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

Photothermal-Driven Droplet Manipulation: A Perspective

Optofluidics, which utilizes the interactions between light and fluids to realize various functions, has garnered increasing attention owing to the advantages of operational simplicity, exceptional flexibility, rapid response, etc. As one of the typical light-fluid interactions, the localized photothermal effect serving as a stimulus has been widely used for fluid manipulation. Particularly, significant progress on photothermal-driven droplet manipulation has been made. In this perspective, recent advancements in localized photothermal effect driven droplet manipulation are summarized. First, the photothermal manipulation of droplets on open surfaces is outlined. An attractive droplet manipulation of light droplet levitation above the gas-liquid interface via localized photothermal effect is then discussed. Besides, the photothermal-driven manipulation of droplets in an immiscible liquid phase is also discussed. Although promising, further development of photothermal-driven droplet manipulation is still needed. The challenges and perspectives of this light droplet manipulation strategy for broad implementation are summarized, which will help future studies and applications.

Smart materials for light control of droplets

Droplet manipulation plays a critical role in both fundamental research and practical applications, especially when combined with smart materials and external fields to achieve multifunctional droplet manipulation. Light control of droplets has emerged as a significant and widely used strategy, driven primarily by photochemistry, photomechanics, light-induced Marangoni effects, and light-induced electric effects. This approach allowing for droplet manipulation with high spatial and temporal resolution, all while maintaining a remote and non-contact mode of operation. This review aims to provide a comprehensive overview of the mechanisms underlying light control of droplets, the design of smart materials for this purpose, and the diverse range of applications enabled by this technique. These applications include merging, splitting, releasing, forwarding, backward movement, and rotation of droplets, as well as chemical reactions, droplet robots, and microfluidics. By presenting this information, we aim to establish a unified framework that guides the sustainable development of light control of droplets. Additionally, this review addresses the challenges associated with light control of droplets and suggests potential directions for future development.

Phototaxis Flight of Microdroplets in a Laser

Phototaxis phenomenon is fundamental and critical for optical manipulation of micro-objects. Here, we report the size-dependent negative or positive phototaxis behaviors for microdroplets containing interfacial energy absorber flying in a laser. The critical diameters for such negative-to-positive turnover are studied through both experiments and simulation with different liquids and absorbers, which establishes the mechanism and reveals the role of both the liquid and the absorber inside the microdroplets. This study offers new insight for the manipulation of the phototaxis behavior of micro-objects, showing potential applications in optical trapping and transporting systems that involve light-microdroplet interactions.

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.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

Amoeba-Inspired Magnetic Venom Microrobots

Nature provides a successful evolutionary direction for single-celled organisms to solve complex problems and complete survival tasks - pseudopodium. Amoeba, a unicellular protozoan, can produce temporary pseudopods in any direction by controlling the directional flow of protoplasm to perform important life activities such as environmental sensing, motility, predation, and excretion. However, creating robotic systems with pseudopodia to emulate environmental adaptability and tasking capabilities of natural amoeba or amoeboid cells remains challenging. Here, this work presents a strategy that uses alternating magnetic fields to reconfigure magnetic droplet into Amoeba-like microrobot, and the mechanisms of pseudopodia generation and locomotion are analyzed. By simply adjusting the field direction, microrobots switch in monopodia, bipodia, and locomotion modes, performing all pseudopod operations such as active contraction, extension, bending, and amoeboid movement. The pseudopodia endow droplet robots with excellent maneuverability to adapt to environmental variations, including spanning 3D terrains and swimming in bulk liquids. Inspired by the "Venom," the phagocytosis and parasitic behaviors have also been investigated. Parasitic droplets inherit all the capabilities of amoeboid robot, expanding their applicable scenarios such as reagent analysis, microchemical reactions, calculi removal, and drug-mediated thrombolysis. This microrobot may provide fundamental understanding of single-celled livings, and potential applications in biotechnology and biomedicine.

Unidirectional self-actuation transport of a liquid metal nanodroplet in a two-plate confinement microchannel

Controllable directional transport of a liquid metal nanodroplet in a microchannel has been a challenge in the field of nanosensors, nanofluidics, and nanofabrication. In this paper, we report a novel design that the self-actuation of a gallium nanodroplet in a two-plate confinement microchannel could be achieved via a continuous wetting gradient. More importantly, suitable channel parameters could be used to manipulate the dynamic behavior of the gallium nanodroplet. The self-actuation transport in the two-plate confinement microchannel is the result of the competition between the driving force from the difference of the Laplace pressure and energy dissipation from the viscous resistance. Furthermore, we have identified the conditions to assess whether the droplet will pass through the contractive cross-section or not. This work can provide guidance for manipulating liquid metal nanodroplets in microchannels.

Research on the Centrifugal Driving of a Water-in-Oil Droplet in a Microfluidic Chip with Spiral Microchannel

Combining the advantages of droplet-based microfluidics and centrifugal driving, a method for centrifugally driving W/O droplets with spiral microchannel is proposed in this paper. A physical model of droplet flow was established to study the flow characteristics of the W/O droplet in the spiral microchannel driven by centrifugal force, and kinematic analysis was performed based on the rigid body assumption. Then, the theoretical formula of droplet flow rate was obtained. The theoretical value was compared with the actual value measured in the experiments. The result shows that the trend of the theoretical value is consistent with the measured value, and the theoretical value is slightly larger than the experimentally measured value caused by deformation. Moreover, it is found that the mode of centrifugal driving with spiral microchannel has better flow stability than the traditional centrifugal driving structure. A larger regulation speed range can be achieved by adjusting the motor speed without using expensive equipment or precise instruments. This study can provide a basis and theoretical reference for the development of droplet-based centrifugal microfluidic chips.

Light-controlled versatile manipulation of liquid metal droplets: a gateway to future liquid robots

The controlled actuation of liquid metal (LM) droplets has recently shown great potential in developing smart actuating systems for applications in robotics. However, there is a lack of a simple approach for the precise manipulation of multiple LM droplets in a 2D plane, which hinders the development of complex control over droplets for realizing useful robotic applications. To overcome this challenge, here, a versatile and powerful light-induced manipulation of LM droplets is presented. The key principle is to selectively activate phototransistors in an electrolyte using infrared laser beams to electrically control LM droplets via Marangoni forces. This approach shows the ability of inducing concurrent motion, splitting, and merging of multiple LM droplets simply using light without complex and bulky systems. Parameters affecting the manipulation of LM droplets are thoroughly investigated. Moreover, a vehicle carrier driven by wheels composed of multiple LM droplets for making a light-controlled relay is demonstrated. We believe such a light-induced control method for manipulating LM droplets has the potential for advancing the development of future field-programmable robotics and droplet-based soft collaborative robots.

Large-area fabrication of superhydrophobic micro-conical pillar arrays on various metallic substrates

Superhydrophobic micro-conical pillar arrays have huge application prospects, from anti-icing to oil/water separation, corrosion resistance, and water droplet manipulation. However, there is still a lack of versatile methods with high processing efficiency to fabricate superhydrophobic micro-conical pillar arrays on various metallic substrates. Herein, a nanosecond laser ablation technology with versatility and high processing efficiency was developed to fabricate large-area superhydrophobic micro-conical pillar arrays. The simulation and experiments indicated that the height and the pillar inclination angle of micro-conical pillars could be easily controlled by adjusting the nanosecond laser parameters or the tilted angles of metallic substrates. The fabricated superhydrophobic micro-conical pillar arrays not only showed good mechanical robustness and chemical stability but also easily reduced the contact time for an impinging water droplet, showing potential application prospects in anti-icing from freezing rain. This kind of method with versatility and high processing efficiency will promote the practical applications of superhydrophobic micro-conical arrays.

External-field-induced directional droplet transport: A review

Directional transport of fluids is crucial for vital activities of organisms and numerous industrial applications. This process has garnered widespread research attention due to the wide breadth of flexible applications such as medical diagnostics, drug delivery, and digital microfluidics. The rational design of functional surfaces that can achieve the subtle control of liquid behavior. Previous studies were mainly dependent on the special asymmetric structures, which inevitably have the problem of slow transport speed and short distance. To improve controllability, researchers have attempted to use external fields, such as thermal, light, electric fields, and magnetic fields, to achieve controllable droplet transport. On the fundamental side, much of their widespread applicably is due to the degree of control over droplet transport. This review provides an overview of recent progress in the last three years toward the transport of droplets with different mechanisms induced by various external stimuli, including light, electric, thermal, and magnetic field. First, the relevant basic theory and typical induced gradient for directional liquid transport are illustrated. We will then review the latest advances in the external-field-induced directional transport. Moreover, the most emerging applications such as digital microfluidics, harvesting of energy and water, heat transfer, and oil/water separation are also presented. Finally, we will outline possible future perspectives to attract more researchers interest and promote the development of this field.

Flexible, superhydrophobic and multifunctional carbon nanofiber hybrid membranes for high performance light driven actuators

Recently, a series of super-hydrophobic materials have been prepared and efforts have been made to further expand their applications, especially in electronics and smart actuators. However, it remains challenging to develop light weight, flexible and super-hydrophobic materials integrating multifunctionalities such as superior photothermal conversion, corrosion resistance, and controllable actuation. Herein, a superhydrophobic and multi-responsive carbon nanofiber (CNF) hybrid membrane with an outstanding photo-thermal effect is fabricated by electrospinning the mixture of polyacrylonitrile and nickel acetylacetonate, followed by two step heat treatment and subsequent fluorination. The superhydrophobic CNF hybrid membrane with outstanding anti-corrosion and self-cleaning performance can float on the water surface spontaneously, thus effectively reducing the motion resistance. The light driven actuation with controllable movement can be achieved by adjusting the laser irradiated location, in which the localized absorption of light is transformed into thermal energy, and hence an imbalanced surface tension is created. The multifunctional hybrid membrane also opens up an arena of applications such as freestanding flexible electronics, drug delivery, and environmental protection.

Photocontrolled directional transport using water-in-oil droplets

Photocontrolled directional transport in both 2D and 3D of water-in-oil droplets was achieved by merocyanine/spiropyran photoisomerization in the droplet.

New insights into unusual droplets: from mediating the wettability to manipulating the locomotion modes

The ability to manipulate droplets can be utilized to develop various smart sensors or actuators, endowing them with fascinating applications for drug delivery, detection of target analytes, environmental monitoring, intelligent control, and so on. However, the stimuli-responsive superhydrophobic/superhydrophilic materials for normal water droplets cannot satisfy the requirements from some certain circumstances, i.e., liquid lenses and biosensors (detection of various additives in water/blood droplets). Stimuli-responsive wetting/dewetting behaviors of exceptional droplets are open issues and are attracting much attention from across the world. In this perspective article, the unconventional droplets are divided into three categories: ionic or surfactant additives in water droplets, oil droplets, and bubble droplets. We first introduce several classical wettability models of droplets and some methods to achieve wettability transition. The unusual droplet motion is also introduced in detail. There are four main types of locomotion modes, which are vertical rebound motion, lateral motion, self-propulsion motion, and anisotropic wettability controlled sliding behavior. The driving mechanism for the droplet motion is briefly introduced as well. Some approaches to achieve this manipulation goal, such as light irradiation, electronic, magnetic, acid-base, thermal, and mechanical ways will be taken into consideration. Finally, the current researches on unconventional droplets extending to polymer droplets and liquid metal droplets on the surface of special wettability materials are summarized and the prospect of unconventional droplet research directions in the field of on-demand transport application will be proposed.