In addition to its use in electronics, the unique physical properties of gallium have led to its use in other areas. Gallium itself is a metal with a very low melting point and is a liquid at just above room temperature (30 °C). Also, gallium is capable of forming several eutectic systems (alloys that have a lower melting point than any of its constituents, including gallium) with a number of other metals. Both pure gallium and these gallium based liquid metal alloys have high surface tension and are considered “non-spreadable” on most surfaces. This renders them difficult to handle, shape, or process, which limits their potential for real-world application. However, a recent discovery may have unlocked the possibility for broader use of gallium in the field of functional materials.

A research team at the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS) in Ulsan, South Korea and the Ulsan National Institute of Science and Technology (UNIST) has invented a new method for incorporating filler particles in liquid gallium to create functional composites of liquid metal. The incorporation of fillers transforms the material from a liquid state into either a paste- or putty-like form (with consistency and “feel” similar to the commercial product “Plasticine”) depending on the amount of added particles. In the case when graphene oxide (G-O) was used as a filler material, G-O content of 1.6~1.8% resulted in a paste-like form, while 3.6% was optimal for putty formation. A variety of new gallium composites and the mechanism of their formation is described in a recent article published in the journal Science Advances.

The mixing of particles inside the gallium based liquid metal alters the physical properties of the material, which allows for much easier handling. First author Chunhui Wang notes: “The ability for liquid gallium composites to form pastes or putties is extremely beneficial. It removes most of the issues of handling of gallium for applications. It no longer stains surfaces, it can be coated or “painted” onto almost any surface, it can be molded into a variety of shapes. This opens up a wide variety of applications for gallium not seen before.” The potential application of this discovery includes situations where soft and flexible electronics are required, such as in wearable devices and medical implants. The study even showed that the composite can be fashioned into a porous foam-like material with extreme heat resistance, with the ability to withstand a blowtorch for 1 minute without sustaining any damage.

In this study, the team was able to identify the factors that would allow the fillers to successfully mix with liquid gallium. Co-corresponding author Benjamin Cunning described the prerequisites: “Liquid gallium develops an oxide ‘skin’ when exposed to air, and this is crucial for mixing. This skin coats the filler particle and stabilizes it inside the gallium, but this skin is resilient. We learned that particles of a large enough size have to be used otherwise mixing cannot occur and a composite cannot be formed.”

The researchers used four different materials as fillers in their study: graphene oxide, silicon carbide, diamond, and graphite. Among these, two of them in particular displayed excellent properties when incorporated in liquid gallium: reduced graphene oxide (rG-O) for electromagnetic interference (EMI) shielding and diamond particles for thermal interface materials. A 13-micron thick coating of Ga/rG-O composite on a reduced graphene oxide film was able to improve the film’s shielding efficiency from 20 dB up to 75 dB, which is sufficient for both commercial (>30 dB) and military (>60 dB) applications. However, the most remarkable property of the composite was its ability to provide EMI shielding property to any everyday common material. The researchers demonstrated that a similar 20-micron thick coating of Ga/rG-O applied on a simple sheet of paper yielded a shielding efficiency of over 70 dB.

Perhaps most exciting was the thermal performance when diamond particles were incorporated into the material. The CMCM team measured the thermal conductivities in collaboration with UNIST researchers Dr. Shalik Joshi and Prof. KIM Gun-ho, and the “real-world” application experiments were carried out by LEE Seunghwan and Prof. LEE Jaeson. The thermal conductivity experiment showed that the diamond containing composite had bulk thermal conductivities of up to ~110 W m-1 K-1, with larger filler particles yielding greater thermal conductivity. This exceeded the thermal conductivity of the commercially available thermal paste (79 W m-1 K-1) by more than 50%. The application experiment further proved the gallium-diamond mixture’s effectiveness as a thermal interface material (TIM) between a heat source and a heat sink. Interestingly, the composite with smaller size diamond particles showed superior real-world cooling capability despite having lower thermal conductivity. The reason for this discrepancy is due to the larger diamond particles being more prone to protruding through the bulk gallium and creating air gaps at the interface of the heat sink or heat source and the TIM, reducing its effectiveness. (Ruoff notes that there are some likely ways to solve this issue in the future.)

Lastly, the group has even created and tested a composite made from a mixture of gallium metal and commercial silicone putty — better known as “Silly Putty” (Crayola LLC). This last type of gallium containing composite is formed by an entirely different mechanism, which involves small droplets of gallium being dispersed throughout the Silly Putty. While it does not have the impressive EMI shielding ability of the above-mentioned Ga/rG-O (the material requires 2 mm of coating to achieve the same 70 dB shielding efficiency), it is compensated with superior mechanical properties. Since this composite uses silicone polymer rather than gallium metal as the base material, it is stretchable in addition to being malleable.

Prof. Rod Ruoff, director of CMCM who conceived of the idea of mixing such carbon fillers with liquid metals notes: “We first submitted this work in September 2019, and it has undergone a few iterations since then. We have discovered that a wide variety of particles can be incorporated into liquid gallium and have provided a fundamental understanding of how particle size plays a role in successful mixing. We found this behavior extends to gallium alloys that are liquids at temperatures below room temperature such as indium-gallium, tin-gallium, and indium-tin-gallium. The capabilities of our UNIST collaborators have demonstrated outstanding applications for these composites, and we hope our work inspires others to discover new functional fillers with exciting applications.”

Story Source:

Materials provided by Institute for Basic Science. Note: Content may be edited for style and length.

A way around the conventional problem is to focus on fluorescence lifetime instead of intensity. When a fluorescent substance is irradiated with a short burst of light, the resulting fluorescence does not disappear immediately but actually “decays” over time in a way that is specific to that substance. The “fluorescence lifetime microscopy” technique leverages this phenomenon — which is independent of experimental conditions — to accurately quantify fluorescent molecules and changes in their environment. However, fluorescence decay is extremely fast, and ordinary cameras cannot capture it. While a single-point photodetector can be used instead, it has to be scanned throughout the sample’s area to be able to reconstruct a complete 2D picture from each measured point. This process involves movement of mechanical pieces, which greatly limits the speed of image capture.

Fortunately, in this recent study published in Science Advances, the aforementioned team of scientists developed a novel approach to acquire fluorescence lifetime images without necessitating mechanical scanning. Professor Takeshi Yasui, from Institute of Post-LED Photonics (pLED), Tokushima University, Japan, who led the study, explains, “Our method can be interpreted as simultaneously mapping 44,400 ‘light stopwatches’ over a 2D space to measure fluorescence lifetimes — all in a single shot and without scanning.” So, how was this achieved?

One of the main pillars of their method is the use of an optical frequency comb as the excitation light for the sample. An optical frequency comb is essentially a light signal composed of the sum of many discrete optical frequencies with a constant spacing in between them. The word “comb” in this context refers to how the signal looks when plotted against optical frequency: a dense cluster of equidistant “spikes” rising from the optical frequency axis and resembling a hair comb. Using special optical equipment, a pair of excitation frequency comb signals is decomposed into individual optical beat signals (dual-comb optical beats) with different intensity-modulation frequencies, each carrying a single modulation frequency, and irradiated on the target sample. The key here is that each light beam hits the sample on a spatially distinct location, creating a one-to-one correspondence between each point on the 2D surface of the sample (pixel) and each modulation frequency of the dual-comb optical beats.

Because of its fluorescence properties, the sample re-emits part of the captured radiation while still preserving the aforementioned frequency-position correspondence. The fluorescence emitted from the sample is then simply focused using a lens onto a high-speed single-point photodetector. Finally, the measured signal is mathematically transformed into the frequency domain, and the fluorescence lifetime at each “pixel” is easily calculated from the relative phase delay that exists between the excitation signal at that modulation frequency versus the one measured.

Thanks to its superior speed and high spatial resolution, the microscopy method developed in this study will make it easier to exploit the advantages of fluorescence lifetime measurements. “Because our technique does not require scanning, a simultaneous measurement over the entire sample is guaranteed in each shot,” remarks Prof. Yasui, “This will be helpful in life sciences where dynamic observations of living cells are needed.” In addition to providing deeper insight into biological processes, this new approach could be used for simultaneous imaging of multiple samples for antigen testing, which is already being used for the diagnosis of COVID-19.

Perhaps most importantly, this study showcases how optical frequency combs, which were only being used as “frequency rulers,” can find a place in microscopy techniques to push the envelope in life sciences. It holds promise for the development of novel therapeutic options to treat intractable diseases and enhance life expectancy, thereby benefitting the whole of humanity.

Since individual cells are almost translucent, microscope cameras must detect extremely subtle differences in the light passing through parts of the cell. Those differences are known as the phase of the light. Camera image sensors are limited by what amount of light phase difference they can detect, referred to as dynamic range.

“To see greater detail using the same image sensor, we must expand the dynamic range so that we can detect smaller phase changes of light,” said Associate Professor Takuro Ideguchi from the University of Tokyo Institute for Photon Science and Technology.

The research team developed a technique to take two exposures to measure large and small changes in light phase separately and then seamlessly connect them to create a highly detailed final image. They named their method adaptive dynamic range shift quantitative phase imaging (ADRIFT-QPI) and recently published their results in Light: Science & Applications.

“Our ADRIFT-QPI method needs no special laser, no special microscope or image sensors; we can use live cells, we don’t need any stains or fluorescence, and there is very little chance of phototoxicity,” said Ideguchi.

Phototoxicity refers to killing cells with light, which can become a problem with some other imaging techniques, such as fluorescence imaging.

Quantitative phase imaging sends a pulse of a flat sheet of light towards the cell, then measures the phase shift of the light waves after they pass through the cell. Computer analysis then reconstructs an image of the major structures inside the cell. Ideguchi and his collaborators have previously pioneered other methods to enhance quantitative phase microscopy.

Quantitative phase imaging is a powerful tool for examining individual cells because it allows researchers to make detailed measurements, like tracking the growth rate of a cell based on the shift in light waves. However, the quantitative aspect of the technique has low sensitivity because of the low saturation capacity of the image sensor, so tracking nanosized particles in and around cells is not possible with a conventional approach.

The new ADRIFT-QPI method has overcome the dynamic range limitation of quantitative phase imaging. During ADRIFT-QPI, the camera takes two exposures and produces a final image that has seven times greater sensitivity than traditional quantitative phase microscopy images.

The first exposure is produced with conventional quantitative phase imaging — a flat sheet of light is pulsed towards the sample and the phase shifts of the light are measured after it passes through the sample. A computer image analysis program develops an image of the sample based on the first exposure then rapidly designs a sculpted wavefront of light that mirrors that image of the sample. A separate component called a wavefront shaping device then generates this “sculpture of light” with higher intensity light for stronger illumination and pulses it towards the sample for a second exposure.

If the first exposure produced an image that was a perfect representation of the sample, the custom-sculpted light waves of the second exposure would enter the sample at different phases, pass through the sample, then emerge as a flat sheet of light, causing the camera to see nothing but a dark image.

“This is the interesting thing: We kind of erase the sample’s image. We want to see almost nothing. We cancel out the large structures so that we can see the smaller ones in great detail,” Ideguchi explained.

In reality, the first exposure is imperfect, so the sculptured light waves emerge with subtle phase deviations.

The second exposure reveals tiny light phase differences that were “washed out” by larger differences in the first exposure. These remaining tiny light phase difference can be measured with increased sensitivity due to the stronger illumination used in the second exposure.

Additional computer analysis reconstructs a final image of the sample with an expanded dynamic range from the two measurement results. In proof-of-concept demonstrations, researchers estimate the ADRIFT-QPI produces images with seven times greater sensitivity than conventional quantitative phase imaging.

Ideguchi says that the true benefit of ADRIFT-QPI is its ability to see tiny particles in context of the whole living cell without needing any labels or stains.

“For example, small signals from nanoscale particles like viruses or particles moving around inside and outside a cell could be detected, which allows for simultaneous observation of their behavior and the cell’s state,” said Ideguchi.