Tunable Superparamagnetic Ring (tSPRing) for Droplet Manipulation

The manipulation of droplets via a magnetic field forms the basis of a fascinating technology that is currently in development. Often, the movement of droplets with magnets involves adding magnetic particles in or around the droplet; alternatively, magneto responsive surfaces may also be used. This work, presents and characterizes experimentally the formation and properties of a tunable superparamagnetic ring (tSPRing), which precisely adjusts itself around a water droplet, due to liquid–liquid interaction, and enables the physical manipulation of droplets. The ring is made of an oil-based ferrofluid, a stable suspension of ferromagnetic particles in an oily phase. It appears spontaneously due to the oil–water interfacial interaction under the influence of a magnetic field. The ferrofluid–water interaction resembles a cupcake assembly, with the surrounding ring only at the base of the droplet. The ring is analogous to a soft matter ring magnet, showing dipole repulsive forces, which stabilizes the droplets on a surface. It enables robust, controllable, and programmable manipulation of enclosed water droplets. This work opens the door to new applications in open surface upside or upside-down microfluidics and lays the groundwork for new studies on tunable interfaces between two immiscible liquids.     

Magnetocontrollable Droplet and Bubble Manipulation on a Stable Amphibious Slippery Gel Surface

Smart manipulation of liquid/bubble transport has garnered widespread attention due to its potential applications in many fields. Designing a responsive surface has emerged as an effective strategy for achieving controllable transport of liquids/bubbles. However, it is still challenging to fabricate stable amphibious responsive surfaces that can be used for the smart manipulation of liquid in air and bubbles underwater. Here, amphibious slippery surfaces are fabricated using magnetically responsive soft poly(dimethylsiloxane) doped with iron powder and silicone oil. The slippery gel surface retains its magnetic responsiveness and demonstrates strong affinity for bubbles underwater but shows small and switching resistance forces with the water droplets in air and bubbles underwater, which is the key factor for achieving the controllable transport of liquids/bubbles. On the slippery gel surface, the sliding behaviors of water droplets and bubbles can be reversibly controlled by alternately applying/removing an external magnetic field. Notably, compared with slippery liquid‐infused porous surfaces, the slippery gel surface demonstrates outstanding stability, whether in air or underwater, even after 100 cycles of alternately applying/removing the magnetic field. This surface shows potential applications in gas/liquid microreactors, gas–liquid mixed fluid transportation, bubble/droplet manipulation, etc.     

Unidirectional Liquid Manipulation Via an Integrated Mesh with Orthogonal Anisotropic Slippery Tracks

The rational manipulation of fluid behavior by functional interfaces plays an indispensable role in the development of advanced materials and devices involving liquid/solid interactions. Previous examples of the liquid “diode” that allows fluid penetration in only one direction rely mainly on the remarkable wettability gradient/contrast. Inspired by the wetting phenomena of the rice leaf and the Pitcher plant, an integrated mesh with orthogonal anisotropic slippery tracks (IMOAS) is presented here that can realize similar unidirectional droplet penetration using a distinct mechanism. The unidirectional droplet penetration can be conveniently switched via the 90° rotation of the IMOAS, showing a highly controllable liquid manipulation. The droplet tends to slip on the surface, which can maximize the contact area between the liquid and the tracks, and complies with the principle of the lowest surface energy. Based on this unique liquid controlling strategy, droplet manipulation of the IMOAS during fog harvesting and droplet self‐regulation has been conducted to illustrate its potential applications. The current design could aid the understanding of liquid unidirectional penetration and unlock additional possibilities for the optimization of fluid‐related systems.     

Electric Field and Gradient Microstructure for Cooperative Driving of Directional Motion of Underwater Oil Droplets

Driving a liquid droplet with control of directional motion on a solid surface, by introducing a surface wettability gradient or external stimuli, has attracted considerable research attention. There still remain challenges, however, due to the slow response rate and slow speed of continuous liquid droplet motion on the structured surface. Here, an approach to continuously drive the underwater oil droplet with control of directional motion by the cooperative effects of an electric field and the gradient of a porous polystyrene microstructure is demonstrated. The gradient microstructure induces the liquid droplet to take on an asymmetrical shape, causing unbalanced pressure on both ends to orient the droplet for motion in a particular direction. Meanwhile, the electric field decreases the contact area and the corresponding viscous drag between the droplet and the gradient‐structured surface. Then, the unbalanced pressure pushes the underwater oil droplet to move directionally and continuously at a certain voltage. This work provides a new strategy to control underwater oil droplets and realize unidirectional motion. It is also promising for the design of new smart interface materials for applications such as electrofluidic displays, biological cell and particle manipulation, and other types of microfluidic devices.     

Droplet and Fluid Gating by Biomimetic Janus Membranes

The ability to gate (i.e., allow or block) droplet and fluid transport in a directional manner represents an important form of liquid manipulation and has tremendous application potential in fields involving intelligent liquid management. Inspired by passive transport across cell membranes which regulate permeability by transmembrane hydrophilic/hydrophobic interactions, macroscopic hydrophilic/hydrophobic Janus‐type membranes are prepared by facile vapor diffusion or plasma treatments for liquid gating. The resultant Janus membrane shows directional water droplet gating behavior in air‐water systems. Furthermore, membrane‐based directional gating of continuous water flow is demonstrated for the first time, enabling Janus membranes to act as facile fluid diodes for one‐way flow regulation. Additionally, in oil‐water systems, the Janus membranes show directional gating of droplets with integrated selectivity for either oil or water. The above remarkable gating properties of the Janus membranes could bring about novel applications in fluid rectifying, microchemical reaction manipulation, advanced separation, biomedical materials and smart textiles.     

Phototunable Underwater Oil Adhesion of Micro/Nanoscale Hierarchical‐Structured ZnO Mesh Films with Switchable Contact Mode

Controllable surface adhesion of solid substrates has aroused great interest both in air and underwater in solving many challenging interfacial science problems such as robust antifouling, oil‐repellent, and highly efficient oil/water separation materials. Recently, responsive surface adhesion, especially switchable adhesion, under external stimulus in air has been paid more and more attention in fundamental research and industrial applications. However, phototunable underwater oil adhesion is still a challenge. Here, an approach to realize phototunable underwater oil adhesion on aligned ZnO nanorod array‐coated films is reported, via a special switchable contact mode between an unstable liquid/gas/solid tri‐phase contact mode and stable liquid/liquid/solid tri‐phase contact mode. The photo‐induced wettability transition to water and air exists (or does not) in the micro/nanoscale hierarchical structure of the mesh films, playing important role in controlling the underwater oil adhesion behavior. This work is promising in the design of novel interfacial materials and functional devices for practical applications such as photo‐induced underwater oil manipulation and release, with loss‐free oil droplet transportation.     

A Versatile Approach for Direct Patterning of Liquid Metal Using Magnetic Field

Patterning of liquid metal (LM) is usually an integral step toward its practical applications. However, the high surface tension along with surface oxide makes direct patterning of LM very challenging. Existing LM patterning techniques are designed for limited types of planar substrates, which require multiple‐step operation, delicate molds and masks, and expensive equipment. In this work, a simple, versatile, and equipment‐free approach for direct patterning of LM on various substrates using magnetic field is reported. To achieve this, magnetic microparticles are dispersed into LM by stirring. When a moving magnetic field is applied to the LM droplet, the aggregated magnetic microparticles deform the droplet to a continuous line. In addition, this approach is also applicable to supermetallophobic substrates since the applied magnetic field significantly enhances the contact between LM and substrate. Moreover, remote manipulation of the magnetic microparticles allows direct patterning of LM on nonplanar surfaces, even in a narrow and near closed space, which is impossible for the existing techniques. A few applications are also demonstrated using the proposed technique for flexible electronics and wearable sensors.     

Electro-Elastic Wetting: A Method to Migrate and Deform Droplets

A new wetting mechanism, termed electro-elastic wetting, and methods to exploit it for droplet manipulation are proposed and demonstrated. The system consists of a droplet of dielectric liquid, an elastic and conductive membrane as its shell, and an electrode-dielectric composite as its substrate. Activation is by an electric field applied between the membrane and the substrate. The equilibrium shape of the droplet is determined by the balance of membrane tension and electrostatic attraction. It is shown that the contact angle of the droplet is governed by a modified Young–Lipmann Equation. It is then demonstrated that it is possible to transport the droplet along a controlled direction, as well as to actively tune its shape, topography, and position by manipulating the spatial distribution of the electrical force.     

Programmable Droplet Bouncing on Bionic Functional Surfaces for Continuous Electricity Generation

The natural phenomenon of droplets bouncing on various surfaces holds remarkable potential for applications like water transportation, self-cleaning, antifreezing, etc. However, achieving precisely controlled patterned droplet bouncing on functional surfaces with accurately controlled factors like bouncing velocity and trajectory in three dimensions remains a formidable challenge. In this context, a concept of bionic hydrophobic functional surfaces composed of mushroom-like microstructures is introduced. These microstructures are crafted using the projection microstereolithography (PµSL) based 3D printing technique, subsequently coated with a hydrophobic spray. By finely adjusting the geometric attributes and inclination angles of these micromushrooms, the ability is gained to meticulously manipulate the bouncing velocity and trajectory of water droplets. The most optimal performance is demonstrated by a droplet exhibiting a maximal jumping distance and height respectively of 2.5 and 7.1 mm with 50° inclined micromushrooms. Notably, these specially designed micromushrooms orchestrate diverse behaviors in droplet bouncing, encompassing patterned bouncing, antigravity jumps, and directional water transportation. Additionally, the functional surface's adaptable self-cleaning capability facilitates the harnessing of energy from rainfall on large surfaces, offering potential applications in realms, such as self-cleaning mechanisms, droplet capture, water conveyance, and clean energy generation.     

Energy minimization-based analysis of electrowetting for microelectronics cooling applications

Electrowetting (EW)-induced droplet motion has been studied over the last decade in view of its promising applications in the field of microfluidics. The objective of the present work is to analyze the physics underlying two specific EW-based applications for microelectronics thermal management. The first of these involves heat absorption by liquid droplets moving on the surface of a chip under EW actuation. Droplet motion between two flat plates under the influence of an electrowetting voltage is analyzed. An energy minimization framework is employed to predict the actuation force on a droplet. This framework, in combination with semi-analytical models for the forces opposing droplet motion, is used to develop a model that predicts transient EW-induced droplet motion. The second application is targeted at hot-spot thermal management and relies on the control of droplet states on artificially structured surfaces through an applied EW voltage. The influence of an electrowetting voltage in determining and altering the state of a static droplet resting on a rough surface is analyzed. An energy minimization-based modeling approach reveals the influence of interfacial energies, surface roughness parameters and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The model is used to establish preliminary criteria to design rough surfaces for use in the hot-spot mitigation application. The concept of an electrically tunable thermal resistance switch for hot-spot cooling applications is introduced and analyzed.     

Noncontact All-In-Situ Reversible Reconfiguration of Femtosecond Laser-Induced Shape Memory Magnetic Microcones for Multifunctional Liquid Droplet Manipulation and Information Encryption

Shape memory polymers can change their shapes in a controlled way under external stimuli and thus promote the development of smart devices, soft robotics, and microfluidics. Here, a kind of iron particles (IPs) doped shape memory microcone is developed for noncontact all-in-situ reversible tuning between the tilted and upright state under near-infrared (NIR) irradiation and magnetic field (MF) actuation. The magnetic microcones are simply fabricated by a laser-ablated replica-molding strategy so that their height can be precisely controlled by adjusting the laser machining parameters. In addition, it is found that the droplets can be transported unidirectionally on the tilted microcones, which is related to the variation of the adhesion force induced by the length of the triple contact line (TCL). Finally, other multifunctional applications have also been realized, such as, selective droplet release, information encryption, rewritable display, and reusable temperature switch. This work may provide a facile strategy for developing multiresponsive smart devices based on shape memory polymers.     

Bioinspired Self‐Propulsion of Water Droplets at the Convergence of Janus‐Textured Heated Substrates

Controlled propulsion of liquid droplets on a solid surface offers viable applications in fog harvesting, heat transfer, microfluidics, and microdevice technologies. A prerequisite for the propulsion of liquid droplets is to break the wetting symmetry of a droplet and contact‐line pinning on the surface by harnessing surface energy gradient. Here, a series of Janus‐textured substrates is constructed to investigate the self‐propulsion of Leidenfrost droplets. It is found that the self‐propulsion of droplets occurs only on two special Janus‐textured substrates. Those are nanostructured silicon substrate bounded by smooth silicon substrate and the nanowire‐decorated microstructured silicon substrate bounded by micropillars with smooth surfaces. The difference in roughness between the two sides of the Janus‐textured substrates creates various numbers and sizes of vapor bubbles. The vapor bubbles cause the droplets to become turbulent, and a pressure gradient is generated. The sufficiently large pressure gradient propels the Leidenfrost droplet to move directionally. The propulsion direction is always toward areas with low roughness.     

Ferrofluid Droplets as Liquid Microrobots with Multiple Deformabilities

Developing microrobots with multiple deformabilities has become extremely challenging due to the lack of materials that are soft enough at the microscale level and the inability to be reconfigured after fabrication. In this study, it is aimed to prove that liquid microrobots composed of ferrofluid droplets are inherently deformable and they can be controlled, individually or in aggregate, with multiple programmable deformabilities. For example, the liquid‐microrobot monomer (LRM) can pass through narrow channels via elongation and achieve scaling via splitting and coalescence. LRMs can also reassemble into various kinds of functional liquid‐robot aggregates, such as microsticks, micropies, microtrains, microkayaks, and microrollingpins. Thus, they can respond to multi‐terrain surfaces or perform various complex tasks. Moreover, the authors' physics‐based theoretical model demonstrates dynamic self‐assembly and group behavior of a multiple LRM system, which is conducive to investigating the mechanisms behind it. These ferrofluid droplet robots provide novel solutions for some potential applications, such as untethered micromanipulation and targeted cargo delivery.     

Magnetic Responsive On-Site Switching of the Directional Droplet Self-Transport in Shape Memory Slippery Tube

The phenomenon of controlled droplet transport has promising application prospects in various fields. Active droplet transport mode is controllable through continuous external stimuli. By contrast, self-transport is a more environmentally friendly and energy-efficient passive transport mode but lacks controllability. In this study, controlled self-transport is achieved by constructing a shape memory polymer (SMP) tube with a lubricated magnetic-responsive gel inner surface. The asymmetrical shape of the tube, combined with the lubricated inner surface, enables directional self-transport of droplets without external stimuli. Furthermore, the resistance on the inner gel surface can be altered by regulating the magnetic field to achieve effective active control during the self-transport process. Thus, smart in situ control of droplet transport can be achieved by integrating the macroscale shape variation of the tube with the dynamic control of the inner surface microstructure. Owing to the fast magnetic responsivity and in situ controllability of the self-transport process, the SMP lubricated tube demonstrates the ability to transport a variety of liquids and can be designed as a micro-reactor for step-by-step droplet detection. The findings of this study may provide guidance for the development of intelligent interface materials and microfluidic devices.     

Measurement of liquid droplet parameters using optical fiber

The measurement of liquid droplet parameters such as size, number, concentration, viscosity, and refractive index is reported. The droplets are sprayed either from a pressure nozzle or a gas atomizing nozzle. The parameters are measured by detecting the clad mode power in the leaky ray zone of a three-region fiber by a new clad-probing technique, using normal core-clad fiber. The refractive index of the liquid is close to that of cladding. Taking the loss characteristics into account, the variation of output power with the deposition of droplets on the fiber is analyzed and compared with experimental results. The measurement sensitivity for different probing conditions is shown experimentally and verified by theoretical analysis. The change in bound power with the number of liquid droplets depositing on unclad fiber is also shown     

Solidification behavior of highly supercooled polycrystalline silicon droplets

Polycrystalline silicon balls are popularly used for solar cells to lower cost and improve their light collection capability. This article investigates on the microstructure evolution during solidification of polycrystalline silicon. The uniform droplet spray process, a controlled capillary jet break-up process, which enables stringent control of the nucleation and microstructure evolution during solidification of alloy droplets in a thermal spray process, was used to produced mono-sized silicon droplets. The experimental parameters for production of silicon droplets were established and the solidification behavior of silicon droplets was investigated using the modification of the free dendritic growth model and the dendritic fragmentation model. This enabled us to correctly establish the transition supercooling for transformation from lateral growth mode to continuous growth mode to be between 81 K and 172 K. The model was used to predict the microstructure of polycrystalline silicon droplets for solar cells produced by a droplet based manufacturing method, enabling greater control over process parameters and its relation to the final microstructure.     

激光微织构表面水滴撞击动力学行为特性研究

为了探究低韦伯条件下,微织构超疏水表面的液滴撞击动力学行为特性,利用激光微织构技术在航空用材Ti 6Al4V试样表面用不同扫描速度的纳秒激光制备出三角纹理微纳织构,借助高速摄像实验平台研究水滴撞击水平表面和倾斜表面动力学行为特性。实验结果表明,水滴冲击平表面高度越高,空气越不容易进入片层,使最大铺展系数增加;与平表面相比,相同高度的斜表面滑移时更多空气进入片层,最大铺展系数最小;其中在扫描速度为100mm/s的工况下,表面凸起占比达到64611,表面纳米颗粒最大,微米颗粒最多,表面的静态接触特性与动态接触特性最优。受表面结构和表面能协同作用共同影响水滴弹离表面的状态。本实验可为航空领域制备超疏水、主动防除冰表面提供一定参考。     

Self-Intercalated Magnetic Heterostructures in 2D Chromium Telluride

Emerging 2D magnetic heterojunctions attract substantial interest due to their potential applications in spintronics. Achieving magnetic phase engineering with structural integrity in 2D heterojunctions is of paramount importance for their magnetism manipulation. Herein, starting with chromium ditelluride (CrTe₂) as the backbone framework, various lateral and vertical magnetic heterojunctions are obtained via self-intercalated 2D chromium telluride (Cr_xTe_y). A Cr₂Te₃-Cr₅Te₈ lateral heterojunction prototype is demonstrated for the manipulation of magnetic moments under different magnitudes of magnetic excitation, showing a sharply stepped hysteresis loop with a dual spin-flip transition at high Curie temperatures up to 150 and 210 K by magneto-optical Kerr measurement. High-resolution scanning transmission electron microscopy and first-principles calculations reveal a preferred random location of Cr intercalants at the phase boundary, allowing lowering energy associated with crystal field splitting. The overall structural rigidity of chromium-telluride heterostructure with magnetic phase decoupled behaviors is promising for 2D spintronic devices.     

A Liquid Gripper Based on Phase Transitional Metallic Ferrofluid

Magnetic fields enable dexterous, precise, and real-time control of ferromagnetic materials. However, most materials, including glasses, organics, and metals, are nonmagnetic and often do not respond to a magnetic field. Here, a transitional ferrofluid (TF) made by embedding magnetic iron particles into pure gallium through the treatment of highly concentrated HCl solutions, as well as its switchable interlocking force to objects during the phase change, is introduced to achieve magnetic manipulation of non-magnetic objects. A gripper made by liquid TF enables intimate contact with arbitrarily shaped objects and then generates a strong interlocking force of as high as 1168 N (using only 10 g TF) upon solidification at room temperature, which can be reversibly eliminated (F < 0.01 N) through melting. Owing to electrical conductivity and magnetism, a solid TF can be melted through electromagnetic induction heating. By coupling the switchable physical force during the phase transition and magnetism of TF, embedded non-magnetic objects can be manipulated using an applied magnetic field and become impervious to magnetic stimuli again after heating and releasing the TF. This study is expected to inspire numerous potential applications in the reversible magnetic actuation of soft robotics, remote operation systems, drug delivery, and liquid grippers.     

Investigation of DNA sequencing droplet trajectory observation and analysis

In this study, DNA sequencing droplet trajectories are observed and analyzed using an application-specific integrated circuit system and a micro-electromechanical system structure jetting chip. We investigate a droplet jetting technique and apply it to biomedical test chips. This use of a sprayed chip structure can reduce sequencing time by two-thirds compared with traditional methods. The structure is a three-dimensional, multi-channel drive system that can control droplets more precisely. The circuit's input control signal was generated and placed on a chip based on the quantitative design of liquid droplets. Delay time and frequency were used to set the LED light sources. In this study, we would use frequencies of 4 kHz, 8 kHz, and 12 kHz combined with two groups of different orifices and frequencies for ink droplet outlet velocity.