Light-driven 3D droplet manipulation on flexible optoelectrowetting devices fabricated by a simple spin-coating method

Open and closed microfluidics for biosensing

Biosensing is vital for many areas like disease diagnosis, infectious disease prevention, and point-of-care monitoring. Microfluidics has been evidenced to be a powerful tool for biosensing via integrating biological detection processes into a palm-size chip. Based on the chip structure, microfluidics has two subdivision types: open microfluidics and closed microfluidics, whose operation methods would be diverse. In this review, we summarize fundamentals, liquid control methods, and applications of open and closed microfluidics separately, point out the bottlenecks, and propose potential directions of microfluidics-based biosensing.

Investigation into the optoelectrowetting droplet transport mechanism

To explore the optoelectronic wetting droplet transport mechanism, a transient numerical model of optoelectrowetting (OEW) under the coupling of flow and electric fields is established. The study investigates the impact of externally applied voltage, dielectric constant of the dielectric layer, and interfacial tension between the two phases on the dynamic behavior of droplets during transport. The proposed model employs an improved Young's equation to calculate the instantaneous voltage and contact angle of the droplet on the dielectric layer. Results indicate that, under the influence of OEW, significant variations in the interface contact angle of droplets occur in bright and dark regions, inducing droplet movement. Moreover, the dynamic behavior of droplet transport is closely associated with various parameters, including externally applied voltage, dielectric layer material, and interfacial tension between the two phases, all of which impact the contact angle and, consequently, the transport process. By summarizing the influence patterns of the three key parameters studied, the optimization of droplet transport performance is achieved. The study employs two-dimensional simulation models to emulate the droplet motion under the influence of the electric field, investigating the OEW droplet transport mechanism. The continuous movement of droplets involves three stages: initial wetting, continuous transport, and reaching a steady position. The findings contribute theoretical support for the efficient design of digital microfluidic devices for OEW droplet movement and the selection of key parameters for droplet manipulation.

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

Directional Manipulation of Drops and Solids on a Magneto-Responsive Slippery Surface

The cloaking of the droplet and solid spheres by a thin ferrofluid layer forms a ferrofluid-wetting ridge, enabling the magnet-assisted directional manipulation of droplets and solid spheres on the magneto-responsive slippery surface. Understanding the interplay of various forces governing motion unravels the manipulation mechanism. The transportation characteristics of droplets and solid spheres on such surfaces enable their controlled manipulation in multiple applications. Here, we prepare magneto-responsive slippery surfaces by using superhydrophobic coatings on glass slides, creating a porous network and impregnating them with ferrofluid. Using a permanent magnet (and its translation) in the proximity of these surfaces, we manipulate the motion of liquid drops and solid spheres. Upon dispensing the droplet on the magneto-responsive slippery surface, the droplet is cloaked by a thin ferrofluid layer and forms a ferrofluid wetting ridge. Incorporating the magnetic field creates a magnetic-nanoparticle-rich zone surrounding the ferrofluid ridge. Thereafter, the motion of the magnet gives rise to the movement of the droplet. We found that the interplay of the magnetic force and viscous drag leads to the magnetic manipulation of droplets in a controlled fashion up to a certain magnet speed. Increasing the magnet speed further results in the uncontrolled motion of the droplet, where the droplet cannot follow the magnet trajectory. Moreover, we delineate multifunctional droplet manipulations, such as trapping, pendant droplet manipulation, coalescence, and microchemical reactions, which have wide engineering applications.

Multifunctional droplet handling on surface-charge-graphic-decorated porous papers

Developing a fluidic platform that combines high-throughput with reconfigurability is essential for a wide range of cutting-edge applications, but achieving both capabilities simultaneously remains a significant challenge. Herein, we propose a novel and unique method for droplet manipulation via drawing surface charge graphics on electrode-free papers in a contactless way. We find that opposite charge graphics can be written and retained on the surface layer of porous insulating paper by a controlled charge depositing method. The retained charge graphics result in high-resolution patterning of electrostatic potential wells (EPWs) on the hydrophobic porous surface, allowing for digital and high-throughput droplet handling. Since the charge graphics can be written/projected dynamically and simultaneously in large areas, allowing for on-demand and real-time reconfiguration of EPWs, we are able to develop a charge-graphic fluidic platform with both high reconfigurability and high throughput. The advantages and application potential of the platform have been demonstrated in chemical detection and dynamically controllable fluidic networks.

Optimizing Optical Dielectrophoretic (ODEP) Performance: Position- and Size-Dependent Droplet Manipulation in an Open-Chamber Oil Medium

An optimization study is presented to enhance optical dielectrophoretic (ODEP) performance for effective manipulation of an oil-immersed droplet in the floating electrode optoelectronic tweezers (FEOET) device. This study focuses on understanding how the droplet's position and size, relative to light illumination, affect the maximum ODEP force. Numerical simulations identified the characteristic length (Lc) of the electric field as a pivotal factor, representing the location of peak field strength. Utilizing 3D finite element simulations, the ODEP force is calculated through the Maxwell stress tensor by integrating the electric field strength over the droplet's surface and then analyzed as a function of the droplet's position and size normalized to Lc. Our findings reveal that the optimal position is xopt= Lc+ r, (with r being the droplet radius), while the optimal droplet size is ropt = 5Lc, maximizing light-induced field perturbation around the droplet. Experimental validations involving the tracking of droplet dynamics corroborated these findings. Especially, a droplet sized at r = 5Lc demonstrated the greatest optical actuation by performing the longest travel distance of 13.5 mm with its highest moving speed of 6.15 mm/s, when it was initially positioned at x0= Lc+ r = 6Lc from the light's center. These results align well with our simulations, confirming the criticality of both the position (xopt) and size (ropt) for maximizing ODEP force. This study not only provides a deeper understanding of the position- and size-dependent parameters for effective droplet manipulation in FEOET systems, but also advances the development of low-cost, disposable, lab-on-a-chip (LOC) devices for multiplexed biological and biochemical analyses.

Flexible droplet transportation and coalescence via controllable thermal fields

Flexible droplet transportation and coalescence are significant for lots of applications such as material synthesis and analytical detection. Herein, we present an effective method for controllable droplet transportation and coalescence via thermal fields. The device used for droplet manipulation is composed of a glass substrate with indium tin oxide-made microheaers and a microchannel with two transport branches and a central chamber, and it's manipulated by sequentially powering the microheaters located at the bottom of microchannel. The fluid will be unevenly heated when the microheater is actuated, leading to the formation of thermal buoyancy convection and the decrease of interfacial tension of fluids. Subsequently, the microdroplets can be transported from the inlets of microchannel to the target position by the buoyancy flow-induced Stokes drag. And the droplet migration velocity can be flexibly adjusted by changing the voltage applied on the microheater. After being transported to the center of central chamber, the coalescence behaviors of microdroplets can be triggered if the microheater located at the bottom of central chamber is continuously actuated. The droplet coalescence is the combined effect of decreased fluid interfacial tension, the shortened droplet distance by buoyancy flow and the increased instability of droplet under the elevated temperature. The droplet coalescence efficiency is also related to the voltage of microheater, by increasing the voltage from 3.5 V to 7 V, the needed time for droplet coalescence dramatically decrease from 220s to 1.4 s. Finally, by the droplet coalescence-triggered calcium hydroxide precipitation reaction, we demonstrate the applicability of the droplet manipulation method on specific sample detection. Therefore, this approach used for droplet transportation and coalescence can be attractive for many droplet-based applications such as analytical detection.

Light-manipulated binary droplet transport on a high-energy surface

Flexible and precise manipulation of droplet transport is of significance for scientific and engineering applications, but real-time and on-demand droplet manipulation remains a challenge. Herein, we report a strategy using light for the outstanding manipulation of binary droplet motion on a high-energy surface and reveal the underlying mechanism. Upon irradiation to a substrate by a focused light beam, the substrate can provide a localized heating source via photothermal conversion, and a binary droplet can be flexibly transported on a high-energy surface with free contact-line pinning, exhibiting light-propelled droplet transport. We theoretically showed that the surface tension gradient across the droplet interface resulting from the localized photothermal effect is responsible for actuating droplet transport. Remarkably, the high reconfigurability and flexibility of light allowed for binary droplet transport with dynamically tunable velocity and direction as well as arbitrary trajectory assisted by 2D channels on a high-energy surface. Complex droplet transportation, controllable droplet coalescence, and anti-gravity motion were realized. The promising applicability of this light-fueled droplet platform was also demonstrated by directional transport of biosample droplets containing DNA molecules and cells, as well as successional microreactions.

An arrayed optofluidic system for three-dimensional (3D) focal control via electrowetting

A new lens capability for three-dimensional (3D) focal control is presented using an optofluidic system consisting of n × n arrayed liquid prisms. Each prism module contains two immiscible liquids in a rectangular cuvette. Using the electrowetting effect, the shape of the fluidic interface can be rapidly adjusted to create its straight profile with the prism's apex angle. Consequently, an incoming ray is steered at the tilted interface due to the refractive index difference between two liquids. To achieve 3D focal control, individual prisms in the arrayed system are simultaneously modulated, allowing incoming light rays to be spatially manipulated and converged on a focal point located at P_focal (f_x, f_y, f_z) in 3D space. Analytical studies were conducted to precisely predict the prism operation required for 3D focal control. Using three liquid prisms positioned on the x-, y-, and 45°-diagonal axes, we experimentally demonstrated 3D focal tunability of the arrayed optofluidic system, achieving focal tuning along lateral, longitudinal, and axial directions as wide as 0 ≤ f_x ≤ 30 mm, 0 ≤ f_y ≤ 30 mm, and 500 mm ≤ f_z ≤ ∞. This focal tunability of the arrayed system allows for 3D control of the lens's focusing power, which could not be attained by solid-type optics without the use of bulky and complex mechanical moving components. This innovative lens capability for 3D focal control has potential applications in eye-movement tracking for smart displays, autofocusing of smartphone cameras, or solar tracking for smart photovoltaic systems.

Combining sensors and actuators with electrowetting-on-dielectric (EWOD): advanced digital microfluidic systems for biomedical applications

The concept of digital microfluidics (DMF) enables highly flexible and precise droplet manipulation at a picoliter scale, making DMF a promising approach to realize integrated, miniaturized "lab-on-a-chip" (LOC) systems for research and clinical purposes. Owing to its simplicity and effectiveness, electrowetting-on-dielectric (EWOD) is one of the most commonly studied and applied effects to implement DMF. However, complex biomedical assays usually require more sophisticated sample handling and detection capabilities than basic EWOD manipulation. Alternatively, combined systems integrating EWOD actuators and other fluidic handling techniques are essential for bringing DMF into practical use. In this paper, we briefly review the main approaches for the integration/combination of EWOD with other microfluidic manipulation methods or additional external fields for specified biomedical applications. The form of integration ranges from independently operating sub-systems to fully coupled hybrid actuators. The corresponding biomedical applications of these works are also summarized to illustrate the significance of these innovative combination attempts.

A review of optoelectrowetting (OEW): from fundamentals to lab-on-a-smartphone (LOS) applications to environmental sensors

Electrowetting-on-dielectric (EWOD) has been extensively explored as an active-type technology for small-scale liquid handling due to its several unique advantages, including no requirement of mechanical components, low power consumption, and rapid response time. However, conventional EWOD devices are often accompanied with complex fabrication processes for patterning and wiring of 2D arrayed electrodes. Furthermore, their sandwich device configuration makes integration with other microfluidic components difficult. More recently, optoelectrowetting (OEW), a light-driven mechanism for effective droplet manipulation, has been proposed as an alternative approach to overcome these issues. By utilizing optical addressing on a photoconductive surface, OEW can dynamically control an electrowetting phenomenon without the need for complex control circuitry on a chip, while providing higher functionality and flexibility. Using commercially available spatial light modulators such as LCD displays and smartphones, millions of optical pixels are readily generated to modulate virtual electrodes for large-scale droplet manipulations in parallel on low-cost OEW devices. The benefits of the OEW mechanism have seen it being variously explored in its potential biological and biochemical applications. This review article presents the fundamentals of OEW, discusses its research progress and limitations, highlights various technological advances and innovations, and finally introduces the emergence of the OEW technology as portable smartphone-integrated environmental sensors.

Flexible on-chip droplet generation, switching and splitting via controllable hydrodynamics

Flexible droplet preparation and manipulation are significant for lots applications such as immunoreaction and monocellular culture. Herein, we present a novel method for effective on-chip droplet generation, splitting and switching via controllable hydrodynamics. The microchannel of the designed chip has 6 inlets and 3 three outlets. The water solution is injected from a specific inlet (inlet d), and the other 5 inlets are used to inject oil fluids. Under the shearing effect of immiscible oils, the water phase breaks into dispersed droplets first, and the generated droplets can be further split into daughter drops or switched into side outlets from the middle outlet. To investigate the hydrodynamic droplet manipulation behaviors, a two-dimensional simulation model based on phase-field method is established. Utilizing the computational model, we systematically analyze the influences of the flow rates of continuous and dispersed fluids and the manipulation modes on droplet generation, splitting and switching. The numerical results indicate that the droplets can be generated with controlled sizes. For instance, at Q_d = 5 μL/min and Q_c1,2 = 5 μL/min, the droplet diameter decreases from 89.2 μm to 49.2 μm as Q_s1,2 gradually rises from 15 μL/min to 40 μL/min. Moreover, the prepared droplets can realize on-demand splitting and switching. When Q_d, Q_c1,2, and Qs_1,2 are fixed at 5 μL/min, 5 μL/min and 25 μL/min, respectively, the generated droplet is split into different proportional daughter drops with the rising of Q_s3 (or Q_s4) at first, and finally it is switched into the side-outlets when Q_s3 (or Q_s4) is higher than 80 μL/min. Therefore, this proposed droplet manipulation approach will be promising for various applications, and the numerical simulations can provide useful guidelines on the design and operation of droplet-based microfluidic systems.

Three-Dimensional Droplet Manipulation with Electrostatic Levitation

An active and precise method for three-dimensional (3D) droplet manipulation is introduced. By modulating the local electrostatic force acting on droplets in carrier oil between needle plate electrodes, the vertical motion of droplets can be controlled, including the droplet levitation at the interface between the carrier oil and the air. Levitated droplets can be translated horizontally with high efficiency by the motion of the needle electrode. With controllable local deformation on the flexible plate electrode, selective manipulation can be realized for multiple droplets. Applying the manipulation method proposed, a platform is built and various droplet handling, such as transport, merging, and mixing, is performed effectively. Complex droplet transport trajectories are achieved by moving the needle electrode. The droplet transport velocity can reach up to 37 mm/s. The introduced method has fundamental advantages of avoiding cross-contamination between droplets, enhancing the flexibility, eliminating the transport track constraint, and lowering costs with straightforward and precise droplet manipulation.

Magnetic Field-Assisted Fission of a Ferrofluid Droplet for Large-Scale Droplet Generation

With the presence of an external magnetic field, a ferrofluid droplet exhibits a rich variety of interesting phenomena notably different from nonmagnetic droplets. Here, a ferrofluid droplet impacting on a liquid-repellent surface is systematically investigated using high-speed imaging. The pre- and post-impact, including the droplet stretching, maximum spreading diameter, and final impact modes, are shown to depend on the impact velocity and the magnitude of the external magnetic field. A scaling relation involving the Weber and magnetic Bond numbers is fitted to predict the maximum spreading diameter based on the magnetic field-induced effective surface tension. The impact outcome is also investigated and classified into three patterns depending on the occurrence of the rim interface instability and the fission phenomenon. Two types of fission (i.e., evenly and unevenly distributed sizes of the daughter droplets) are first identified, and the corresponding mechanism is revealed. Last, according to Rayleigh-Taylor instability, a semiempirical formula is proposed to estimate the number of the daughter droplets in the regime of evenly distributed size, which agrees well with the experimental data. The present study can provide more insight into large-scale droplet generation with monodispersive sizes.

Magnetowetting dynamics of sessile ferrofluid droplets: a review

The fascinating behavior of ferrofluids in a magnetic field has been intriguing researchers for many years. With the advancement in digital microfluidics, ferrofluid droplets have been extensively used in different applications ranging from biomedical to mechanical systems. Notably, the magnetic field can change the wetting dynamics of sessile ferrofluid droplets, leading to a plethora of interesting hydrodynamic phenomena. In the recent past, the spatiotemporal evolution of the droplet shape and contact line dynamics of a ferrofluid droplet in different magnetowetting scenarios has been explored widely. The relevant studies elucidate several critical aspects, such as the role of magnetic nanoparticles, carrier fluid, and the interaction of the magnetic fluid with the solid surface, among many others. Hence a systematic review of the progress made in understanding the fundamental and practical aspects of magnetowetting in the past decade (2010-2020) would be a helpful resource to the scientific community in the near future. Drawn by this motivation, an honest effort has been made in this Review to highlight the significant scientific findings concerning the sessile droplet magnetowetting phenomena within the timeline of interest. Several cutting-edge applications developed from the scientific findings in the purview of magnetowetting have also been discussed before outlining the conclusions and future areas of scope.

Optical Dielectrophoretic (DEP) Manipulation of Oil-Immersed Aqueous Droplets on a Plasmonic-Enhanced Photoconductive Surface

We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications.

Optoelectronic tweezers: a versatile toolbox for nano-/micro-manipulation

This review covers the fundamentals, recent progress and state-of-the-art applications of optoelectronic tweezers technology, and demonstrates that optoelectronic tweezers technology is a versatile and powerful toolbox for nano-/micro-manipulation.

Upper Limit of Light-Levitated Droplet Motion

The light-enabled droplet levitation shows promising potential in applications in biotechnology, clinical medicine, and nanomaterials. In particular, light-levitated droplets have good followability with a moving laser beam, resulting in flexibility in manipulating their motion. However, it is still unclear whether there exists an upper limit to the light-levitated droplet motion with a moving laser beam. Therefore, the motion of light-levitated droplets above the free interface is studied to determine the upper limit of motions of the droplets with a moving laser beam. We demonstrate that an inefficient interface temperature response because of a very high moving speed of the laser beam and the resultant small upward vertical component of vapor flow are responsible for the existence of an upper-limit velocity. Above the upper limit, the light-levitated droplets are unable to stably move with the laser beam and finally disappear. By contrast, the droplets can stably move with the laser beam in a wide range at or below this upper limit. In addition, an almost linear relationship between the upper-limit velocity of the light-levitated droplets and the input laser power is presented. The findings of the present study are informative for the implementation of this light levitation technology.

Universal Plasma Jet for Droplet Manipulation on a PDMS Surface towards Wall-Less Scaffolds

Droplet manipulation is important in the fields of engineering, biology, chemistry, and medicine. Many techniques, such as electrowetting and magnetic actuation, have been developed for droplet manipulation. However, the fabrication of the manipulation platform often takes a long time and requires well-trained skills. Here we proposed a novel method that can directly generate and manipulate droplets on a polymeric surface using a universal plasma jet. One of its greatest advantages is that the jet can tremendously reduce the time for the platform fabrication while it can still perform stable droplet manipulation with controllable droplet size and motion. There are two steps for the proposed method. First, the universal plasma jet is set in plasma mode for modifying the manipulation path for droplets. Second, the jet is switched to air-jet mode for droplet generation and manipulation. The jetted air separates and pushes droplets along the plasma-treated path for droplet generation and manipulation. According to the experimental results, the size of the droplet can be controlled by the treatment time in the first step, i.e., a shorter treatment time of plasma results in a smaller size of the droplet, and vice versa. The largest and the smallest sizes of the generated droplets in the results are about 6 µL and 0.1 µL, respectively. Infrared spectra of absorption on the PDMS surfaces with and without the plasma treatment are investigated by Fourier-transform infrared spectroscopy. Tests of generating and mixing two droplets on a PDMS surface are successfully achieved. The aging effect of plasma treatment for the proposed method is also discussed. The proposed method provides a simple, fast, and low-cost way to generate and manipulate droplets on a polymeric surface. The method is expected to be applied to droplet-based cell culture by manipulating droplets encapsulating living cells and towards wall-less scaffolds on a polymeric surface.

Magnetically Responsive Film Decorated with Microcilia for Robust and Controllable Manipulation of Droplets

Droplet manipulations are critical for applications ranging from biochemical analysis, medical diagnosis to environmental controls. Even though magnetic actuation has exhibited great potential, the capability of high-speed, precise manipulation, and mixing improvement covering a broad droplet volume has not yet been realized. Herein, we demonstrated that the magnetic actuation could be conveniently achieved via decorating the magnetically responsive film with microcilia. Under magnetic field, the film can quickly response with localized deformation, along with the microcilia to realize the surface superhydrophobicity for droplet manipulation with velocity up to ∼173 mm/s covering a broad volume of 2-100 μL. The robust system further allows us to realize rapid and complete droplet mixing within ∼1.6 s. In addition, the microcilia decorated surface can preserve the robust superhydrophobicity after various stability tests, for example, normal pressing, chemical corrosion, and mechanical abrasion, exhibiting the possibility toward the long-term and real applications. With the multifunctional demonstrations such as obvious mixing improvement, parallel manipulation, and serial dilution, we believe that the methodology can open up a magnetic field-based avenue for future applications in digital microfluidics, and biochemical assays, etc.

Designing Splicing Digital Microfluidics Chips Based on Polytetrafluoroethylene Membrane

As a laboratory-on-a-chip application tool, digital microfluidics (DMF) technology is widely used in DNA-based applications, clinical diagnosis, chemical synthesis, and other fields. Additional components (such as heaters, centrifuges, mixers, etc.) are required in practical applications on DMF devices. In this paper, a DMF chip interconnection method based on electrowetting-on-dielectric (EWOD) was proposed. An open modified slippery liquid-infused porous surface (SLIPS) membrane was used as the dielectric-hydrophobic layer material, which consisted of polytetrafluoroethylene (PTFE) membrane and silicone oil. Indium tin oxide (ITO) glass was used to manufacture the DMF chip. In order to test the relationship between the splicing gap and droplet moving, the effect of the different electrodes on/off time on the minimum driving voltage when the droplet crossed a splicing gap was investigated. Then, the effects of splicing gaps of different widths, splicing heights, and electrode misalignments were investigated, respectively. The experimental results showed that a driving voltage of 119 V was required for a droplet to cross a splicing gap width of 300 μm when the droplet volume was 10 μL and the electrode on/off time was 600 ms. At the same time, the droplet could climb a height difference of 150 μm with 145 V, and 141 V was required when the electrode misalignment was 1000 μm. Finally, the minimum voltage was not obviously changed, when the same volume droplet with different aqueous solutions crossed the splicing gap, and the droplet could cross different chip types. These splicing solutions show high potential for simultaneous detection of multiple components in human body fluids.

Nonmonotonic contactless manipulation of binary droplets via sensing of localized vapor sources on pristine substrates

Droplet motion on surfaces influences phenomena as diverse as microfluidic liquid handling, printing technology, and energy harvesting. Typically, droplets are set in motion by inducing energy gradients on a substrate or flow on their free surface. Current configurations for controllable droplet manipulation have limited applicability as they rely on carefully tailored wettability gradients and/or bespoke substrates. Here, we demonstrate the nonmonotonic contactless long-range manipulation of binary droplets on pristine substrates due to the sensing of localized water vapor sources. The droplet-source system presents an unexpected off-centered equilibrium position. We capture the underlying mechanism behind this symmetry breaking with a simplified model based on the full two-dimensional functional form of the surface tension gradient induced by the source on the droplet's free surface. This insight on the transport mechanism enables us to demonstrate its versatility for applications by printing, aligning, and reacting materials controllably in space and time on pristine substrates.

Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

Fabrication of artificial biomimetic materials has attracted abundant attention. As one of the subcategories of biomimetic materials, artificial cells are highly significant for multiple disciplines and their synthesis has been intensively pursued. In order to manufacture robust "alive" artificial cells with high throughput, easy operation, and precise control, flexible microfluidic techniques are widely utilized. Herein, recent advances in microfluidic-based methods for the synthesis of droplets, vesicles, and artificial cells are summarized. First, the advances of droplet fabrication and manipulation on the T-junction, flow-focusing, and coflowing microfluidic devices are discussed. Then, the formation of unicompartmental and multicompartmental vesicles based on microfluidics are summarized. Furthermore, the engineering of droplet-based and vesicle-based artificial cells by microfluidics is also reviewed. Moreover, the artificial cells applied for imitating cell behavior and acting as bioreactors for synthetic biology are highlighted. Finally, the current challenges and future trends in microfluidic-based artificial cells are discussed. This review should be helpful for researchers in the fields of microfluidics, biomaterial fabrication, and synthetic biology.

Light-driven motion of water droplets with directional control on nanostructured surfaces

Discrete droplet transport has drawn much interest in a broad range of applications. Controlling the motion direction in droplet transport, however, is a long-lasting challenge. In this work, a simple yet efficient approach is demonstrated to realize the motion of droplets with directional control on nanostructured surfaces with predefined channels. Light is used as the external stimulus to induce the uneven thermal expansion of the substrate, which leads to the tilting of nanostructured channels so that the droplet is driven to move along the channel. Due to the easy manipulation of light, including both the light position and power density, this study demonstrates the controllable entrance of static water droplets into targeted channels and the simultaneous control of the motion of multiple droplets in multi-channel systems, using just one light source. Besides static droplets, this approach can also be applied for the directional control of moving droplets in multi-channel systems. As a proof-of-concept, such an approach has been utilized for efficient multiplexed reactions for chemical sensing or microreactor applications. This work offers an alternative approach for the manipulation of droplet movement in applications that involve the control of droplet motion.

Advances towards programmable droplet transport on solid surfaces and its applications

We review progress towards the programmable transport of droplets on surfaces together with its applications in chemistry and materials science.

Electromagnetic three dimensional liquid metal manipulation

In this paper, we report three-dimensional (3-D) liquid metal manipulation using electromagnets, which can be applied to electrical switching applications. The liquid metal droplet was coated with iron (Fe) particles by chemical reaction with hydrochloric acid (HCl), and thus it became responsive to the magnetic field, becoming a magnetic liquid metal marble. Using electromagnets, the magnetic field was turned on and off on-demand. We investigated an average velocity and the maximum working distance of the horizontal and vertical electromagnetic field-driven manipulation of the magnetic liquid metal marble. Linear (1-D) and plane (2-D) manipulation of the marble was successfully demonstrated and 3-D manipulation was verified for electrical switching.

Modes and break periods of electrowetting liquid bridge

In this paper, we propose a microscale liquid oscillator using electrowetting-on-dielectric (EWOD). Specifically, a mesoscale liquid bridge (LB) between two horizontal surfaces with EWOD is considered. When EWOD is applied, the solid surface becomes more hydrophilic, and hence the contact angle (CA) is reduced. Following the activation of EWOD, the LB can remain connected or it can break into either symmetric or asymmetric shapes depending on the initial liquid volume and wettability of the two surfaces. The LB dynamics activated by EWOD is studied using the multibody dissipative particle dynamics (MDPD) method. Our numerical results show that the behavior of an LB under EWOD can be interpreted via three modes. In the first mode, the LB does not break after applying EWOD. In the second mode, the LB breaks and does not reform. The third mode happens when, depending on the interplay of the volume of the liquid and CA manipulation, the LB continuously breaks, recoils, and reforms. For asymmetric cases, it was observed that the LB may completely detach from one surface and may not reform. It was also observed that decreasing the wettability of a surface, for cases with a continuous breaking and reformation behavior, increases the connecting time interval and decreases the breaking time interval in one break-reform cycle. The results provided in this investigation facilitate fundamental understanding of LB dynamics and their application for the design of microscale liquid oscillators using EWOD.

An automated 3D-printed smartphone platform integrated with optoelectrowetting (OEW) microfluidic chip for on-site monitoring of viable algae in water

A sudden increase of algae and their associated toxins in aquatic ecosystems can detrimentally affect the quality of the water, causing serious socio-economic and public health problems. To prevent the spread of harmful algae in aquatic ecosystems, it is essential to track the water's quality through rapid and in-situ monitoring systems. Conventional methods of algae quantification such as microscopy, hemocytometry, and UV-vis spectroscopy, however, are often unsuitable or inconvenient for in-situ assessment as they require skilled labor and expensive equipment. In this study, we developed a three-dimensional (3D)-printed smartphone platform integrated with a light-driven microfluidic chip operated by optoelectrowetting (OEW). This OEW-driven microfluidic chip not only allows multiplexed drop-wise functions such as droplet transportation, merging, mixing, immobilization on a detection zone, for on-chip water sample preparation but also fluorescent detection and counting of target algae cells using a commercially-available smartphone. Two freshwater algae (C. reinhardtii and M. aeruginosa) and two marine water algae (Amphiprora sp and C. closterium) were employed to validate the 3D-printed smartphone platform in this study. The fluorescence images of viable algae and the cell counting from the microfluidic chip were comparable to the results from a hemocytometer (P > 0.05). We have further conducted tests with spiked samples using freshwater and marine water that were directly collected from environmental samples, showing the same order of magnitude of cell numbers in the spiked and control cultures of algae cells (10⁶ cell/mL, P > 0.05). Unlike traditional quantification methods, the 3D-printed smartphone platform integrated with the OEW offers a highly portable, user-friendly, low-cost tool that enables simple on-chip sample preparation and detection of viable algae. Thus, this stand-alone technology has the potential for rapid and in-situ monitoring of water quality, while using the smartphone's wireless communication capabilities to report the quality of the water in real-time.

Magnetically Actuated Heterogeneous Microcapsule-Robot for the Construction of 3D Bioartificial Architectures

Core-shell microcapsules as one type of the most attractive carriers and reactors have been widely applied in the fields of drug screening and tissue engineering owing to their excellent biocompatibility and semi-permeability. Yet, the spatial organization of microcapsules with specific shapes into three-dimensional (3D) ordered architectures still remains a big challenge. Here, we present a method to assemble shape-controllable core-shell microcapsules using an untethered magnetic microcapsule-robot. The microcapsule-robot with the shape-matching design can grab the building components tightly during the transportation and assembly processes. The core-shell feature of the microcapsule effectively prevents the magnetic nanoparticles from interacting with bioactive materials. The assembly results of cell-loaded heterogeneous microcapsules reveal that this strategy not only allows the magnetic microcapsule-robot to work in different workspaces in vitro for the creation of 3D constructions but also offers a noninvasive and dynamical manipulation platform by remotely controlling the position and orientation of the soft and liquid-like microcapsule components individually.

Droplet manipulation with polarity-dependent low-voltage electrowetting on an open slippery liquid infused porous surface

This paper reports an open-loop method for highly efficient and precise droplet manipulation with polarity-dependent low-voltage electrowetting on a perfluorinated silane modified slippery liquid infused porous surface (SLIPS) in which a droplet can be driven between individual square electrodes. The electrowetting phenomenon on modified SLIPS was investigated first, and it exhibited an up to 55° contact angle difference with respect to voltage polarity while the threshold voltage was reduced to only 2 V. Then, a coplanar electrode experiment was designed to study the performance of droplet manipulation on several modified SLIPS samples with different vertically placed times and silicon oil viscosities. The optimal condition for preparing a modified SLIPS membrane is that a sample is placed vertically for 2 hours after infusing 10 cSt silicon oil, on which the droplet can be driven with the fastest velocity, and the activation voltage for moving a droplet is only 8 V. Finally, multi-droplet simultaneous and continuous manipulation on modified SLIPS in bi-direction on a loop of square electrodes was achieved. Interestingly, unlike asymmetric electrowetting, actuation methods on a solid insulator and hydrophobic layers, the droplet actuation velocity was not limited by the contact angle saturation effect and always increased with the applied voltage on modified SLIPS. This method achieves a very wide range of droplet continuous manipulation velocities from 0.075 mm s^-1 to 123 mm s^-1 under 20 V to 500 V applied voltage and the continuous droplet actuation voltage exhibits at least a 15-fold decrease compared to that of an unmodified SLIPS membrane.

Transport of a Sessile Aqueous Droplet over Spikes of Oil Based Ferrofluid in the Presence of a Magnetic Field

Droplets can be used as carrier vehicles for the transportation of biological and chemical reagents. Manipulation of water- and oil-based ferromagnetic droplets in the presence of a magnetic field has been well-studied. Here, we elucidate the transport of a sessile aqueous (diamagnetic) droplet placed over spikes of oil-based ferrofluid (FF) in the presence of a nonuniform magnetic field. An oil-based FF droplet, dispensed over a rigid oleophilic surface, interacts with a magnetic field to get transformed into an array of spikes which then act as a carrier for the transportation of the aqueous droplet. Our study reveals that transportation phenomena is governed by the interplay of three different forces: magnetic force F_m, frictional force F_f, and interfacial tension force F_i, which is expressed in terms of the magnetic Laplace number ( La_m) and magnetic Bond number ( Bo_m) as La_m^?1 = ( F_f1/ F_{m, x}) and Bo_m La_m^?1 = ( F_f2/ F_i). Based on the values of the dimensionless numbers, three different regimes, steady droplet transport, spike extraction, and magnet disengagement, are identified. It is found that steady droplet transport is observed for La_m^?1 ? 1 and Bo_m La_m^?1 ? 1, whereas extraction of spikes is observed for La_m^?1 ? 1 and Bo_m La_m^?1 > 1 and magnet disengagement is observed for La_m^?1 > 1. In the steady droplet transport regime, velocity of the aqueous droplet U_ds was found to be dependent on the volumes of the aqueous droplet V_w and FF droplet V_FF following U_ds ? V_w^?0.19 V_FF^0.36. A simple model is presented that accurately predicts the aqueous droplet velocity U_ds within 5% of the corresponding experimental data. In the spike extraction regime, the spike extraction distance L_se was found to vary with V_w, V_FF, and the magnet velocity U_ms following L_se ? V_w^?1.75 V_FF^0.75 U_ms^?1.56.

Gecko toe pads inspired in situ switchable superhydrophobic shape memory adhesive film

Recently, smart adhesive superhydrophobic surfaces have attracted much attention. However, it is still a challenge to obtain a superhydrophobic surface with shape memory adhesive performance. Herein, inspired by the special back-scrolling/unfolding ability of gecko toe pads and corresponding tunable adhesion, we report such a film produced by sticking a layer of superhydrophobic pillar structured polyurethane (s-PU) onto a shape memory polyurethane-cellulose nanofiber (PU-CNF) substrate to mimic the hair-like skin structure and underlying muscle of the gecko toe pads, respectively. Similar to the muscle of the gecko toe pads, the excellent shape memory effect of the PU-CNF substrate can help the obtained film to memorize and repeatedly display different shapes and solid/water contact models. Thus reversible switching between multiple states from the low-adhesive rolling performance to the high-adhesive pinning performance can be realized. Meanwhile, based on its smart wetting performance, not only the traditional in situ capture/release of one microdroplet, but also the step-by-step release of different droplets can be realized on our film. This work reports a new superhydrophobic shape memory adhesive film, which offers a novel strategy for surface adhesion control and meanwhile opens a new road for applications in controlled droplet manipulation.

Highly Flexible and Ultraprecise Manipulation of Light-Levitated Femtoliter/Picoliter Droplets

Ultraprecise manipulation of the droplets at the microscale is a promising paradigm for broad implications in reagent transport and element analysis, but the existing strategies still suffer from cross-contamination or the miscellaneous auxiliaries. Inspired by the levitation, we develop a method for excellently manipulating levitated femtoliter/picoliter droplets by a single focused laser. We show that the unique light-induced vapor flow in association with the interface morphology is responsible for creation and manipulation of levitated droplets. In particular, we demonstrate that the levitated droplets formed by this light method show extraordinary motility. The highly accurate two-dimensional labyrinth movement of the levitated droplets with designed trajectories above the free surface is easily realized by scanning the light. These results demonstrate that a single focused light can function as an "optical baton" to enable us to construct a wide variety of the long-sought precise manipulation systems for bioassays, pharmacy, and chemosynthesis.

Electrowetting-driven solar indoor lighting (e-SIL): an optofluidic approach towards sustainable buildings

Optofluidics is an emerging research field that combines the two disciplines of microfluidics and optics. By using microfluidic technologies for light control, optofluidic devices can offer several advantages over solid-type optical components, including optical-grade smoothness at the fluidic interface and a high degree of optical tunability without bulky and complex mechanical moving parts. These features have made optofluidic devices more versatile and reconfigurable to improve their optical performances. In this paper, we present a novel optofluidic sunlight manipulation technology for solar indoor lighting using the electrowetting principle. Rooftop sunlight is collected by a solar concentrator and guided to individual rooms along an optical fiber (waveguide) on the bottom of which tunable liquid prisms are linearly integrated. In the light-off mode, electrowetting controls the apex angle of the prisms to be φ = 0°. Under this condition, incoming sunlight experiences total internal reflection and thus keeps propagating along the optical fiber without leaking to the prism bottom for indoor lighting. In contrast, when liquid prisms are controlled to have the angle at φ > 0°, incoming sunlight is partially transmitted to the bottom surface of the arrayed prisms to contribute to interior illumination. Simulation studies validate that our electrowetting-driven solar indoor lighting (e-SIL) system is capable of variably tuning the lighting power from 0% to 98.6% of the input solar power by controlling the prism angle and varying the refractive index of prism materials. For experimental studies, we fabricated an array of 5 prisms filled with silicone oil and water. Using a fiber illuminator as a white light source that includes visible light with various incident angles, we have demonstrated two important lighting functions, (1) light on/off and (2) illumination power control. Lighting performance can be further enhanced by lowering the aspect ratio of the prism as well as increasing the number of prisms. The e-SIL technology based on tunable liquid prisms offers a new approach towards sustainable buildings that are able to reduce their electricity usage as well as provide a healthy and comfortable indoor environment under illumination of natural sunlight.

Shape evolution and splitting of ferrofluid droplets on a hydrophobic surface in the presence of a magnetic field

We elucidate the phenomena of dynamic wetting, shape evolution and splitting of ferrofluid (FF) droplets on a hydrophobic surface under the influence of a magnetic field. In the case of a FF droplet interacting with a magnetic field, both surface energy and magnetic energy contribute to the total Gibb's free energy and hence the wetting phenomena. The nanoparticles in the FF droplet migrate and get accumulated at the apex of the droplet which enhances the magnetic interaction causing large deformation of the droplet. The FF droplet deformation and subsequent splitting are governed by the interplay between the magnetic Fm and surface tension Fs forces. The ratio of the forces km = (Fm/Fs) was found to be a function of the magnetic Bond number Bom and non-dimensional gap g* as km ∼ (Bom)0.3(g*)-0.86. Splitting of the FF droplets was observed for km > 1 and for km < 1, an equilibrium droplet shape was observed. The wetting behavior of the FF droplets was found to be strongly dependent on the FF concentration c - concentrated (c = 1.2%) FF droplets exhibit contact line (CL) pinning and decrease in contact angle (CA) θ with time throughout, while diluted (c = 0.6%) FF droplets show a mixed mode (CL pinning followed by constant CA). In splitting of FF droplets, the ratio of the volume of the daughter droplet to that of the parent droplet i.e. (Vd/Vp), was found to decrease with an increase in the parent droplet size Vp.

A droplet energy harvesting and actuation system for self-powered digital microfluidics

When a water droplet slides down a hydrophobic surface, a major energy it possesses is kinetic energy. However, people may ignore another important energy source: triboelectrification. To quantify and utilize triboelectrification energy, a phenomenon is presented in this study: one droplet slides down a tilted chip with a hydrophobic coating and patterned electrodes, triboelectrification happens and the induced charges are transferred to another horizontally placed chip with copper wires, on which another droplet is actuated by the transferred charges. The mechanism of this phenomenon is triboelectrification, electrostatic induction and EWOD (electrowetting on dielectrics). When an 80 μL droplet slides down the chip, the induced charges build up a potential difference between the electrodes of 46 V. With this potential difference, the droplet actuation is achieved not only on the horizontal chip, but also on the vertical chip. By patterning a comb-shaped electrode, functions for droplet manipulations are achieved. Theoretical analysis is conducted to quantify the frictional force, gravitational force and driving force (EWOD force). The presented concept and device could be employed for a self-powered digital microfluidics (DMF) system, replacing the bulky and energy consuming voltage sources which are commonly used in DMF devices.

Droplet Translation Actuated by Photoelectrowetting

In traditional electrowetting-on-dielectric (EWOD) devices, droplets are moved about a substrate using electric fields produced by an array of discrete electrodes. In this study, we show that a drop can be driven across a substrate with a localized light beam by exploiting the photoelectrowetting (PEW) effect, a light-activated variant of EWOD. Droplet transport actuated by PEW eliminates the need for electrode arrays and the complexities entailed in their fabrication and control, and offers a new approach for designing lab-on-a-chip applications. We report measurements of the maximum droplet speed as a function of frequency and magnitude of the applied bias, intensity of illumination, volume of the droplet, and viscosity and also introduce a model that reproduces these data.

Smartphone integrated optoelectrowetting (SiOEW) for on-chip sample processing and microscopic detection of water quality

With the increasing capabilities and ubiquity of smartphones and their associated digital cameras, this study presents a smartphone integrated optoelectrowetting (SiOEW) device as a simple, portable tool capable of on-chip water sample preparation and microscopic detection of the target cells in water samples, which significantly reduce the detection time and the labor cost required for water quality monitoring. A commercially available smartphone is used as a low-intensity portable light source to perform optoelectrowetting (OEW)-based microfluidic operations such as droplet transportation, merging, mixing, and immobilization on a hydrophobic detection zone. Furthermore, a built-in smartphone camera allows on-chip microscopic detection of water quality with a 45× magnification. We have experimentally demonstrated that the SiOEW platform is able not only to automate the sample processing of marine water including the target algae cells (Amphiprora sp.) and staining reagents fluorescein diacetate (FDA) and 5-chloromethylfluorescein diacetate (CMFDA), but also detect the fluorescence signals emitted from the target cells in water samples and count their populations. Using the smartphone, the collected information (e.g. the location of the water sample collected and the time it was detected, the number of the target cells, etc.) can be rapidly and wirelessly shared with a central host such as an environmental regulation agency for real-time monitoring and further management of water quality.

Microfluidic Technology for the Generation of Cell Spheroids and Their Applications

A three-dimensional (3D) tissue model has significant advantages over the conventional two-dimensional (2D) model. A 3D model mimics the relevant in-vivo physiological conditions, allowing a cell culture to serve as an effective tool for drug discovery, tissue engineering, and the investigation of disease pathology. The present reviews highlight the recent advances and the development of microfluidics based methods for the generation of cell spheroids. The paper emphasizes on the application of microfluidic technology for tissue engineering including the formation of multicellular spheroids (MCS). Further, the paper discusses the recent technical advances in the integration of microfluidic devices for MCS-based high-throughput drug screening. The review compares the various microfluidic techniques and finally provides a perspective for the future opportunities in this research area.

A Study of Dip-Coatable, High-Capacitance Ion Gel Dielectrics for 3D EWOD Device Fabrication

We present a dip-coatable, high-capacitance ion gel dielectric for scalable fabrication of three-dimensional (3D) electrowetting-on-dielectric (EWOD) devices such as an n × n liquid prism array. Due to the formation of a nanometer-thick electric double layer (EDL) capacitor, an ion gel dielectric offers two to three orders higher specific capacitance (c ≈ 10 μF/cm²) than that of conventional dielectrics such as SiO₂. However, the previous spin-coating method used for gel layer deposition poses several issues for 3D EWOD device fabrication, particularly when assembling multiple modules. Not only does the spin-coating process require multiple repetitions per module, but the ion gel layer also comes in risks of damage or contamination due to handling errors caused during assembly. In addition, it was observed that the chemical formulation previously used for the spin-coating method causes the surface defects on the dip-coated gel layers and thus leads to poor EWOD performance. In this paper, we alternatively propose a dip-coating method with modified gel solutions to obtain defect-free, functional ion gel layers without the issues arising from the spin-coating method for 3D device fabrication. A dip-coating approach offers a single-step coating solution with the benefits of simplicity, scalability, and high throughput for deposition of high-capacitance gel layers on non-planar EWOD devices. An ion gel solution was prepared by combining the [EMIM][TFSI] ionic liquid and the [P(VDF-HFP)] copolymer at various wt % ratios in acetone solvent. Experimental studies were conducted to fully understand the effects of chemical composition ratios in the gel solution and how varying thicknesses of ion gel and Teflon layers affects EWOD performance. The effectiveness and potentiality of dip-coatable gel layers for 3D EWOD devices have been demonstrated through fabricating 5 × 1 arrayed liquid prisms using a single-step dip-coating method. Each prism module has been individually controlled to achieve spatial beam steering without the need for bulky mechanical moving parts.

Single-Sided Digital Microfluidic (SDMF) Devices for Effective Coolant Delivery and Enhanced Two-Phase Cooling

Digital microfluidics (DMF) driven by electrowetting-on-dielectric (EWOD) has recently been attracting great attention as an effective liquid-handling platform for on-chip cooling. It enables rapid transportation of coolant liquid sandwiched between two parallel plates and drop-wise thermal rejection from a target heating source without additional mechanical components such as pumps, microchannels, and capillary wicks. However, a typical sandwiched configuration in DMF devices only allows sensible heat transfer, which seriously limits heat rejection capability, particularly for high-heat-flux thermal dissipation. In this paper, we present a single-sided digital microfluidic (SDMF) device that enables not only effective liquid handling on a single-sided surface, but also two-phase heat transfer to enhance thermal rejection performance. Several droplet manipulation functions required for two-phase cooling were demonstrated, including continuous droplet injection, rapid transportation as fast as 7.5 cm/s, and immobilization on the target hot spot where heat flux is locally concentrated. Using the SDMF platform, we experimentally demonstrated high-heat-flux cooling on the hydrophilic-coated hot spot. Coolant droplets were continuously transported to the target hot spot which was mitigated below 40 K of the superheat. The effective heat transfer coefficient was stably maintained even at a high heat flux regime over ~130 W/cm², which will allow us to develop a reliable thermal management module. Our SDMF technology offers an effective on-chip cooling approach, particularly for high-heat-flux thermal management based on two-phase heat transfer.

A review of digital microfluidics as portable platforms for lab-on a-chip applications

Following the development of microfluidic systems, there has been a high tendency towards developing lab-on-a-chip devices for biochemical applications. A great deal of effort has been devoted to improve and advance these devices with the goal of performing complete sets of biochemical assays on the device and possibly developing portable platforms for point of care applications. Among the different microfluidic systems used for such a purpose, digital microfluidics (DMF) shows high flexibility and capability of performing multiplex and parallel biochemical operations, and hence, has been considered as a suitable candidate for lab-on-a-chip applications. In this review, we discuss the most recent advances in the DMF platforms, and evaluate the feasibility of developing multifunctional packages for performing complete sets of processes of biochemical assays, particularly for point-of-care applications. The progress in the development of DMF systems is reviewed from eight different aspects, including device fabrication, basic fluidic operations, automation, manipulation of biological samples, advanced operations, detection, biological applications, and finally, packaging and portability of the DMF devices. Success in developing the lab-on-a-chip DMF devices will be concluded based on the advances achieved in each of these aspects.