May 20, 2016 01:51 AM
Switch and stick: Potential new adhesive can be turned on and off
https://www.sciencedaily.com/releases/20...100713.htm
RELEASE: Some adhesives may soon have a metallic sheen and be particularly easy to unstick. Researchers at the Max Planck Institute for Intelligent Systems in Stuttgart are suggesting gallium as just such a reversible adhesive. By inducing slight changes in temperature, they can control whether a layer of gallium sticks or not. This is based on the fact that gallium transitions from a solid state to a liquid state at around 30 degrees Celsius. A reversible adhesive of this kind could have applications everywhere that temporary adhesion is required, such as industrial pick-and-place processes, transfer printing, temporary wafer bonding, or for moving sensitive biological samples such as tissues and organs. Switchable adhesion could also be suitable for use on the feet of climbing robots.
The principle is actually quite simple: Above 30 degrees Celsius, gallium metal is liquid, and below 30 degrees it is solid. So if a drop of liquid gallium is introduced between two objects and then cooled to less than 30 degrees, the gallium layer solidifies and sticks the two objects together. When it is time to separate the objects, the temperature is raised to transition the gallium layer to its liquid state and they can be pulled apart with a small amount of unloading force. As an adhesive, gallium works in a similar fashion to hot glue, widely used in DIY applications. The difference is that far less heating and cooling are sufficient in the case of gallium, it lifts much more easily and cleanly from the surface, it is highly repeatable, and it is electrically conductive.
For their experiments, scientists working with Metin Sitti, Director at the Max Planck Institute for Intelligent Systems, wet the tip of a cylindrical elastomer rod with liquid gallium. They then brought the gallium droplet into contact with different materials such as glass, plastic and gold. After cooling the tip to 23 degrees, they found that the solidified gallium formed a strong bond between the elastomer and each of the materials.
Tests on particularly rough or damp surfaces
The researchers also took direct measurements of the effective binding power of gallium in both its liquid and solid phases. "The behaviour of these two values tells us something about the true reversibility and switchability of the adhesion process," explains Metin Sitti. The greater the difference of the binding power between the liquid and solid state, the easier it is to reverse and switch the adhesive effect.
The team deliberately tested gallium on particularly rough and damp surfaces as well. "These are surface conditions that showed up as major weaknesses of reversible micro/nanostructured adhesives proposed recently," says Sitti. How so? Adhesives that have yielded strong binding values on rough or wet surfaces to date have always had poor reversibility. Not so with the new gallium approach. The Stuttgart-based team have become convinced of its effectiveness in damp conditions, even testing it under water. Its binding power and reversibility when wet were reduced compared to dry conditions, but they still remained relatively strong for a wide range of applications.
Application wherever careful and reversible adhesion is required
Metin Sitti emphasises that gallium's performance in damp conditions makes it ideal for biological applications. The scientist and engineer foresees a time when gallium may be used to move individual cells, tissue samples or even organs, for example in laboratory or hospital settings.
Another possible field of application is industrial manufacturing, especially where fragile components such as ultra-thin graphene membranes or tiny electronic chips are involved. These components could be picked up by gallium-coated grippers and then set down at the precise location where they are required, e.g. a circuit board. In technical jargon, this kind of assembly technology is called "pick and place." It already exists today, but is generally based on the use of vacuum suction.
Metin Sitti believes the temperature-controlled gallium adhesive has two advantages. "Wetting an object with a metallic liquid such as gallium that forms a bond when cooled slightly is a far more gentle process for fragile materials than sucking them up using a vacuum," he expounds, adding that the new methods are also more energy-efficient. Once an object adheres to the gallium layer, no more energy is required to sustain the adhesive bond. Only when it is time to reverse the adhesion must the metal be quickly heated to 30 degrees. The vacuum technique, however, requires the constant use of suction in order to maintain the adhesive effect.
Temperature control for phase change of gallium
To achieve rapid heating and cooling as required in their tests, the team in Stuttgart connected a Peltier element to their experiment set-up. This element can either release or absorb heat when an electric current is applied. However, for practical applications in the future, the scientists anticipate that the adhesive bond could also be reversed using infrared radiation remotely or using electrical Joule heating by integrating conductive wiring to the surface.
Metin Sitti sees robotics as another possible application for this adhesive. For example, climbing robots such as those that may one day ascend wind turbines for maintenance purposes could benefit from reversible adhesives. By activating the adhesive, the robot foot would be fixed to the wall of the turbine, and for the next step, the adhesive layer between the foot and the wall would be briefly heated by means of an integrated heating element.
An adhesive that doesn't run out
A consideration of major importance for practical applications is that the material should be able to be used for as many cycles as possible without the need to replace it. Gallium conforms to this requirement, because the liquid metal lifts completely from the substrate with proper loading and unloading conditions. No residues are left on the surface, and the adhesive loses none of its own substance. This is by no means something to be taken for granted. "Good adhesives are generally hard to separate from the substrate," states Sitti, explaining that in gallium's case, the material forms a fine oxide layer in air. This shell of gallium oxide retains the gallium and ensures that no residues are left behind when the adhesion is reversed.
And that's not all. Gallium has even more to offer: "We can use it at different scales, from the nanometre range to microelectronics, and right up to larger applications," says Sitti with a smile. In theory, it could even be used to lift a fully-grown person as long as the contact surface was sufficiently large. However, it would be most cost-effective, energy efficient, and practical with smaller objects.
Metin Sitti believes that this method could be used in practical applications in the near future. And his team has started exploring some of these potential applications already. At the same time, they are working to optimize the technique. Until now, for example, the gallium was applied to an elastomer rod around two millimetres in diameter for all tests. "We want to test other elastomer geometries and designs with different length scales and see if we can enhance the binding strength as we do so," says Sitti. The scientists also plan to study alloys of gallium with other metals such as indium, but they will be watching closely to ensure that the melting point is close to normal ambient temperature.
New angles on visual cloaking of everyday objects
https://www.sciencedaily.com/releases/20...120708.htm
RELEASE: Using the same mathematical framework as the Rochester Cloak, researchers at the University of Rochester have been able to use flat screen displays to extend the range of angles that can be hidden from view. Their method lays out how cloaks of arbitrary shapes, that work from multiple viewpoints, may be practically realized in the near future using commercially available digital devices.
The Rochester researchers have shown a proof-of-concept demonstration for such a setup, which is still much lower resolution than the nearly perfect imaging achieved by the Rochester Cloak lenses. But with increasingly higher resolution displays becoming available, the "digital integral cloak" they describe in their new Optica paper will continue to improve.
While the Rochester Cloak offered a simple way of cloaking, it was limited by the cloaking working only over small angles, and cloaking large objects would require large, expensive lenses.
By breaking up the information into distinct pieces, it becomes possible to use currently available digital cameras and digital displays. The Rochester researchers use a camera to scan a background and then encode the information in such a way that every pixel on a screen offers a unique view of a given point on the background for a given position of a viewer. By doing this for many views and using lenticular lenses -- a sheet of plastic with an array of thin, parallel semicylindrical lenses -- they can recreate multiple images of the background, each corresponding to a viewer at a different position. So if the viewer moves from side to side, every part of the background moves accordingly as if the screen was not there, "cloaking" anything in the space between the screen and the background.
In the current system, it takes PhD student Joseph Choi and his advisor Professor of Physics John Howell several minutes to scan, process and update the image on the screen, i.e. to update the background. But Choi explains they are hoping soon to be able to do this in real-time, even if at lower resolution.
Their mathematical framework and their proof-of-concept setup also demonstrates how any object of a fixed size can be cloaked, even when in motion -- so long as the shape of the object remains fixed and does not deform. To do this one side of the object would be covered in an array of sensors -- effectively cameras -- and the other side in pixels with tiny lenses over them. Choi's and Howell's approach could then be used to identify which sensors need to feed into which pixels so as to show the background as if an object wasn't there. A similar trick has been used in advertising, but for one viewing angle only. However, by using the Rochester group's setup, a car, for example, could be made invisible to viewers from multiple positions, not just to a person at a predetermined position.
Scientists create 'rewritable magnetic charge ice'
https://www.sciencedaily.com/releases/20...144533.htm
RELEASE: A team of scientists working at the U.S. Department of Energy's (DOE) Argonne National Laboratory and led by Northern Illinois University physicist and Argonne materials scientist Zhili Xiao has created a new material, called "rewritable magnetic charge ice," that permits an unprecedented degree of control over local magnetic fields and could pave the way for new computing technologies.
The scientists' research report on development of magnetic charge ice is published in the May 20, 2016 issue of the journal Science. With potential applications involving data storage, memory and logic devices, magnetic charge ice could someday lead to smaller and more powerful computers or even play a role in quantum computing, Xiao said.
Current magnetic storage and recording devices, such as computer hard disks, contain nanomagnets with two polarities, each of which is used to represent either 0 or 1 -- the binary digits, or bits, used in computers. A magnetic charge ice system could have eight possible configurations instead of two, resulting in denser storage capabilities or added functionality unavailable in current technologies.
"Our work is the first success achieving an artificial ice of magnetic charges with controllable energy states," said Xiao, who holds a joint appointment between Argonne and NIU. "Our realization of tunable artificial magnetic charge ices is similar to the synthesis of a dreamed material. It provides versatile platforms to advance our knowledge about artificial spin ices, to discover new physics phenomena and to achieve desired functionalities for applications."
Over the past decade, scientists have been highly interested in creating, investigating and attempting to manipulate the unusual properties of "artificial spin ices," so-called because the spins have a lattice structure that follows the proton positioning ordering found in water ice.
Scientists consider artificial spin ices to be scientific playgrounds, where the mysteries of magnetism might be explored and revealed. However, in the past, researchers have been frustrated in their attempts to achieve global and local control of spin-ice magnetic charges.
To overcome this challenge, Xiao and his colleagues decoupled the lattice structure of magnetic spins and the magnetic charges. The scientists used a bi-axis vector magnet to precisely and conveniently tune the magnetic charge ice to any of eight possible charge configurations. They then used a magnetic force microscope to demonstrate the material's local write-read-erase multi-functionality at room temperature.
For example, using a specially developed patterning technique, they wrote the word, "ICE," on the material in a physical space 10 times smaller than the diameter of a human hair.
Magnetic charge ice is two-dimensional, meaning it consists of a very thin layer of atoms, and could be applied to other thin materials, such as graphene. Xiao said the material also is environmentally friendly and relatively inexpensive to produce.
Yong-Lei Wang, a former postdoctoral research associate of Xiao's, is first author and co-corresponding author on the Science article. He designed the new artificial magnetic ice structure and built custom instrumentation for the research.
"Although spin and magnetic charges are always correlated, they can be ordered in different ways," said Wang, who now holds a joint appointment with Argonne and Notre Dame. "This work provides a new way of thinking in solving problems. Instead of focusing on spins, we tackled the magnetic charges that allow more controllability."
There are hurdles yet to overcome before magnetic charge ice could be used in technological devices, Xiao added. For example, a bi-axis vector magnet is required to realize all energy state configurations and arrangements, and it would be challenging to incorporate such a magnet into commercial silicon technology.
But in addition to uses in traditional computing, Xiao said quantum computing could benefit from magnetic monopoles in the charge ice. Other potential applications of magnetic charge ice might include enhancement of the current-carrying capability of superconductors.
https://www.sciencedaily.com/releases/20...100713.htm
RELEASE: Some adhesives may soon have a metallic sheen and be particularly easy to unstick. Researchers at the Max Planck Institute for Intelligent Systems in Stuttgart are suggesting gallium as just such a reversible adhesive. By inducing slight changes in temperature, they can control whether a layer of gallium sticks or not. This is based on the fact that gallium transitions from a solid state to a liquid state at around 30 degrees Celsius. A reversible adhesive of this kind could have applications everywhere that temporary adhesion is required, such as industrial pick-and-place processes, transfer printing, temporary wafer bonding, or for moving sensitive biological samples such as tissues and organs. Switchable adhesion could also be suitable for use on the feet of climbing robots.
The principle is actually quite simple: Above 30 degrees Celsius, gallium metal is liquid, and below 30 degrees it is solid. So if a drop of liquid gallium is introduced between two objects and then cooled to less than 30 degrees, the gallium layer solidifies and sticks the two objects together. When it is time to separate the objects, the temperature is raised to transition the gallium layer to its liquid state and they can be pulled apart with a small amount of unloading force. As an adhesive, gallium works in a similar fashion to hot glue, widely used in DIY applications. The difference is that far less heating and cooling are sufficient in the case of gallium, it lifts much more easily and cleanly from the surface, it is highly repeatable, and it is electrically conductive.
For their experiments, scientists working with Metin Sitti, Director at the Max Planck Institute for Intelligent Systems, wet the tip of a cylindrical elastomer rod with liquid gallium. They then brought the gallium droplet into contact with different materials such as glass, plastic and gold. After cooling the tip to 23 degrees, they found that the solidified gallium formed a strong bond between the elastomer and each of the materials.
Tests on particularly rough or damp surfaces
The researchers also took direct measurements of the effective binding power of gallium in both its liquid and solid phases. "The behaviour of these two values tells us something about the true reversibility and switchability of the adhesion process," explains Metin Sitti. The greater the difference of the binding power between the liquid and solid state, the easier it is to reverse and switch the adhesive effect.
The team deliberately tested gallium on particularly rough and damp surfaces as well. "These are surface conditions that showed up as major weaknesses of reversible micro/nanostructured adhesives proposed recently," says Sitti. How so? Adhesives that have yielded strong binding values on rough or wet surfaces to date have always had poor reversibility. Not so with the new gallium approach. The Stuttgart-based team have become convinced of its effectiveness in damp conditions, even testing it under water. Its binding power and reversibility when wet were reduced compared to dry conditions, but they still remained relatively strong for a wide range of applications.
Application wherever careful and reversible adhesion is required
Metin Sitti emphasises that gallium's performance in damp conditions makes it ideal for biological applications. The scientist and engineer foresees a time when gallium may be used to move individual cells, tissue samples or even organs, for example in laboratory or hospital settings.
Another possible field of application is industrial manufacturing, especially where fragile components such as ultra-thin graphene membranes or tiny electronic chips are involved. These components could be picked up by gallium-coated grippers and then set down at the precise location where they are required, e.g. a circuit board. In technical jargon, this kind of assembly technology is called "pick and place." It already exists today, but is generally based on the use of vacuum suction.
Metin Sitti believes the temperature-controlled gallium adhesive has two advantages. "Wetting an object with a metallic liquid such as gallium that forms a bond when cooled slightly is a far more gentle process for fragile materials than sucking them up using a vacuum," he expounds, adding that the new methods are also more energy-efficient. Once an object adheres to the gallium layer, no more energy is required to sustain the adhesive bond. Only when it is time to reverse the adhesion must the metal be quickly heated to 30 degrees. The vacuum technique, however, requires the constant use of suction in order to maintain the adhesive effect.
Temperature control for phase change of gallium
To achieve rapid heating and cooling as required in their tests, the team in Stuttgart connected a Peltier element to their experiment set-up. This element can either release or absorb heat when an electric current is applied. However, for practical applications in the future, the scientists anticipate that the adhesive bond could also be reversed using infrared radiation remotely or using electrical Joule heating by integrating conductive wiring to the surface.
Metin Sitti sees robotics as another possible application for this adhesive. For example, climbing robots such as those that may one day ascend wind turbines for maintenance purposes could benefit from reversible adhesives. By activating the adhesive, the robot foot would be fixed to the wall of the turbine, and for the next step, the adhesive layer between the foot and the wall would be briefly heated by means of an integrated heating element.
An adhesive that doesn't run out
A consideration of major importance for practical applications is that the material should be able to be used for as many cycles as possible without the need to replace it. Gallium conforms to this requirement, because the liquid metal lifts completely from the substrate with proper loading and unloading conditions. No residues are left on the surface, and the adhesive loses none of its own substance. This is by no means something to be taken for granted. "Good adhesives are generally hard to separate from the substrate," states Sitti, explaining that in gallium's case, the material forms a fine oxide layer in air. This shell of gallium oxide retains the gallium and ensures that no residues are left behind when the adhesion is reversed.
And that's not all. Gallium has even more to offer: "We can use it at different scales, from the nanometre range to microelectronics, and right up to larger applications," says Sitti with a smile. In theory, it could even be used to lift a fully-grown person as long as the contact surface was sufficiently large. However, it would be most cost-effective, energy efficient, and practical with smaller objects.
Metin Sitti believes that this method could be used in practical applications in the near future. And his team has started exploring some of these potential applications already. At the same time, they are working to optimize the technique. Until now, for example, the gallium was applied to an elastomer rod around two millimetres in diameter for all tests. "We want to test other elastomer geometries and designs with different length scales and see if we can enhance the binding strength as we do so," says Sitti. The scientists also plan to study alloys of gallium with other metals such as indium, but they will be watching closely to ensure that the melting point is close to normal ambient temperature.
New angles on visual cloaking of everyday objects
https://www.sciencedaily.com/releases/20...120708.htm
RELEASE: Using the same mathematical framework as the Rochester Cloak, researchers at the University of Rochester have been able to use flat screen displays to extend the range of angles that can be hidden from view. Their method lays out how cloaks of arbitrary shapes, that work from multiple viewpoints, may be practically realized in the near future using commercially available digital devices.
The Rochester researchers have shown a proof-of-concept demonstration for such a setup, which is still much lower resolution than the nearly perfect imaging achieved by the Rochester Cloak lenses. But with increasingly higher resolution displays becoming available, the "digital integral cloak" they describe in their new Optica paper will continue to improve.
While the Rochester Cloak offered a simple way of cloaking, it was limited by the cloaking working only over small angles, and cloaking large objects would require large, expensive lenses.
By breaking up the information into distinct pieces, it becomes possible to use currently available digital cameras and digital displays. The Rochester researchers use a camera to scan a background and then encode the information in such a way that every pixel on a screen offers a unique view of a given point on the background for a given position of a viewer. By doing this for many views and using lenticular lenses -- a sheet of plastic with an array of thin, parallel semicylindrical lenses -- they can recreate multiple images of the background, each corresponding to a viewer at a different position. So if the viewer moves from side to side, every part of the background moves accordingly as if the screen was not there, "cloaking" anything in the space between the screen and the background.
In the current system, it takes PhD student Joseph Choi and his advisor Professor of Physics John Howell several minutes to scan, process and update the image on the screen, i.e. to update the background. But Choi explains they are hoping soon to be able to do this in real-time, even if at lower resolution.
Their mathematical framework and their proof-of-concept setup also demonstrates how any object of a fixed size can be cloaked, even when in motion -- so long as the shape of the object remains fixed and does not deform. To do this one side of the object would be covered in an array of sensors -- effectively cameras -- and the other side in pixels with tiny lenses over them. Choi's and Howell's approach could then be used to identify which sensors need to feed into which pixels so as to show the background as if an object wasn't there. A similar trick has been used in advertising, but for one viewing angle only. However, by using the Rochester group's setup, a car, for example, could be made invisible to viewers from multiple positions, not just to a person at a predetermined position.
Scientists create 'rewritable magnetic charge ice'
https://www.sciencedaily.com/releases/20...144533.htm
RELEASE: A team of scientists working at the U.S. Department of Energy's (DOE) Argonne National Laboratory and led by Northern Illinois University physicist and Argonne materials scientist Zhili Xiao has created a new material, called "rewritable magnetic charge ice," that permits an unprecedented degree of control over local magnetic fields and could pave the way for new computing technologies.
The scientists' research report on development of magnetic charge ice is published in the May 20, 2016 issue of the journal Science. With potential applications involving data storage, memory and logic devices, magnetic charge ice could someday lead to smaller and more powerful computers or even play a role in quantum computing, Xiao said.
Current magnetic storage and recording devices, such as computer hard disks, contain nanomagnets with two polarities, each of which is used to represent either 0 or 1 -- the binary digits, or bits, used in computers. A magnetic charge ice system could have eight possible configurations instead of two, resulting in denser storage capabilities or added functionality unavailable in current technologies.
"Our work is the first success achieving an artificial ice of magnetic charges with controllable energy states," said Xiao, who holds a joint appointment between Argonne and NIU. "Our realization of tunable artificial magnetic charge ices is similar to the synthesis of a dreamed material. It provides versatile platforms to advance our knowledge about artificial spin ices, to discover new physics phenomena and to achieve desired functionalities for applications."
Over the past decade, scientists have been highly interested in creating, investigating and attempting to manipulate the unusual properties of "artificial spin ices," so-called because the spins have a lattice structure that follows the proton positioning ordering found in water ice.
Scientists consider artificial spin ices to be scientific playgrounds, where the mysteries of magnetism might be explored and revealed. However, in the past, researchers have been frustrated in their attempts to achieve global and local control of spin-ice magnetic charges.
To overcome this challenge, Xiao and his colleagues decoupled the lattice structure of magnetic spins and the magnetic charges. The scientists used a bi-axis vector magnet to precisely and conveniently tune the magnetic charge ice to any of eight possible charge configurations. They then used a magnetic force microscope to demonstrate the material's local write-read-erase multi-functionality at room temperature.
For example, using a specially developed patterning technique, they wrote the word, "ICE," on the material in a physical space 10 times smaller than the diameter of a human hair.
Magnetic charge ice is two-dimensional, meaning it consists of a very thin layer of atoms, and could be applied to other thin materials, such as graphene. Xiao said the material also is environmentally friendly and relatively inexpensive to produce.
Yong-Lei Wang, a former postdoctoral research associate of Xiao's, is first author and co-corresponding author on the Science article. He designed the new artificial magnetic ice structure and built custom instrumentation for the research.
"Although spin and magnetic charges are always correlated, they can be ordered in different ways," said Wang, who now holds a joint appointment with Argonne and Notre Dame. "This work provides a new way of thinking in solving problems. Instead of focusing on spins, we tackled the magnetic charges that allow more controllability."
There are hurdles yet to overcome before magnetic charge ice could be used in technological devices, Xiao added. For example, a bi-axis vector magnet is required to realize all energy state configurations and arrangements, and it would be challenging to incorporate such a magnet into commercial silicon technology.
But in addition to uses in traditional computing, Xiao said quantum computing could benefit from magnetic monopoles in the charge ice. Other potential applications of magnetic charge ice might include enhancement of the current-carrying capability of superconductors.