Galinstan Droplets Research Articles

The collective transportation of many self-propelled liquid metal droplets in Tesla valve

Gallium-based liquid metal droplet is active particle that maintains a liquid phase at room temperature and perform Self-propelled motion in electrolyte solutions. Unlike passive rigid particles, the collective transportation of swarming liquid metal droplets is strongly influenced by the surface tension, the shape deformation, the collision dynamics and the coalescence process of liquid metal droplets. We drive the swarming micron-sized Galinstan droplets to pass a Tesla microfluidic valve by applying external electric field, the diodicity of liquid metal droplet flow in Tesla valve is verified experimentally, which increases with respect to an increasing voltage. Theoretical simulations of flow field within the Tesla microfluidic valve suggested different turbulent flow field with opposite input flows. The coalescence of liquid metal droplets was enhanced when the Tesla valve blocks the flow. The collective transportation of swarming self-propelled liquid metal droplets has promising applications in biomedics, flexible electronics and swarm microrobotic technology.

Magnetic Liquid Metal Droplet Robot with Multifunction and High Output Force in Milli-Newton.

Magnetically actuated miniature robots have immeasurable potential in lab-on-a-chip and biomedical due to their ability to navigate in constrained space. However, current soft robots made by elastomers have limited functionalities and are prevented from very narrow environments such as channel much smaller than their size because of their non- or limited deformability. In this study, a soft and multifunctional robot based on liquid metal (magnetic liquid-metal droplet robot [MLDR]) with high output force is reported. It is fabricated by engulfing iron particles into a Galinstan droplet. By changing the shape and motion of permanent magnets, the MLDR can be reshaped and moved. The MLDR can also be split in batches and merged efficiently. It shows good softness and flexibility when navigating freely in a narrow channel, and thus can pass through a confined space smaller than its size easily. Furthermore, the MLDR can also push and spread the accumulated liquid in a desired path, and manipulate the motions of small objects well. Benefiting from the solidification-like phenomenon, an MLDR can output milli-Newton-level force much higher than the output force of ferrofluid droplet robots in micro-Newton level. The demonstrated capabilities of the MLDR are promising for the applications in lab-on-a-chip or biomedical devices.

Homogeneity of liquid metal polymer composites: impact on mechanical, electrical, and sensing behavior.

Liquid metal polymer composites (LMPCs) are formed by dispersing eutectic gallium-indium-tin (galinstan) droplets within a soft polymer matrix, such as polydimethylsiloxane (PDMS), resulting in an insulating composite that is suitable for dielectric applications, including wearable sensors and actuators. LMPCs offer a unique combination of robust mechanical performance and desirable electrical properties. While much research has focused on the effects of rigid fillers in polymer composites, the behavior of liquid metal fillers, particularly the impact of homogeneity, has received limited attention. The density disparity between galinstan and the polymer matrix (6.44 g cm-3 compared to 0.97 g cm-3) results in the settling of galinstan droplets before curing, especially in matrices with low viscosity, leading to an inhomogeneous composition that may affect material performance. To address this, an innovative approach was introduced that enabled a spatially uniform (homogeneous) dispersion of galinstan droplets in PDMS while preserving the non-conductive nature of the composites. Work described herein evaluates the influence of homogeneity on electrical and mechanical properties as well as performance of LMPCs as pressure sensors. It was found that homogeneity has minimal effect on permittivity and dielectric loss but exhibits a complex behavior with respect to other parameters, including dielectric strength, which is often exacerbated at higher concentrations (≥50 vol%). These findings provide valuable insight that contributes to improved control over the material properties of LMPCs and expands their potential applications in soft robotics and stretchable electronics.

A Reusable Pseudo-Liquid-to-Solid Inertial Switch Based on Hetero-Coated Galinstan Droplets

Compared to solid-to-solid inertial switches, the devices based on liquid-to-solid contact can avoid the signal bouncing and contact wear caused by the rigid contact between the proof mass and the stationary electrode. However, most of the current liquid-to-solid inertial switches can only support one-time use. To resolve this issue, we, for the first time, present a pseudo-liquid-to-solid inertial switch that not only has a soft electronic contact but also is reusable. We utilize one graphene-coated galinstan droplet (GGD) with high conductivity as the proof mass and two polytetrafluoroethylene (PTFE)-coated galinstan droplets (PGDs) with excellent elasticity and mechanical robustness as springs to restrain the GGD. Moreover, the coated powder shell can prevent galinstan droplets from being oxidized thus enables superior performance in terms of nonstick property and high mobility. As a result, a pseudo-liquid-to-solid inertial switch with a threshold of 20 g is designed and fabricated, and it exhibits high reliability, reusability and stability with a contact time of 1 ms. In summary, this novel type of inertial switch combines the advantages and overcomes the limitations of both liquid-to-solid type and solid-to-solid type, therefore is of great potential for practical applications. [2022-0151]

How an Oxide Layer Influences the Impact Dynamics of Galinstan Droplets on a Superhydrophobic Surface.

When exposed to air, gallium-based alloys rapidly form a thin oxide layer with viscoelasticity and high adhesion. Although previous work demonstrated that an oxide layer inhibits liquid metal droplet rebound, there is still a lack of a quantitative study to elaborate how an oxide layer affects the impact dynamics. To address this issue, we experimentally investigate Galinstan droplet impingement on a superhydrophobic CuO nanoblade surface and physically explain the difference in the dynamic characteristics of oxidized and unoxidized droplets. Experimental results show that the effect of an oxide layer becomes prominent during the retraction phase. The high adhesion significantly suppresses retraction and rebound, while the elastic response prevents a droplet from sufficiently stretching and maintains the stability of the morphology. More importantly, we systematically and quantitatively explore the influence of an oxide layer on several critical impact parameters, which contributes to a comprehensive understanding of the impact dynamics of liquid metal droplets. It is indicated that an oxide layer has little effect on the maximum spreading factor and spreading time, whereas it causes a 45% reduction of the restitution coefficient and a 36% increase in contact time. Notably, the scaling laws that describe the critical impact parameters of unoxidized droplets show good agreement with the ones known from ordinary Newtonian fluids.

Line Tension and Drop Size Dependence of Contact Angle at the Nanoscale

Despite considerable research efforts, the influence of contact line tension during wetting at the nanoscale and its experimental determination remain challenging tasks. So far, molecular dynamics simulations and atomic force microscope measurements have contributed to the understanding of these phenomena. However, a direct measurement of the size dependence of the contact angle and the magnitude of the apparent line tension has not been realized so far. Here, we show that the contact angle is indeed dependent on the drop size for small drop diameters and determine the magnitude of the apparent line tension via liquid-metal based measurements of advancing and receding contact angle inside a scanning electron microscope. For this purpose, a robotic setup inside an electron microscope chamber and oxide-free Galinstan droplets—produced via an electromigration-based and focused ion beam irradiation-assisted process—are employed. Using the first-order correction of Young’s equation, we find an apparent line tension value of 4.02 × 10−7 J/m for Galinstan© on stainless steel.

Spreading and contact-line arrest dynamics of impacting oxidized liquid-metal droplets

A phenomenological study is presented of the impact, spreading, and arrest of Galinstan droplets on a rigid substrate. The maximum diameter at which contact line arrest occurs is found to be a power law of the speed and a model is presented to rationalize the observations.

Experimental investigation of droplet breakup of oxide-forming liquid metals

This work investigates the breakup of liquid metal droplets experimentally using a shock tube. Droplets of Field's metal melt are produced and their flow-induced deformation and rupture are captured by a high-speed camera. Results are compared to previous data on Galinstan droplet breakup using image sequences and deformation data. Regarding differences, we find that Field's metal droplets show slightly larger deformations and breakup into a larger number of smaller fragments, especially at low Weber numbers. We expect this to be an effect of different oxidation rates. However, both oxidizing metals show a very similar behavior with respect to the breakup morphology, transition between modes, and the timing of the deformation across the investigated Weber number range of 10–100. In addition, core features that distinguish the breakup of metals from that of conventional, water-like liquids are confirmed. Based on the similarities, we propose that the findings can be generalized to also represent other oxide-forming metals. Weber number-dependent fits are presented for the initial deformation time, the time of the onset of breakup, and the maximum cross-stream diameter. In addition, we provide an empirical fit for the time-dependent cross-stream deformation and evaluate it against experimental data and models from the literature. The fits can be used directly in numerical studies or help improve current breakup models.

Secondary Atomization of Liquid Metal Droplets at Moderate Weber Numbers

Atomization of liquid metal is an essential process in a variety of production methods such as spray forming or laser sintering. A critical part of all atomization processes is the breakup of single droplets, also termed secondary atomization. While it has been widely analyzed for conventional liquids, studies focusing on the influence of the specific properties of liquid metals remain rare. To identify differences, this work investigates single liquid metal Galinstan droplets exposed to a shock-induced crossflow by recording the breakup with a high-speed camera. The experimental test series covers a Weber number range of 11-104, and results show that the breakup morphology of Galinstan droplets follows the known sequence of bag, bag-and-stamen, multimode, and shear stripping breakup. We identify transition Weber numbers of ~15, ~35, and ~80, respectively, but also show that transition between modes is a continuous process with gradual changes. Compared to conventional liquids, the initial deformation of Galinstan droplets is very similar with respect to the shape, the initial deformation time, and the maximum cross-stream diameter. In contrast, later stages of the breakup process show clear differences. We observe that the onset of breakup appears significantly earlier in non-dimensional time, that Galinstan bags inflate much less, and that the bag breakup does not exhibit the same phenomenology as that of a water droplet. Further differences in the droplet shape and fragmentation suggest that the elastic oxide layer forming on Galinstan plays an essential role.

Liquid-metal capillary switch for electrical power application

Liquid-metal (LM) electrical switches have been desired for decades to overcome intrinsic shortcomings of traditional mechanical switches equipped with solid contacts. Existing LM electrical switches, functioning via solid-liquid wetting or liquid droplet actuation, mostly focus on low current and small signal switches. In this Letter, a LM switch aiming at a power-level current switch has been proposed. A proof-of-concept prototype was fabricated via encapsulating Galinstan droplets into a silicone matrix. The device switches on a circuit when LM droplets are squeezed into a capillary channel to coalesce. The switch-off process with arc burning is realized by capillary breakup of the LM bridge and high-speed contact separation driven by a pressure difference contributed by Laplace pressure, electrical arc pressure, and atmospheric pressure. Switching off 0.5 A–5 A current under DC 220 V within 0.13–1.69 ms, the device demonstrates a high-speed current interruption ability compared to two kinds of commercial push-button switches. A high-speed camera is utilized to visualize the LM switch's current interruption process. In such a situation, the LM contacts exhibit a separation speed up to 3.85 m/s. Apart from the pressure difference, a slip layer between the LM and capillary wall also plays an important role in driving LM contacts. The LM switch has potential for applications in communication and control equipment acquiring a fast switch of the electrical power signal. This paper offers a framework for designing the LM switch dealing with intense electrical discharge.

Flexible pressure and touch sensor with liquid metal droplet based on gallium alloys

Galinstan is a liquid metal based on gallium alloys consisting of gallium, indium and tin at a ratio of 67.5:22.5:10 by weight. It has been received great attention recently since it is fluidic and conductive at room temperature, even safe to humans. By utilizing a fringe capacitance of liquid galinstan, a pressure sensor and an object approach detecting sensor were fabricated. In a pressure sensor, the capacitance increases as the pressure increases and also the performance of the device is reversible by using an elastic PDMS substrate. In an object detecting approach sensor, the device can detect an approach of the grounded object by using fringe capacitance of liquid galinstan droplet and the performance of the device is also reversible.

Analysis and Transformations of Room‐Temperature Liquid Metal Interfaces – A Closer Look through Interfacial Tension

AbstractThe front cover artwork is provided by the group of Prof. Xuechang Zhou at Shenzhen University. The image shows the chemical modification of Galinstan droplets by oxygen and water molecules as well as oxonium and hydroxide ions, which was analyzed by pendant drop tensiometry. Read the full text of the Article at 10.1002/cphc.201800129.

Thermomechanical-stress-free interconnection of solar cells using a liquid metal

Research efforts to overwhelmingly fortify the economics of photovoltaics have focused on increasing the light-to-electricity conversion efficiency and reducing the overall manufacturing costs. Since lowering the consumption of silicon has the potential to greatly curtail the material costs, studies on wafering thin silicon substrates have been performed. However, current methods to interconnect solar cells with metal ribbons are not sufficient to employ thin silicon wafers. Therefore, this study explores a novel route to interconnect solar cells with metal ribbons without thermomechanical stress using a highly conductive liquid metal. Galinstan, which is notoriously difficult to print, is successfully harnessed to be printable by suspending submillimetre-sized Galinstan droplets in the carrier vehicle, as aided by the yield stress. By mechanically sintering the Galinstan paste with gentle mechanical pressure, solar cells are interconnected with metal ribbons at room temperature. Albeit the averaged maximum power of the unit modules interconnected using the Galinstan paste was significantly degraded after just 100 thermal cycles due to the possible formation of resistive intermetallic compounds of Galinstan, the power degradation of the best unit module using the Galinstan paste is −0.9%, which is smaller than the averaged one of the unit modules using a conventional soldering method (-1.35%), implying the thermomechanical-stress-free interconnection method using a liquid metal has a chance to be improved for being a comparable competitor to the conventional soldering method.

Metallic nanoemulsion with galinstan for high heat-flux thermal management

Galinstan is a eutectic alloy of gallium, indium, and tin, of which thermal conductivity is ∼27 times higher than that of water, while the dynamic viscosity is only twice. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the direct use of galinstan can suffer from practical issues such as oxidation and low specific heat. Therefore, galinstan is mixed with a coolant as an additive to form a colloidal fluid; i.e., dispersion of nanoscale galinstan droplets in a coolant to enhance the thermal conductivity. It is expected that this “metallic nanoemulsion” can contribute to substantial improvement in heat transfer capability. Also, the common issues with colloidal fluids such as rapid sedimentation, erosion, and clogging, can be minimized by the “fluidity” of the liquid metal.It was shown that ultrasonic emulsification can yield nanodroplets with a few hundreds of nanometer or smaller. However, the long exposure of galinstan to oxygen in water inevitably results in severe oxidation of the droplets. Theoretical analysis was also conducted to examine the feasibility of the metallic nanoemulsion as a microchannel heat-sink working fluid. Effective medium theory was used to evaluate the thermal conductivity of the mixture. The viscosity change was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Under one-dimensional laminar flow assumption, mass, momentum, and energy conservation equations were analytically solved. The effect of high thermal conductivity of galinstan was evident; the convection heat transfer capability was greatly enhanced, while the high viscosity due to the nanoscale blending and the low specific heat of galinstan reduce the flow rate and thus increase the caloric thermal resistance.

Continuous transfer of liquid metal droplets across a fluid-fluid interface within an integrated microfluidic chip.

Micro scale liquid metal droplets have been hailed as the potential key building blocks of future micro-electro-mechanical systems (MEMS). However, most of the current liquid metal enabled systems involve millimeter scale droplets, which are manually injected onto the desired locations of the microchip. Despite its simplicity, this method is impractical for patterning large arrays or complex systems based on micro scale droplets. Here, we present a microfluidic chip, which integrates continuous generation of micro scale galinstan droplets in glycerol, and the hydrodynamic transfer of these droplets into sodium hydroxide (NaOH) solution. Observation via high-speed imaging along with computational fluid dynamics (CFD) analysis are utilised to comprehend the lateral migration of droplets from the glycerol to NaOH fluid. This platform is simple, can be readily integrated into other microfluidic systems, and creates flexibility by separating the continuous phase for droplet generation from the eventual target carrier fluid within a monolithic chip.

A Super-Lyophobic 3-D PDMS Channel as a Novel Microfluidic Platform to Manipulate Oxidized Galinstan

We report a 3-D super-lyophobic polydimethylsiloxane (PDMS) microfluidic channel patterned with an array of multi-scale surface texture as a novel microfluidic platform to mobilize naturally oxidized Galinstan. Galinstan is a liquid metal that has multiple advantages over mercury such as non-toxicity, higher thermal conductivity, and lower electrical resistivity. However, Galinstan gets easily oxidized in an air environment and it becomes a viscoelastic liquid that wets almost any solid surface. We studied the feasibility of developing super-lyophobic surfaces against Galinstan, using various flat and textured surfaces including PDMS micropillar and microridge arrays by measuring static and dynamic contact angles. The highest advancing angle of 175 ° and receding angle of 163 ° were achieved on a surface patterned with micropillars, each of which was textured with additional roughness. Pitch distance between pillars was 175 μm. An extremely simple PDMS-PDMS bonding technique was used to fabricate a 3-D super-lyophobic channel structure as a microfluidic platform for oxidized Galinstan droplets. The driving force to actuate a ~ 3-μL Galinstan droplet in the 3-D super-lyophobic channel was 3.11±0.23 mN.

Performance improvement of a micro thermomechanical generator by incorporating Galinstan® micro droplet arrays

In previous research we have demonstrated a micro thermomechanical pyroelectric generator (µTMPG) as an alternative to thermoelectric generators to harvest ambient heat energy. In such a device, a thermal mass oscillates between a hot and a cold side by virtue of the bistability of its mechanical mount, thus generating a temporal thermal gradient over a pyroelectric material in between. The operational frequency as a major factor deciding the power output of the µTMPG is in turn dependent on the thermal contact resistance (TCR) present at the mating regions of thermal mass, hot and cold sides. Hence, we have investigated the incorporation of an array of Galinstan droplets at the mating interfaces to reduce the TCR. These arrays are fabricated by selective deposition of Galinstan on a laser-micromachined silicon substrate. After incorporating such an array the operational frequency of the µTMPG increases by at least 50%.

Characterization of Nontoxic Liquid-Metal Alloy Galinstan for Applications in Microdevices

We have obtained interfacial properties of Galinstan, a nontoxic liquid-metal alloy, to help replace mercury in miniature devices. To prevent formation of an oxide skin that severely hinders the fluidic behavior of small Galinstan droplets and leads to inaccurate property data, we performed our experiments in a nitrogen-filled glove box. It was found that only if never exposed to oxygen levels above 1 part per million (ppm) would Galinstan droplets behave like a liquid. Two key properties were then investigated: contact angles and surface tension. Advancing and receding contact angles of Galinstan were measured from sessile droplets on various materials: for example, 146.8 and 121.5, respectively, on glass. Surface tension was measured by the pendant-drop method to be 534.6 10.7 mN/m. All the measurements were done in nitrogen at 28 with oxygen and moisture levels below 0.5 ppm. To help design droplet-based microfluidic devices, we tested the response of Galinstan to electrowetting-on-dielectric actuation.