How birds avoid crashes

The secret of how birds zip flawlessly through narrow spaces without crashing into obstacles has been unlocked by Australian scientists.

Their discovery could be used to design ‘bird-safe’ buildings and windmills, and improve the versatility of pilotless aircraft.

Researchers at The Vision Centre have found that birds weave rapidly and safely through dense forests and narrow corridors by using their eyes to sense the speed of background image flow on both sides and adjust their flight according to it.

“As animals travel forward, things that are close seem to speed by, and things that are farther away seem to travel more slowly,” says Professor Mandyam Srinivasan from The Vision Centre and The University of Queensland, whose team made the discovery. “It’s the same for birds. We found that they try to achieve a safe ‘balance’ by ensuring that the background images are passing at the same speed in both eyes.

“This means that if the bird flies closer to obstacles on one side, the near eye will see things passing by faster while those seen by its other eye will pass more slowly. This imbalance prompts the bird to veer away to even out the speed of image flow in both eyes.”

To find out how birds navigate through narrow passages and away from danger, the research group trained budgerigars to fly along a corridor with walls lined with horizontal or vertical stripes, says Dr Partha Bhagavatula of The Vision Centre and The National Vision Research Institute.


“We found that birds fly the fastest when both walls are lined horizontal stripes, because the stripes are parallel to the bird’s flight direction, and the birds don’t ‘see’ a strong image flow in the background,” Dr Bhagavatula says. “But when both walls have vertical stripes, birds slow down significantly due to the strong image motion, which shows that birds also regulate their flight speed according to what they see.”

When the walls were set up with different orientations – one with vertical and the other with horizontal stripes, the group found that birds flew significantly closer to the horizontal stripes.

“As vertical stripes project a stronger image flow to their corresponding eye, they veer away to restore the balance between the flows experienced by their two eyes,” says Dr Bhagavatula. “This was also demonstrated when one wall was left completely blank. Then the birds flew very close to, and occasionally collided with, the blank side.”

Prof. Srinivasan says that flight behaviour in birds is very similar to insects such as honeybees, bumblebees and flies: “This suggests that this principle of visual guidance may be shared by all day-active flying animals.

“Furthermore we believe these findings can contribute to the technology of guiding unmanned aerial vehicles where aircraft have to fly through obstacles in cluttered environments, or through canyons and gorges, or under bridges.

“Another potential application is the design of urban structures that are more bird-friendly to minimise the risk of bird fatalities through collisions with window panes. We can also think about decorating windmill blades with patterns that generate motion signals to repel birds.”

The group’s paper “Optic flow cues guide flight in birds” by Partha Bhagavatula, Charles Claudianos, Michael Ibbotson and Mandyam Srinivasan has been published in the latest issue of Current Biology.

Polymer Composites Containing Nanotubes (CNTs)

1. Introduction
Polymer composites containing carbon nanotubes (CNTs) have emerged as advanced multifunctional materials in view of exceptional mechanical, thermal and electrical properties associated with CNT [1]. They have become attractive structural materials not only in the weight-sensitive aerospace industry, but also in the marine, armor, automobile, railway, civil engineering structures, and sporting goods industries because of their high specific strength and specific stiffness. Different polymer/CNT nanocomposites have been synthesized by incorporating carbon nanotubes (CNTs) into various polymer matrices, such as polyamides [2], polyimides [3–5], epoxy [6], polyurethane [7–8] and polypropylene [9–11]. The presence of the nanotubes can improve the properties of polymers as well as add multi-functionality to polymer based composite systems [12-14]. Among the resins, epoxy resins have good stiffness, specific strength, dimensional stability and chemical resistance, and show considerable adhesion to the embedded filler [15].
Many studies have been conducted on multiwalled carbon tube (MWNT) based composites earlier and recently on aligned carbon tube (ACNT). MWNT composites have shown improved result in tensile modulus and yield strength [16]. However some studies reported decrease in flexural strength and pull out of CNTs in those composites [17]. In order to detect orientation and deformation of the CNTs in the nanocomposites, the tensile behaviour of both random and aligned MWNTs/Polystyrene nanocomposites was investigated by Thostenson and Chou [18]. They found the aligned CNTs composites showed more improved mechanical properties than random CNTs composites. Therefore, the comparison of different CNTs to obtain an efficient composite is very important. The purpose is to compare the suitability of these fillers in a polymer matrix that can be aimed for structural applications in aerospace/ automotive industries. In view of this in the present study, two types of nanotubes were dispersed in epoxy matrix. The mechanical properties of CNTs reinforced epoxy composites were measured using the flexural and
hardness tests. Fracture behaviour and crack propagation of the nanocomposites are studied by scanning electron microscopy. The pure epoxy samples were also prepared and subjected to the tests for comparison.
2. Experiments
A. Materials
Multiwall carbon nanotubes (MWCNTs) used for the preparation of nanocomposites was obtained from MER corporation, USA. They are produced by arc plasma method (purity 95%, length 1-5 μm and diameters 20-70 nm). SEM morphology of the products (Fig. 1) was carried out with a "JEOL JSM-6480 LV Scanning Microscope". They are highly entangled and randomly oriented.
Fig.1 SEM of Random carbon nanotubes (RCNTs)
Aligned carbon nanotubes (ACNTs) used for the preparation of nanocomposites was obtained from ARCI, Hyderabad, India. They are produced by chemical vapor deposition method diameters 10-20 nm. SEM morphology of the products (Fig.2) was carried out with a “JEOL JSM 5410 Scanning Microscope".
Fig.2 SEM of Aligned carbon nanotubes (ACNTs)
Epoxy polymer matrix was prepared by mixing epoxy resin (Ciba-Geigy, araldite LY-556 based on Bisphenol A) and hardener HY-951 (aliphatic primary amine) in wt. ratio 100/12. Epoxy resin (5.3-5.4 equiv/kg) was of low processing viscosity and good overall mechanical properties.
B. Nanocomposite preparation
Two types of nanocomposites involving Randomly oriented multiwalled carbon nanotubes (RCNTs) and aligned multiwalled carbon nanotubes (ACNTs) were prepared. First the nanotubes of both types were treated in ethanol for the deagglomeration of the tube bundles. The treated tubes (0.5 %) were then added to the epoxy resin and sonicated for 2hrs at room temperature.Hardener was added to that mixture and stirred. Then the mixture was poured into a mold and cured under vacuum at 900 C for 10 hrs. Pure resin samples have also been prepared for comparison purpose.
C. Flexural strength test
From each sample, five rectangular specimens were taken for three-point bend test as per ASTM D790 (width=2.7cm, thickness=0.7cm, span=11.2cm, length=12cm). Flexural tests were carried out at ambient temperature using Instron 1195 keeping the cross-head speed 2 mm/min. Flexural modulus of each sample was determined from the average value of five specimens.
D. Hardness of nanotube composites
The hardness of all samples was measured using a micro-hardness tester. A total of 10 points on each of the nanotube composites were measured in order to get average readings. The unit and magnitude of the hardness are defined by Vickers hardness, Hv and determined by measuring the average diagonal length, d of the indentation (mm).
E. Surface topography characterization
Scanning electron microscope (JEOL JSM-6480 LV) was used to conduct the dispersion behaviour and fracture surface topography characterization. After mechanical test fracture surfaces were coated with a thin platinum layer.
3. Results and Discussion
A. Flexural measurements
Flexural modulus of pure resin, RCNT composite & ACNT composite are
shown in Fig.3. Both the composite samples are showing greater modulus than pure resin sample that is attributed to the high mechanical strength of CNT. Flexural Test02004006008001000EpoxyEpoxy/RCNTEpoxy/ACNTSample TypesFlexural Modulus in MPa
Fig 3 Flexural moduli of resin as well as composite samples
The flexural modulus was found to be 136.86 MPa in case of epoxy/RCNT composite which is about six times than the flexural modulus of plane epoxy sample (24.52 MPa). Increase in modulus is more pronounced in epoxy/ACNT composite i. e. 837.42 MPa which is more than six times that of epoxy/RCNT composite and thirty four times that of epoxy sample. This may be due to efficient load transfer from matrix to aligned CNT in
axial direction. Local stiffening due to nanotubes results in improved load transfer at the fibre/matrix interface [18]. It had been reported that the increase in elastic modulus between the random and aligned nanocomposite is a consequence of the nanotube orientation, not polymer chain orientation.
B. Hardness measurements Microhardness Test 0102030405060EpoxyEpoxy/RCNTEpoxy/ACNTSamplesHardness in MPa
Fig. 4 Hardness of epoxy and epoxy/ nanotube composites
A considerable enhancement of hardness is observed by the nanocomposites in comparison to pure resin sample (Fig.4). Pure resin samples showed hardness of 12 MPa. Epoxy/RCNT had hardness value of 18 MPa which is 50% more and epoxy/ACNT had 49 MPa that is about four times that of epoxy sample. High strength and long nanotube reinforcements may result in forming a network structure that improves the hardness of the composites.
C. Surface topography analysis
The investigation of the fractured surfaces to analyse the micro deformation and crack propagation mechanism has been carried out using Scanning electron microscope (SEM). The micrographs corresponding to nanocomposites obtained by incorporating RCNTs and ACNTs are presented. From Fig.5a & b, the fracture surface of epoxy/RCNT appears to be rough than epoxy/ACNT composites.
Fig.5 Fracture surface of (a) epoxy/RCNT (b) epoxy/ACNT composite sample
The resultant failure mechanism of the epoxy/RCNT interface was analysed by observing the crack propagation regions within the composite (Fig. 6a). An agglomeration of several carbon nanotubes was observed in the fracture surface near the crack region. At a low stress level, the agglomerated particle increases the stiffness of the material, but at a high stress level, the stress concentration caused by the agglomerated particle initiates a crack, which make the sample fail quickly. Some nanotubes were observed to be pulled out which might be the result of a poor interfacial bonding between the nanotubes and matrix. Therefore, the nanotubes inside the composites could not take
up the load, which resulted in the decrease of flexural strength of the nanotube composite beams [19]. In comparison to that in case of epoxy/ ACNT composites, no crack was observed though the surface was seen to undergo less intensive fracture with smaller crack lengths in vertical direction (Fig. 6b).
Fig.6 Crack features of (a) epoxy/RCNT (b) epoxy/ACNT composite sample
Higher magnification showed a crack interacting with the nanotube reinforcement. RCNT matrix pullout was observed along with extension and bridging of RCNTs across the crack (Fig.7a). In epoxy/ACNT composites, the cracks were spanned by the nanotubes causing enhanced resistance to the crack propagation process. The bridging of the nanotubes as a mechanism of inhibiting the crack initiation in polymer and ceramic based nanocomposites has been well illustrated in literature [20-24].
Fig.7 Bridging mechanism of (a) epoxy/RCNT (b) epoxy/ACNT composite sample
From the above discussion, the evident difference between fracture surfaces indicated that the reinforcing role of nanotubes in the two kinds of polymer nanocomposites was different. The mechanical tests described above further supported this.
4. Conclusions
All the composite samples demonstrated enhanced mechanical properties than pure resin samples that were attributed to addition of high strength nanotubes. In addition, aligned nanotube composites resulted in significantly improved flexural modulus and hardness indicating that there is efficient load transfer between the polymer matrix and the nanotube reinforcement along axial direction. Reduction in flexural modulus and hardness value in epoxy/ RCNT composites was due to formation of agglomerates of nanotubes inside polymer matrix that reduced reinforcing effects of the CNTs by acting as flaws in the resin. Investigation of fracture surface in nanocomposite revealed that narrower crack-tips underneath the advancing cracks were more efficiently bridged by the nanotubes in epoxy/ACNT resulting in an increased resistance against crack propagation.

Applications Abound For Nano Field

“If you ask 10 people what I study, you may get 10 answers,” says Krystyn Van Vliet. That’s because the general phenomenon she explores — how a material’s mechanical properties, such as stiffness, can affect its chemistry, and vice versa — is applicable to so many disciplines. Van Vliet’s work has already, for example, led to important insights on everything from the formation of blood vessels to the structural dynamics of cement.

Key to her approach: studying these chemomechanical interactions at the nanoscale, or billionths of a meter.

Van Vliet, the Paul M. Cook Career Development Associate Professor of Materials Science and Engineering and Biological Engineering, is at the forefront of a relatively new field called chemomechanics. The tools that can probe materials’ mechanical properties at the most basic level of atomic features have been around only since the early 1990s.

Van Vliet recalls needing to travel to Lawrence Livermore National Lab to run experiments for her MIT doctorate because the Institute didn’t have the equipment. Now, she’s faculty director of the Department of Materials Science and Engineering’s NanoMechanical Technology Laboratory, “which is filled with these machines.”

Van Vliet first used these tools, coupled with computer simulations, to show how a stress or force can initiate a defect in an otherwise perfect crystal of metal. Such work is important as devices get smaller and smaller — a seemingly minuscule flaw can compromise their performance.

She’d always had a strong interest in biology, however, so before joining the MIT faculty she spent her postdoc years in a lab at Children’s Hospital. “I felt that there were measurements I could take down at the nanoscale to explore how mechanics might affect cancer biology and
vascular biology,” she says. “I don’t pretend that I knew how I was going to do that, but it seemed like it should be possible.”

She succeeded, and biology is now a major focus of Van Vliet’s lab. “Small changes in the chemistry of the interface [between a cell and the material it is next to] can affect the mechanical adhesion of the cell to that material, for example,” says Van Vliet. Similarly, “small changes in the mechanics, like the fact that the periphery of a tumor is stiffer, can change the biochemistry — how quickly, for example, enzymes can affect the speed of certain reactions.

“It’s that back-and-forth between chemistry and mechanics that interests me,” she says.

Among other advances, she and colleagues have shown how certain cells surrounding capillaries may use mechanical forces — contractions — to initiate angiogenesis, or the growth of new blood vessels, a process key to wound healing and the growth of cancerous tumors. The team is currently exploring how these contractions affect the growth rate, shape, and chemical secretions of the cells key to angiogenesis.

Van Vliet emphasizes, however, the range of projects in her lab. These include not only studies of cells, but of inorganic materials like new scratch-resistant coatings for cars, and even polymer nanocomposites that replicate the mechanical response of real human tissues. So, for example, the Army could develop a new protective garment without testing on a live body.

“So many amazing discoveries have been made at MIT,” says Van Vliet. “It’s inspiring just to be around all that history of great thinking and enthusiasm for learning.”