A drop of water glides across the flat surface like quicksilver, moving effortlessly from place to place as the surface is tilted. It's hard to believe that the little bead is water, for it doesn't wet the surface as it races around, seemingly without friction. The little drop in this impromptu laboratory demonstration isn't on an ordinary surface. It's riding on "nanograss," a bed of upright silicon posts a thousand times thinner than a human hair. Nanograss is a whole new class of structure.. It is also possible to alter the properties of nanograss on the fly by changing the temperature, applying ultrasound or a small voltage, or other means. A voltage builds up an electrical field at the tips of the nanograss, and that changes its wettability through an effect called electrowetting. That could allow the electrodes and electrolytes in a battery to remain separated until the battery is needed, extending its shelf life indefinitely. Nanograss can be employed to develop a "smart" heat sink that can change its cooling properties as needs change, The idea is noteworthy because chip makers have found that heat dissipation is one of the greatest obstacles to making silicon circuits smaller than the current generation, at 90 nanometers. It's possible to construct tiny, cheap liquid lenses whose focal lengths and other properties can be changed very quickly by the application of electrical fields. Nanograss can also be used to make, filters, multiplexers and other devices in order to manipulate light in ways that are difficult to do by conventional means. Nanograss is an important technology because it combines a materials breakthrough with electronic properties that enhance the material and allow it to be used as a platform for a slew of applications.
Introduction:
Scientists at Bell Labs (Murray Hill, N.J.), the R&D arm of Lucent Technologies, have discovered a new method to control the behavior of tiny liquid droplets by applying electrical charges to specially engineered silicon surfaces that resemble blades of grass. The new technique of manipulating fluids has many potential applications. This specially engineered silicon surface is covered with billions of tiny posts, a forest of with each blade 30 times thinner than a red blood cell. Each post is covered with a non-stick, water-repellent material similar to the Teflon that coats a non-stick pan. The surface developed is termed as NANOGRASS as it is made of an array of nanosized posts attached to a flat surface and appears as an evenly cut lawn of grass.
Liquid interact with nanograss surface in a novel way, there by providing a way to precisely control their effects its surface. Physically, this technique reduces the surface area that the droplet feels, and reduces the interaction between the liquid and the substrate by a factor of a hundred to thousand. By applying a small voltage, however the behavior of droplets could be altered, making them sink in and wet the surface as directed. The droplets also respond to a change in temperature in the similar way. Some basic physical phenomenon related to nanograss is that it behaves as hydrophobic as well as hydrophilic surfaces under certain applied conditions.
In its uncharged state nanograss surface is hydrophobic in nature, if a drop of water is put on the surface, it forms a nearly perfect ball, and gets suspended on the tips of tiny blades of nanograss. This happens because the surface area felt by the liquid droplet reduces drastically, and the cohesive force between the molecules of the liquid overcomes the force of adhesion between the droplet and the surface of nanograss. The Fig 2 shows the hydrophobic behavior of the nanograss surface with a water droplet positioned on it.
Fig 3: Nanograss in it's Hydrophilic nature
When a small charge is applied to the surface it’s behavior changes and it becomes hydrophilic in nature. That is the force of adhesion between the molecules of nanograss and droplet increase above the force of cohesion between the molecules of the droplet. Fig 3 shows the hydrophilic nature of the nanograss surface. Due to hydrophilic nature of the surface the liquid droplet appears glued to the surface.
Comparing the images in Fig 2 and Fig 3 the contrasting nature of nanograss surface can be better understood. This drastic change in the surface nature is caused by the application of small electrical charge to the nanograss surface. The nanograss surface behaves in the similar manner to the change in the surface temperature. That is at normal temperature the surface is hydrophobic in nature, but as the temperature of the surface rises it gradually becomes hydrophobic.
Fabrication:
Materials and Methods:
m, 1.75 m, and 4.0 m) were investigated at the Bell Labs.
In the above shown fig each post has a diameter of about 350 nm and a height of about 7 m. The distance between posts varied from 1 m to 4 m. Three examples are shown: (a) pitch of 1.05 m (b) pitch of 1.75 m, and (c) pitch of 4 m.
The dots were printed on 200 mm single crystal Silicon wafers using 248 nm wavelength photolithography to create 4 x10 mm fields with the various pitches. Deep Silicon dry etching was used to etch the posts into the Silicon. To electrically isolate the posts 50 nm of thermal oxide was grown at 10500C in O2. To create a hydrophobic surface an additional fluorocarbon layer (approximately 20 nm thick) was deposited by plasma assisted chemical vapour deposition using C4F8 as the precursor.
Common liquids on nanostructured substrates:
180 .
Lower surface tension liquids such as (c) cyclopentanol, = 33 mN/m and (d) octanol, = 28 mN/m form immobile droplets with much lower contact angle. The unusual octahedral shape of the base of the droplets reflects the underlying symmetry of the nanopost array. Finally, the liquids with even lower surface tension such as (e) isopropanol, = 24 mN/m completely wetted the nanostructured areas of the substrate. The difference in the droplet shading between the left and the right halves of image (e) is due to the difference in the pitch of the underlying nanostructured substrate. A thin Pt wire inserted in the droplets was intended for use in subsequent electrowetting experiments.
a, In case where no voltage was applied the liquid formed a local contact angle 0 > 90 with the walls of the nanoposts. No liquid penetrated inside the nanostructured layer. b, When 4 sufficient voltage was applied between the liquid and the substrate, the local contact angle was reduced below 90 and the liquid wetted the nanostructured layer. c, Each nanoposts had a conductive core covered by a dielectric oxide and, then, by a conformal layer of low surface energy fluoropolymer.
Applications:
Integrated Chip Cooling
The latest trend in the microprocessor industry is to increase the computational capability of the chip and shrink it in size. As the computational capability of chip increases it draws more and more current from the power supply, which in turn results in temperature rise of the chip.
Inefficiency related with conventional cooling:
The image in Fig 10 shows the thermal profile of a chip with conventional cooling system employed. Different colors on the profile shows that the chip surface is not at uniform temperature. Which may hamper the normal working of the processors. Thus it proves that the conventional cooling systems face some drawbacks
Drawback in Immobile segment:
Heat is unevenly distributed across the chip. That is the temperature rise is not same along the surface of the chip. The chi may develop some spots with temperature 150 OC while rest of the chip is at 550C. The problem is that the hottest spot on the chip determines the cooling requirements. Even though the chip has few hot spots at 1500 C the whole chip is to be treated as if it is at 1500 C. hence a lot of resources is wasted in cooling the chip by conventional forced air cooling method.
Drawback in Mobile segment:
In cooling duct technique the duct covers the maximum possible area of the chip that may develop hotspot .But the uncovered area develops hotspots if the system is continuously used for a long time thereby degrading the system performance. In such circumstances the cooling duct technique fails to attain its goal of providing optimum temperature for the proper working of chip /processor.
Nanograss technique for IC cooling:
In this technique the whole nanograss surface is considered as a (M)X(N) matrix where each stack of nanograss is represented by a matrix cell (m*n ).
The droplet of coolant can be maneuvered over the surface of nanograss by applying charge to the nanograss stacks in sequential manner &; be positioned over the desired hotspot.
Let the liquid droplet present at point A on the nanograss surface & let the hot spot on the chip covered with nanograss surface denoted by point B .By applying charges sequentially to stacks between point A to point B, starting from point A. the liquid droplet can be maneuvered to point B.
Once the coolant reaches point B the application of charges could be stopped .Due to the presence of hot spot the temperature of Nanograss surface rises to at point B. and the Nanograss surface turns hydrophilic from hydrophobic. The coolant droplet gets drawn towards the base of nanograss thereby cooling it .As the temp. at point B drops back to normal the nanograss surface returns to it’s initial hydrophobic state.
Hence this technique promises better cooling option for the power hungry processors in a more efficient way using minimum possible resources.
Long Lasting Batteries:
The Fig 12 shows the basic construction of a battery. The construction of the system is such that the electrolyte and electrode of the batteries are continuously in contact with each other. Though slow but continuous contact between electrolyte and electrode, drains out the chemical energy stored in battery. This uncontrolled chemical reaction limits the shelf life of batteries that is batteries can only be stored for certain limited duration of time before becoming useless. The problem of battery shelf life can be solved if the electrode and the electrolyte could be isolated from each other till energy is not required, thereby lowering the rate of degradation of the system.
If the electrode of the battery system is coated with Nanograss, effective isolation between the electrode and electrolyte could be obtained. As nanograss is hydrophobic in its uncharged state it will restrict the electrolyte and electrode union.
Whenever energy is required to be drawn from the battery a small charge is to be applied to nanograss surface, which will turn it the surface hydrophilic thereby allowing electrode and electrolyte to come in contact and generate the required energy from the resulting electrolytic reaction.
The nanograss battery can reach high power rapidly and has a long shelf life, Compared with
a lithium-based battery typically used as a backup power supply that can take several seconds to activate, the nanograss battery, which uses zinc chloride and is smaller, activates in microseconds
This principle could be used to manufacture micro batteries with extended life. the question arises that if the battery is initially in isolation, from where should the energy be drawn to get the battery system out of the isolation. The answer lies in nuclear batteries. At Corenell
University researchers are trying to make a nuclear powered battery with a very long life span. The prototype batteries developed at Corenell University uses spike of Nickel 63 (a radio active isotope) to vibrate a tiny cantilever, the cantilever could be
made from a piece of piezoelectric material, which could supply power to the nanograss surface whenever required. Nickel 63 has a half life of around 100 year, so it could provide power for several decades. This combination of nuclear and nanograss batteries will prove a boon to soldiers and space missions. A typical soldier carries between 20 and 30 pounds of batteries, and in a satellite system approximately 20% of the weight is of batteries.
Optical Switching:
The rate of failure decreases as the complexity of the system decreases. Using nanograss very simple and efficient optical switch can be designed. By moving a droplet of liquid directly into the path of ray of light the direction of radiation can be altered.
The above diagram shows the main and auxiliary optical fibers with a basic conventional optical switch. Whenever there is a need to switch from main fiber to auxiliary fiber the mirror M is moved such that it bends the light and allows it to enter the auxiliary fiber, thereby switching communication from main fiber to auxiliary fiber.
In nanograss technique the mirror is replaced by a nanograss surface with a droplet of transparent liquid on it. When the droplet is at position A, the light ray directly enters the main fiber without undergoing any change in the path of radiation/propagation. For switching from main fiber to auxiliary fiber charge is applied sequentially from point A to point B such that the liquid droplet present at point A is drawn and positioned at point B. At point B due to the basic property of refraction of light the way of light bends as it passes through the liquid droplet such that it enters the auxiliary fiber. Thus, switching the communication from main fiber to auxiliary fiber.
This method of optical switching does not employ any kind of mechanical or complex circuitry for switching and hence be regard as much more reliable as compared to conventional switching.
Water transport :
60% share. Due to improved deigning and efficient engines on vessels water transport has increased by 4 folds in last 2 decades and has established it self as an multimillion industry. Though Designers are coming up with better and more streamlined hull deigns the basic problem of drop offered to the moving vessel by surrounding water is a matter of concerns, mainly in large oil tankers.
The Fig 16 shows the most common vessel Design used by vessel manufacturer around the world. Out of the total surface area of vessel 29 percent (Fig 16(A) ) and 32 percent (Fig
16(B) ) area of vessels are continuously in contact with water. Vessel engine become more fuel efficient as the drag offered to the surface by the surrounding water decreases.
The drag offered to the vessel could be reduced to approximately 6.73% if surface which is permanently in contact with water is coated with water repellent nanograss surface. Applying this technique would not increase the operating complexity of system as nanograss in its add considerably to the weight of the vessel.
Defense :
U.S navy is exploring the possible application of dual nature of nanograss to improve its War capabilities under water as the drag offered by water around submarines and torpedoes not only reduces its speed but also limits its effective range.
If the whole surface of under water submarines and torpedoes is covered with nanograss surface retarding effect of water on these bodies will increase their effective range and efficiency.
The Fig shows the schematic representation of an US Submarine moving ahead for a 1800 turn. Trajectory T1 is the normal trajectory that the submarine would follow with conventional fin steering systems. While the trajectory T2 is the calculated trajectory of the submarine covered with nanograss. The nanograss cover over the submarine is partially made hydrophilic. As surrounding water offers more resistance to hydrophilic surface as compared to hydrophobic part it helps in steering this submarine with more ease and comparatively less time is required to complete the 1800 turn. According to the data released by DoD United States almost 27.34 % less time is required to complete the 1800 in an experiment performed under Standard Laboratory Conditions.
Conclusion:
Nanograss technology is still in it’s infancy. But this breakthrough discovery has the potential to revolutionize not only the electronics sector but also the allied consumer market. mPhase technologies has moved into an $400 million contract with Bell Labs for the research and development of micro batteries based on Nanograss. In a press release on 28th September
2004 mPhase technologies announced their first product, micro batteries based on Nanograss to be released in commercial market with in next 12 months. Also American Department Of Defence has agreed to put in $300 million in Nanograss technology by 2007. many more commercial products like self cleaning wind shields, lab-on-chip etc are being developed by different manufacturers around the world. This response from commercial as well as from other allied fields had made sure that this technique will not suffer a premature death.
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