographic microscopy to measure the space-charge field in a y-cut crystal, we demonstrate the difference between electrophoretic and dielectrophoretic particle manipulation. The suggested method enables the creation of arbitrary two-dimensional patterns, circumventing restrictions originating from the crystal asymmetry. Furthermore, it allows the discrimination between charged particles of different signs, thus acting as a charge sensor

We have studied the motion of drops on inclined liquid-impregnated surfaces (LISs) subject to vertical vibrations. The liquid drops comprise distilled water and different aqueous solutions of glycerol of increasing viscosity. The use of weak pinning LISs strongly affects the dynamical phase diagram. First of all, there is no trace of the dominant static region at low oscillating amplitudes reported for oscillating solid surfaces characterized by contact angle hysteresis. On the contrary, at sufficiently low oscillating amplitudes, the drops always move downwards with a velocity that depends only on the drop viscosity. Further increasing the oscillating amplitude may drive the drop upwards against gravity, as reported for dry surfaces. The use of more viscous drops widens this climbing region. Arguably, the main novelty of this work concerns the observation of two distinct descending regimes where the downhill speed differs by a factor of five or more. Fast-rate videos show that the evolution of the drop profile is diverse in the two regimes, likely because the vertical oscillations reduce the effect of the oil meniscus surrounding the drop at high accelerations.

This contribution presents an optofluidic droplet router which is able to route and steer microdroplets using optically induced forces created solely by the bulk photovoltaic effect on a nonlinear substrate. The combination of microfluidic tools with the properties of a photorefractive crystal allows for the generation of dielectrophoretic forces that can be either repulsive, leading to virtual barriers, or attractive, creating virtual rails. The sign of these forces is solely determined by the electrical properties of the liquid medium under investigation. Moreover, the induced structures on the bottom of the microfluidic channel are optically reconfigurable, so that the same device can easily be adopted for different purposes. Appropriate droplet‐generating devices are fabricated by UV illumination of SU‐8 and polydimethylsiloxane replica molding of the master structures. The bottom of the channels is formed by an iron‐doped lithium niobate crystal, whose internal electric fields are induced by structured illumination patterns and exert dielectrophoretic forces on droplets in the microfluidic section.

Iron doped LiNbO 3 crystals with different iron valence states are investigated. An extended x-ray absorption fine structure (EXAFS) spectroscopy study highlights evident changes in the local structure around iron that can be ascribed to the presence of small polarons. In particular, when a Fe 3+ replaced a Li ion, the oxygen octahedron shrinked with respect to the pure material, with an average iron-oxygen bond value very similar to that of Fe 2 O 3 hematite. When adding an electron, it localizes at the Fe site in a configuration very close to the atomic Fe d orbitals, inducing a relaxation of the oxygen cage. The same system was modelled by spin-polarized density functional theory (DFT). Several local as well as hybrid exchange-correlation functionals were probed on the bulk LiNbO 3 structural properties. The computation is then extended to the case of hematite and finally to the Fe defect in LiNbO 3. The calculations reproduced with good accuracy the large lattice relaxation of the oxygen ligands associated to the electronic capture at the Fe center that can be interpreted as due to the polaron formation. The calculations reproduce satisfactorily the available EXAFS data, and allow for the estimation of the polaron energies and the optical properties of the defect.

The actuation and control of liquid droplets on a surface have important implications in many industrial applications and microfluidics. In recent years, various strategies have been used. Here, an optofluidic platform that performs the basic droplet handling operations required in a common microfluidic device is presented. It is based on z‐cut, iron‐doped lithium niobate crystals that, when illuminated, generate surface charges of opposite sign at the two crystal faces because of the photovoltaic effect. The face of the crystal in contact with the droplets is coated with a lubricant‐infused layer, which guarantees hydrophobicity and, more importantly, a very slippery and robust surface for prolonged use. In this way, sessile water droplets having volumes of microliters, corresponding to millimeters in size, can be easily actuated, guided, merged, and split by projection on the crystal of suitable static or dynamic light patterns. The actuated droplets can cover distances of centimeters on a timescale of a few seconds. The resulting platform is highly flexible and reconfigurable and does not require moving parts.

In the last decades, optical tweezers have progressively emerged as a unique tool to investigate the biophysical world, allowing to manipulate and control forces and movements of one molecule at a time with unprecedented resolution. In this review, we present the use of optical tweezers to perform single-molecule force spectroscopy investigations from an experimental perspective. After a comparison with other single-molecule force spectroscopy techniques, we illustrate at an introductory level the physical principles underlying optical trapping and the main experimental configurations employed nowadays in single-molecule experiments. We conclude with a brief summary of some remarkable results achieved with this approach in different biological systems, with the aim to highlight the great variety of experimental possibilities offered by optical tweezers to scientists interested in this research field.

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