Dielectrophoresis-electrophoresis transition during the photovoltaic manipulation of water microdroplets on LiNbO3:Fe platform.

The abrupt behaviors of microdroplets during the LN-based photovoltaic manipulation may cause the transient instability and even failure of the microfluidic manipulation. In this paper, we perform a systematical analysis on the responses of water microdroplets to laser illumination on both naked and PTFE-coated LN:Fe surface, and find that the abrupt repulsive behaviors of the microdroplets are due to the electrostatic transition from the dielectrophoresis (DEP) to electrophoresis (EP) mechanism. Charging of the water microdroplets through the Rayleigh jetting from electrified water/oil interface is suggested as the cause of the DEP-EP transition. Fitting the kinetic data of the microdroplets to the models describing the motion of the microdroplets under the photovoltaic field yields the charging amount depending on the substrate configuration (∼1.7 × 10-11 and 3.9 × 10-12 C on the naked and PTFE-coated LN:Fe substrates), and also reveals the dominance of the EP mechanism in the co-existence of the DEP and EP mechanisms. The outcome of this paper will be quite important to the practicalization of the photovoltaic manipulation in LN-based optofluidic chips.

Hydrophobic-substrate based water-microdroplet manipulation through the long-range photovoltaic interaction from a distant LiNbO3:Fe crystal.

Development of photovoltaic water-microdroplet manipulation using LN:Fe crystals has to meet the requirement of the hybrid and heating-avoided design of biological lab-on-chips. To fulfill this, we demonstrate a successful manipulation of a water microdroplet on a hydrophobic substrate by utilizing the long-range photovoltaic interaction from a distant LN:Fe crystal (see Visualization 1). The maximal manipulation distance (MMD) is found to be dependent on the laser-illumination intensity at the LN:Fe crystal and it can be tuned up to a sub-centimeter level (∼4 mm). Basing on the two-center model of light-induced charge transport in the LN:Fe crystal, we establish an analytic model to describe the force balance during the microdroplet manipulation under a long-range photovoltaic interaction. Either shortening the manipulation distance or increasing the illumination intensity can enhance the photovoltaic interaction and increase the velocity of the microdroplet being manipulated. An abrupt shape change followed by a fast repelling movement of the water microdroplet is observed under a strong photovoltaic interaction (see Visualization 2).

Photovoltaic splitting of water microdroplets on a y-cut LiNbO3:Fe crystal coated with oil-infused hydrophobic insulating layers.

We demonstrate the successful photovoltaic splitting of water microdroplets on a $y$y-cut ${{\rm LiNbO}_3}:{\rm Fe}$LiNbO3:Fe substrate coated with an oil-infused hydrophobic layer. The temporal evolution of the microdroplet contact angle upon a central illumination and the distinct behaviors of two sub-droplets during a following boundary illumination reveal that both electrowetting and electroosmotic effects induced by the dipolar photovoltaic potential on the substrate contribute to the water microdroplet splitting. The reciprocal relationship between the splitting time and the illumination intensity verifies the inherent photovoltaic nature of the water microdroplet splitting. The splitting time is found to be linearly dependent on the initial microdroplet size. These points are quite important to the practicalization of lithium niobate (LN)-based microfluidic chips in the biological field.

All-optical splitting of dielectric microdroplets by using a y-cut-LN-based anti-symmetrical sandwich structure.

We demonstrate an all-optical active mode of dielectric microdroplet splitting in a sandwich structure consisting of two anti-symmetrical y-cut LN:Fe substrates. The dynamic process of the microdroplet splitting and the simulation of the electrostatic interaction inside the sandwich gap show that the combination of two anti-symmetrical substrates are capable to provide a sufficient dielectrophoretic force and to reduce the unbalance of the drag forces for a stable and efficient splitting of the microdroplet. The dependences of the splitting time on the illumination intensity and the initial microdroplet size are also studied, and the results show that the microdroplet splitting process is fully governed by the establishment of the superposed photovoltaic field inside the sandwich gap. A key ratio Er/E0, representing the microdroplet splitting difficulty for a given sandwich structure, is found linearly dependent on the initial microdroplet size. These points are quite important to the integration of splitting functionality on the LN-based microfluidic chip.

Visible-light-assisted condensation of ultrasonically atomized water vapor on LiNbO3:Fe crystals.

Optically massive trapping of the moisture in the air into an adjacent surface is a potential technique in the fields of bacterial adhesion and microfluidic generation, which is quite important to the development of LN-based biological lab-on-chips. Here we demonstrate on a LiNbO3:Fe substrate the visible-light-assisted condensation of the water vapor in a flowing stream created by an ultrasonic atomizer. Through analyzing the dynamic processes of the visible-light-assisted water condensation at different illumination intensities, it is found that the extent of the water condensation, the bending angle of water vapor trails and the interaction range of the condensation effect are highly dependent on the illumination intensity. According to these findings and the simulated trajectories of the water vapor stream at different illumination intensities, we propose that this visible-light-assisted water condensation is an aggregation process of tiny water droplets driven by the dielectrophoretic interaction of inhomogeneous photovoltaic field and also an electrostatic screening course of photovoltaic charges through the charged evaporation of condensed water. The prolonged condensation of water vapor after a high-intensity illumination and that of oil vapor at a super-low evaporation rate are also studied, and the agreement between the simulation and experimental results reinforces the above mechanism. The reported technique, employing the inexpensive, safe-for-cell visible laser beam, is quite convenient for the controllable generation of various biological microdroplets, and thus it is promising for the microfluidic functionality integration of LN-based biological lab-on-chips.

Photovoltaic Rotation and Transportation of a Fragile Fluorescent Microrod Toward Assembling a Tunable Light-Source System.

Continuous rotation of a fragile, photosensitive microrod in a safe, flexible way remains challenging in spite of its importance to microelectro-mechanical systems. We propose a photovoltaic strategy to continuously rotate a fragile, fluorescent microrod on a LiNbO3/Fe (LN/Fe) substrate using a continuous wave visible (473 nm) laser beam with an ultralow power (few tens of µW) and a simple structure (Gaussian profile). This strategy does not require the laser spot to cover the entire microrod nor does it result in a sharp temperature rise on the microrod. Both experiments and simulation reveal that the strongest photovoltaic field generated beside the laser spot firmly traps one corner of the microrod and the axisymmetric photovoltaic field exerts an electrostatic torque on the microrod driving it to rotate continuously around the laser spot. The dependence of the rotation rate on the laser power indicates contributions from both deep and shallow photovoltaic centers. This rotation mode, combined with the transportation mode, enables the controllable movement of an individual microrod along any complex trajectory with any specific orientation. The tuning of the end-emitting spectrum and the photothermal cutting of the fluorescent microrod are also realized by properly configuring the laser illumination. By taking a microrod as the emitter and a polystyrene microsphere as the focusing lens, we demonstrate the photovoltaic assembly of a microscale light-source system with both spectrum and divergence-angle tunabilities, which are realized by adjusting the photoexcitation position along the microrod and the geometry relationship in the system, respectively.

Efficient Surfactant-Mediated Photovoltaic Manipulation of fL-Scale Aqueous Microdroplets for Diverse Optofluidic Applications on LiNbO3 Platform.

The electrodeless biocompatible manipulation of femtoliter-scale aqueous microdroplets remains challenging. The appropriate isolation of electrostatic charges from femtoliter-scale aqueous microdroplets is crucial for electrodeless optoelectronic manipulation based on space-charge-density modulation. Here, surfactant-mediated photovoltaic manipulation is proposed, where the surfactant layers self-assembled at the water-oil and oil-Lithium niobate interfaces are employed to isolate photovoltaic charges. The reduced electrostatic attenuation, remarkable hydrophobicity, and strong electrical breakdown suppression of the surfactant layers enable the stable and swift manipulation of femtoliter-scale aqueous microdroplets using µW-level laser in oil media. By virtue of the surfactant-mediated photovoltaic manipulation, a controllable merging/touching/detaching switch of aqueous microdroplets by adjusting the laser illumination intensity and position is realized and the cascading biochemical operations and microreactions of aqueous microdroplets and microdroplet strings are demonstrated. To demonstrate its potential in photonic Micro-Electro-Mechanical-System assemblies, the end coupling of a focused-laser-beam into a ZnO microrod leveraging the refraction effect occurring at the water/oil interface is demonstrated. Moreover, because of the selective permeability of the droplet-interface-bilayer developed between the touching microdroplets, in situ adjustment of the size of the microdroplets and the fluorescent solute contained in the microdroplets are achieved, aiming at constructing multicomponent fluorescent microdroplets with tunable whispering-gallery-mode characteristics.

3D Photovoltaic Router of Water Microdroplets Aiming at Free-Space Microfluidic Transportation.

So far, microfluidic navigation based on space-charge modulation is limited in a two-dimensional (2D) substrate plane. In this paper, a three-dimensional (3D) photovoltaic water-microdroplet router based on a superhydrophobic LiNbO3:Fe crystal is reported. This router employs the repulsive electrostatic force induced by the positive photovoltaic charges generated under focused laser illumination and permits traveling microdroplets to be routed in both in-plane and out-of-plane ways. By analyzing the dynamic process of microdroplet routing, it is found that the microdroplets can gain positive charges through traveling on a superhydrophobic surface and that the positive photovoltaic charges exert an electrophoretic (EP) force on the microdroplets being charged and make them either routed inside the 2D substrate plane or jump out of the 2D plane through electrostatic ballistic ejection. The laser-illumination and microdroplet-size dependence of the deflecting parameters of the in-plane microdroplet routing as well as the jumping trajectory of the out-of-plane routing are investigated. An electrostatic kinetic model is established for both routing ways, and the simulation based on this model predicts well the experimental dependence. A few examples of cascaded free-space microfluidic transportation using the 3D photovoltaic router are demonstrated, showing the potential of this technique in future biological applications.