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.

Extracellular vesicles (EVs) are double-layered lipid membrane vesicles released by cells. Currently, EVs are attracting a lot of attention in the biological and medical fields due to their role as natural carriers of proteins, lipids, and nucleic acids. Thus, they can transport useful genomic information from their parental cell through body fluids, promoting cell-to-cell communication even between different organs. Due to their functionality as cargo carriers and their protein expression, they can play an important role as possible diagnostic and prognostic biomarkers in various types of diseases, e.g., cancers, neurodegenerative, and autoimmune diseases. Today, given the invaluable importance of EVs, there are some pivotal challenges to overcome in terms of their isolation. Conventional methods have some limitations: they are influenced by the starting sample, might present low throughput and low purity, and sometimes a lack of reproducibility, being operator dependent. During the past few years, several microfluidic approaches have been proposed to address these issues. In this review, we summarize the most important microfluidic-based devices for EV isolation, highlighting their advantages and disadvantages compared to existing technology, as well as the current state of the art from the perspective of the use of these devices in clinical applications.

The actuation of droplets on a surface is extremely relevant for microfluidic applications. In recent years, various methodologies have been used. A promising solution relies on iron-doped lithium niobate crystals that, when illuminated, generate an evanescent electric field in the surrounding space due to the photovoltaic effect. This field can be successfully exploited to control the motion of water droplets. Here, we present an experimental method to determine the attractive force exerted by the evanescent field. It consists of the analysis of the elongation of a pendant droplet and its detachment from the suspending syringe needle, caused by the illumination of an iron-doped lithium niobate crystal. We show that this interaction resembles that obtained by applying a voltage between the needle and a metallic substrate, and a quantitative investigation of these two types of actuation yields similar results. Pendant droplet tensiometry is then demonstrated to offer a simple solution for quickly mapping out the force at different distances from the crystal, generated by the photovoltaic effect and its temporal evolution, providing important quantitative data for the design and characterization of optofluidic devices based on lithium niobate crystals.

The introduction of slippery lubricated surfaces allows for the investigation of the flow of highly viscous fluids, which otherwise will hardly move on standard solid surfaces. Here we present the study of the gravity induced motion of small drops of polymeric fluids deposited on inclined lubricated surfaces. The viscosity of these fluids decreases with increasing shear rate (shear thinning) and, more importantly, they exert normal forces on planes perpendicular to shear directions (elasticity). Despite the homogeneity of the surface and of the fluids, drops of sufficiently elastic fluids move downward with an oscillating instantaneous speed whose frequency is found to be directly proportional to the average speed and inversely to the drop volume. The oscillatory motion is caused by the formation of a bulge at the rear of the drop, which will be dragged along the drop free contour by the rolling motion undergone by the drop. This finding can be considered as a kind of new Weissenberg effect applied to moving drops that combines dynamic wetting and polymer rheology.

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