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.

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.

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.

Significance: Understanding the molecular forces driving the unfolded polypeptide chain to self-assemble into a functional native structure remains an open question. However, identifying the states visited during protein folding (e.g., the transition state between the unfolded and native states) is tricky due to their transient nature. Here, we introduce calorimetric force spectroscopy in a temperature jump optical trap to determine the enthalpy, entropy, and heat capacity of the transition state of protein barnase. We find that the transition state has the properties of a dry molten globule, that is, high free energy and low configurational entropy, being structurally similar to the native state. This experimental single-molecule study characterizes the thermodynamic properties of the transition state in funneled energy landscapes.

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.

Optoelectronic tweezers (OET) are a promising technique for the realization of reconfigurable systems suitable to trap and manipulate microparticles. In particular, dielectrophoretic (DEP) forces produced by OET represent a valid alternative to micro-fabricated metal electrodes, as strong and spatially reconfigurable electrical fields can be induced in a photoconductive layer by means of light-driven phenomena. In this paper we report, and compare with the experimental data, the results obtained by analyzing the spatial configurations of the DEP-forces produced by a 532 nm laser beam, with Gaussian intensity distribution, impinging on a Fe-doped Lithium Niobate substrate. Furthermore, we also present a promising preliminary result for water-droplets trapping, which could open the way to the application of this technique to biological samples manipulation.