SALi based cushion  =  lots of compressible capsules
 +”incompressible” matrix fluid
+ low stretch packaging

The SALi fluid behaves like a type of material that does not exist in nature. You can envisage it as either a compressible liquid, or a remarkably stiff gas.

SALi based cushions offer four damage mitigation mechanisms:

• The capsules absorb energy when they suffer bulk compression during an impact.
•  Viscous damping converts impact energy into heat when the matrix fluid swirls round the compressing capsules. This reduces the “kickback” when the capsules spring back into shape in the milliseconds following the impact.
•  The hydraulic characteristics of the matrix fluid distribute impact loading over the surface being protected.
•  Shock waves are scattered at the capsule-fluid interfaces.

The compressive stiffness of the device depends on the type of capsules used. Published research papers [2, 3, 4.] describe the use of air capsules made from bubble packing to produce very soft cushions, with stiffer expanded polystyrene beads being the most versatile for use in protective clothing. For very violent crash impacts, expanded metal foam beads can be used. Open ended hollow tube capsules are also described in Courtney's patent literature.

Thick liquids having a viscosity similar to treacle offer good viscous damping, but silicone gel is usually preferred because this eliminates the problem of liquid leakage if the packaging is ruptured.

Shear thickening SALi
The use of shear thickening (dilatant) liquids that stiffen up during impact is described in our patent literature and recorded research. [1, 4, 6, 9.]

Packaging
The packaging used for the published research varies from stout cotton bags to piston and cylinder arrangements. [1, 2,3, 4, 5.] A key feature of the SALi concept is that stretching of the packaging must be minimised in order to maximise the capsule compression and viscous damping processes.

Graphene looks like becoming the SALi packaging material for the future, for sportswear and other body armour applications because of its strength, flexibility, good thermal conductivity and antibacterial properties.

In principle, the shear thickening gel versions of SALi that lock up under impact don't require a low stretch packaging. But unpackaged gel SALi is less effective than the original packaged version.

 Q. Why is the SALi concept referred to as a "technology"?

A. We want to emphasise that a broad collection of materials and ways of blending and packaging is involved. The global idea is to create a composite material that shares the impact protection characteristics of solids, liquids and gasses.
But, to absorb useful amounts of impact energy, some form of low stretch, variable volume packaging is required.

Prior to the invention of SALi, engineers had  developed a range of useful impact and virbration energy absorbing devices by combinig any two of the three basic states of matter,( solids, liquids and gases) . These emergent properties are summarised along the sides of the triangle below. SALi is the next logical step, combining all three basic states of matter.

SALi was originally conceived as a cash cow invention to fund Bill’s planned research into cool running heat engines, to protect the planet from global warming.

Contents

1         The basic SALi mechanism

2        The load spreading benefits of SALi cushions

3        Reducing the weight of SALi cushions

4        Illustrative proposed applications

5        Barriers to product development

References

1     The basic SALi mechanism

Figure 2. During an impact the capsules shrink in size as they are compressed on all sides by the matrix fluid. The capsules, lubricated by the fluid, re-arrange themselves inside the package, so that the front face of the package takes up the shape of the impacting body.

The matrix fluid In principle any liquid or gel which allows the hydraulic transfer of pressure can be used as the matrix fluid. In experiments[1-4] silicone oil, glycol anti-freeze, wall-paper paste, Vaseline, mastic sealant and a range of engineering greases have been used as the matrix fluid. In order to maximise viscous damping very thick liquids having a viscosity similar to treacle are preferred. Gooey mastics or silicone gels are good because there are no leakage problems if the packaging is damaged.

 Shear thickening fluids allow the package to change shape easily when deformed slowly, but stiffen up during violent impacts. This feature is appealing in protective clothing, for example in pads to protect the spinal column of motorbike or equestrian riders.

 The capsules Elastomeric capsules tested in the published research[1-5] include hollow rubber balls, expanded polystyrene beads, polymeric microspheres, bubbles cut from bubble packing and narrow diameter, open ended, hollow tubes, with filaments of air trapped inside them. The inclusion of hollow glass microspheres has been proposed as a mechanism for adding shear thickening.[9]

 The packaging  If the packaging stretches significantly during an impact then some impact energy is absorbed but overall, energy absorbing efficiency is reduced because the capsules suffer less compression and the matrix fluid provides less viscous damping.

Figure 3. The correct packaging is a vital part of SALi Technology.

2       The load spreading benefits of SALi cushions

Figure 3. Early experiments to demonstrate the load spreading advantages of SALi used very simple equipment.[1]  
Impact patterns produced in the surface of soft clay slabs were compared, when the slabs were protected by different types of cushion.

 Three types of cushion were tested: (i) Sorbothane (a visco-elastic rubber), (ii) elastomeric foam and (iii) SALi filled bags. The Sorbothane and foam cushions both produced distinct impact craters under the impact zone. In contrast, the SALi cushion produced a very shallow indentation over most of the clay slab.

Figure 4. The steel sphere produced a clear impact crater in the Sorbothane. In contrast, the SALi based cushion produced a broad shallow impact pattern. Similar impact craters to those made by Sorbothane were produced when elastomeric foams were tested.

Load spreading occurs throughout the interior of a SALi cushion. Consequently, elastomeric foam based SALi capsules suffer bulk compression. In contrast, if a similar foam is used as the basis of a conventional cushion, the individual air cells are flattened under the impact zone, but cells to the sides of the impact zone are unaffected.

Figure 5. The hydraulic nature of SALi cushions produces a load spreading effect throughout the interior of the cushion. The cell walls inside a SALi beads are subjected to axial compression, making the foam stiffer than the same material in block form.

 [Thought experiment Compressing foam is similar to squashing a squash or tennis ball between your finger and thumb; distorting the shape is fairly easy. But if you try to mimic uniform bulk compression of SALi beads by compressing the same ball between cupped hands, changing the volume of the ball is very difficult.]

3       Reducing the weight of SALi cushions

 The matrix fluid is responsible for most of the weight of SALi based devices. So the key to weight reduction is to minimise the fluid fraction by close packing the capsules.

 3.1 Spherical elastomeric capsules
The basic SALi formulation as illustrated in Figure 1 has a single size range of low density capsules, with the matrix fluid occupying about 34% of the volume. The fluid fraction can be reduced by using two size ranges of capsules, with small capsules occupying part of the void spaces between the larger capsules. This reduces the matrix fluid to about 12% of the volume.

The larger capsules could be expanded polystyrene beads and the smaller ones, polymeric microspheres. The matrix fluid fraction can be further reduced by introducing hollow nano-particles between microspheres.

 If the smallest size of capsules are fairly rigid, compared with the larger ones, the smaller capsules will tend to bunch up during impact, producing a shear thickening effect.[9]

  3.2 Use cube shaped capsules

 Cubes have the most efficient shape for filling a volume.

Figure 7. Close packed cube SALi. The density of the matrix fluid between the cubes is reduced by adding polymeric microspheres.
Polystyrene nano-spheres can be added to create a shear thickening matrix fluid.

During impact the compressed cubes  lose their neat packing structure. But during the recovery phase they realign themselves in order to minimise the potential energy stored inside the package.

Research tip: Cube shaped capsules can be made by dicing up a sheet of closed cell camping mat foam. At least four faces of each cube should be slightly corrugated to create small pockets for the matrix fluid to settle into. Alternatively, if cutting the cubes using a sharp blade, deliberately make the cuts a few degrees away from true right angles so that the jumbled up "cubes" cannot mate together without leaving small wedge shaped gaps for the matrix fluid.

 3.3 Maximising the viscous damping for a given mass of SALi

 In order to maximise viscous damping during an impact the shear movements between adjacent elements of matrix liquid should be maximised. The general rule for doing this is:

 The larger the elastomeric capsule, the lower the bulk compressive stiffness should be.

 For example, in the case of 2-3 centimetre sided cube based SALi, with the pure liquid mass minimised by adding polystyrene beads and polymeric microspheres, the bulk stiffness should be graded as follows:

4                 Some proposed applications

This is a short review. Please follow the links for more detailed information.

4.1    Body armour

Depending on the market requirements, design issues that can be solved by SALi based designs  include low weight, soft feel, flexibility for articulated parts of the body, penetration resistance and temperature control.

Where flexibility is important packaging based on strong poly-cotton, Kevlar or Cuban fibre can be used.

Figure 8. Soft hats incorporating SALi cushioning.
(Designed by Sue Darby-Pankhurst).

Key applications include improved brain protection for rugby players, boxers and epileptics.

For more illustrations visit our Sassy Hats page.

Weight reduction using SALi plus foam If the body armour includes a stiff outer shell, then SALi cushions can be used to protect the most vulnerable body parts with lighter compressible foam protecting other parts.

Figure 9. This prototype footballer’s shin pad incorporates SALi cushioning to protect the tibia (shin bone), but reduces weight by using compressible foam to the sides of the tibia.[1]

The following diagram represents a cross section through a shin bone receiving a kick.

Figure 10. The load spreading characteristics of SALi work in harmony with the load transmitting characteristics of the human bodies own soft tissue.

Hip protection for older people and other osteoporosis sufferers

In 2003, the market leading product was a plastic Hip Shield. This was inserted into the underwear and covered the hip bone area. It looked similar to a footballer's shin pad but included a cushioning layer of elastomeric foam.  A Manchester undergraduate student carried out tests in which he replaced the foam with SALi. The reduction in maximum force experienced during impact was significant.

Figure 11. The impact test results using a basic SALi formulation were encouraging.
 (Paul Featherstone, The University of Manchester, 2003.)

Untested but patented improvements [9] include the incorporation of nano-particles to create shear thickening during a fall.

Based on Featherstone's results, plans were made to bid for funding to design a superior form of hip protection pad that incorporated shear thickening and dispensed wit the plastic hip shield.

Soft shell hip protection for
elderly people

 Dynamic comfort tests 2003.

 Figure 12. The Shear thickening hip protection pad was sufficiently light and flexible that it did not  interfere when playing an energetic game of squash.

 (The match ended in a draw.)

 Unfortunately academic jealousy problems at Manchester University resulted in the funding bid being abandoned in late  2003.

 Keeping cool
To reduce heating problems, the use of a phase change wax as the matrix fluid has been proposed. [1, 6.]

 Graphene would be an ideal packaging material because of its combination of flexibility, strength and outstanding thermal conduction properties. 

4.2    Soft, pedestrian friendly car bumpers (fenders)

 For details of SALi filled bumpers, please visit our PedSALi page.

 This proposal was well received in the motoring media

4.3 Vibration isolation units (vehicle suspension systems)

 Several SALi based designs have been proposed.[1, 5, 6, 7, 10] A common characteristic is that the elastomeric capsules stiffen up as they are compressed. This allows the suspension unit to offer a soft ride when travelling over smooth roads, then automatically stiffening up when moving over rough ground. Here is one of the proposed designs:

Figure 13. Vibration isolators are discussed on our CrashSALi and  battery charging suspension web pages

4.4 Motor vehicle crash protection

 Improving the crash protection offered by vehicle box sections.

Figure 14. The matrix liquid transmits pressure, offering energy adsorption by compression inside the whole length of the box section.
To minimize the liquid weight, three sizes of compressible capsule can be used:
1  large aluminium foam balls fill most of the volume.
2  Expanded polystyrene beads fit into the spaces between the Al balls.
3  Polymeric micro-spheres fit in the spaces between the polystyrene beads.

Using this nested bead system only about 4% of the space inside the box sections is occupied by liquid.

Keeping repair costs down
The front and rear crumple sections can be telescopic, moving into the rigid section during a collision. Following minor bumps, the gas filled capsules will make a full recovery. If necessary, additional SALi fluid can be injected into the cavity, to expand the box section to its full length.

 Converting SALi filled box sections into vehicle batteries

 The compressible capsules can be coated with a thin layer of lead and dilute sulphuric acid used as the matrix fluid.

Figure 15. A short horizontal cross section through a SALi filled vehicle box section that also doubles up as a lead-acid battery. The battery is packaged, allowing it to be removed for replacement during the life of the vehicle. In common with existing lead-acid batteries, the  lead coating is preferably in the form of a grid, with voids in the grid lattice being filled with lead paste.

4.5 Acoustic vibration (sound) reduction

 Work by Valentin LeRoy at Paris Diderot University, France suggests that SALi type materials may employ a process known as Minnaert scattering to reduce sound transmission through walls. [15, 16]

 4.6 Blast mitigation

 LeRoy's work is in line with earlier unpublished SALi research carried out at Cranfield University Royal Military College of Science. Researchers under the supervision of Professor Horsfall have verified that SALi type materials have good blast wave mitigation properties.

 Figure 16. The blast mitigation properties of SALi being tested at Cranfield University RMCS.
The SALi filled bag was ruptured by debris thrown up during the explosion. If the bag is covered by a floating steel under plate, it remains intact during the explosion.

Figure 17. SALi vehicle protection combines shock wave scattering with the other impact mitigation benefits discussed in this article.

4.7 For further defence and civil engineering applications

 Visit the Civil & Defence Engineering page on this web site.

5        Barriers to product development

The first of the following journal papers explains how academic mischief brought SALi development to a grinding halt.

The second paper is an attempt to rescue something good from the mischief.

1. Courtney, W. A. ‘A private researcher’s struggles against research fraud I. A case study’ Journal of Biological Physics and Chemistry 16 (2016) 142 156.
2. Courtney, W. A. ‘A private researcher’s struggles against research fraud II. Suggestions for reducing the fraud problem’ Journal of Biological Physics and Chemistry 17 (2017) 81 – 88.