Fluid pumping by liquid metal droplet utilizing ac electric field

A system for fluid pumping by liquid metal multi-droplets

The transportation and control of microfluidics have an important influence on the fields of biology, chemistry, and medicine. Pump systems based on the electrocapillary effect and room-temperature liquid metal droplets have attracted extensive attention. Flow rate is an important parameter that reflects the delivery performance of the pump systems. In the systems of previous studies, cylindrical structures are mostly used to constrain the droplet. The analysis and quantitative description of the influence of voltage frequency, alternating voltage, direct current voltage bias, and solution concentration on the flow rate are not yet comprehensive. Furthermore, the systems are driven by only one droplet, which limits the increase in flow rate. Therefore, a pump with a cuboid structure is designed and the droplet is bound by pillars, and the flow rate of the pump is increased by more than 200% compared with the cylindrical pump. For this structure, the mechanism of various factors on the flow rate is analyzed. To further enhance the flow rate, a pump system with multi-droplets is proposed. Moreover, the expression of flow velocity of the solution on the surface of each droplet and the relationship between the flow rate, alternating voltage, and the number of droplets are deduced. Finally, the potential of applying the multi-droplet cuboid pump system in drug delivery and analytical chemistry is demonstrated. Additionally, the core of the pump, the droplet area, is modularized, which breaks the overall structural limitations of the liquid metal pump and provides ideas for pump design.

Temperature-responsive peristome-structured smart surface for the unidirectional controllable motion of large droplets

The manipulation of fast, unidirectional motion for large droplets shows important applications in the fields of fog collection and biochemical reactions. However, driving large droplets (>5 μL) to move directionally and quickly remains challenging due to the nonnegligible volume force. Herein, we fabricated a scalable, bionic peristome substrate with a microcavity width of 180 μm using a 3D printing method, which could unidirectionally drive a large water droplet (~8 μL) at a speed reaching 12.5 mm/s by temperature-responsive wettability. The substrate surface was grafted with PNIPAAm, which could reversibly change its wettability in response to temperature, thereby enabling a temperature-responsive smart surface that could regulate droplet movement in real-time by changing the temperature. A series of temperature-responsive smart patterns were designed to induce water transport along specific paths to further realize controllable droplet motion with the antibacterial treatment of predesignated areas. The ability to achieve temperature-responsive unidirectional motion and dynamic control of droplet movement could allow programmable fluidic biosensors and precision medical devices. A temperature-responsive smart surface was produced to control the unidirectional motion of large droplets between spreading and pinning movement by changing the surface wettability.

Bioinspired Underwater Superoleophilic Two-Dimensional Surface with Asymmetric Oleophobic Barriers for Unidirectional and Long-Distance Oil Transport

Unidirectional and long-distance liquid transport is critically important to a range of practical applications, e.g., water harvesting, microfluidics, and chemical reactions. Great efforts have been made on liquid manipulation; most of which, however, are limited in the air environment. It is still a great challenge to achieve unidirectional and long-distance oil transport in an aqueous environment. Herein, we have successfully fabricated an underwater superoleophilic two-dimensional surface (USTS) with asymmetric oleophobic barriers to arbitrarily manipulate oil in aqueous medium. The behavior of oil on USTS was carefully investigated, of which the unidirectional spreading capability was originated from the anisotropic spreading resistance resulted from the asymmetric oleophobic barriers. Accordingly, an underwater oil/water separation device has been developed, which can achieve continuous and efficient oil/water separation and further prevent the secondary pollution caused by oil volatilization.

3D actuation of foam-core liquid metal droplets

Precise manipulation of liquid metal (LM) droplets possesses the potential to enable a wide range of applications in reconfigurable electronics, robotics, and microelectromechanical systems. Although a variety of methods have been explored to actuate LM droplets on a 2D plane, versatile 3D manipulation remains a challenge due to the difficulty in overcoming their heavy weight. Here, foam-core liquid metal (FCLM) droplets that can maintain the surface properties of LM while significantly reducing the density are developed, enabling 3D manipulation in an electrolyte. The FCLM droplet is fabricated by coating LM on the surface of a copper-grafted foam sphere. The actuation of the FCLM droplet is realized by electrically inducing Marangoni flow on the LM surface. Two motion modes of the FCLM droplet are observed and studied and the actuation performance is characterized. Multiple FCLM droplets can be readily controlled to form 3D structures, demonstrating their potential to be further developed to form collaborative robots for enabling wider applications.

A tripodal wheeled mobile robot driven by a liquid metal motor

As a novel driving concept, the liquid metal motor (LMM) has been regarded as a promising actuator due to its unique traits, such as infinitely variable speed, lack of transmission chain, convenient maintenance, and silence. However, at present, driving devices based on this material are still in the preliminary and rudimentary stage, and representative application examples are scarce. Therefore, an 8-shaped tripodal wheeled mobile robot (WMR) completely driven by a LMM is designed in this study to further prove the practicability of this material. Through combining the Marangoni surface flow on a liquid metal droplet (LMD) caused by an electrochemical reaction and the eccentric torque generated by the change in droplet shape and position, the two independently driven wheels of the mobile robot are actuated at differential moving speeds. Additionally, a matching control module, a cell phone application, and a battery have been developed and added for wireless control of three types of driving functions (moving forward, steering, and stopping). It is expected that this work could further advance the development and application of LMMs and bring new ideas to the design of WMRs.

A visual portable microfluidic experimental device with multiple electric field regulation functions

High portability and miniaturization are two of the most important objectives pursued by microfluidic methods. However, there remain many challenges for the design of portable and visual microfluidic devices (e.g., electrokinetic experiments) due to the use of a microscope and power supply. To this end, we report a visual portable microfluidic experimental device (PMED) with multiple electric field regulation functions, which can realize the electric field regulation functions of various basic microfluidic experiments through modular design. The internal reaction process of the microfluidic chip is displayed by a smartphone, and the experimental results are analyzed using a mobile phone application (APP). Taking the induced-charge electroosmosis (ICEO) particle focusing phenomenon as an example, we carried out detailed experiments on PMED and obtained conclusions consistent with numerical simulations. In addition to ICEO experiments, other functions such as alternating electroosmosis (ACEO), thermal buoyancy convection, and dielectrophoresis (DEP) can be realized by replacing module-specific covers. The device expands the application of microfluidic experiments and provides a certain reference for the further integration and portability of subsequent microfluidic experiment devices.