## 3 Passive Systems
### Field-passive Fluids in Energy Dissipating Systems
#### 3.2.2 Passive Sandwich Structures  
In contrast to the case of dampers, the use of STFs in sandwich structures is wider than the use of VEFs, in both impact [170; 171; 172; 173] and vibration applications [174; 122; 175]. In these cases, the sandwich structure was designed to be activated exclusively by the stress due to impact or the vibration. The design of the sandwich structure tested by [PERSON] and co-workers for vibration control [174; 122; 175] is very similar to the ones reported for the use of active fluids, i.e., two skin layers containing a fluid layer in between, having all of them a thickness much smaller than the other two dimensions of the plates. The shear thickening behaviour of the fluid is activated whenever the local shear rate, which is given as a result of the combination of shear strain \(\gamma\) and frequency of vibration \(\omega\), exceeds the critical shear rate \(\dot{\gamma}_{c}\). Alternatively, for impact protection purposes, the core of the sandwich may consist of an open cell foam filled with a STF [176], and the STF will be activated whenever the local shear rate exceeds the critical shear rate, i.e., \(\dot{\gamma}_{c}\leq\dot{\gamma}_{local}\sim\frac{\dot{u}}{d}\), being \(u\) the speed of the impact and \(d\) the cell size. In the STF-filled foams, successive impacts damage partly the foam, resulting in a decrease of properties [177]. Recently, [PERSON] et al. [178] proposed a new concept of sandwich structure consisting of one microagglomerated cork lamina engraved with a microfluidic pattern, filled with STF and covered with another cork lamina (Figure 5a). The idea is to offer an alternative to the synthetic cellular material currently used by promoting the use of cork as a natural cellular foam in technical applications. Micro-agglomerated cork has clear advantages as a safety padding material for impact-resistance applications, e.g., motorcycle/bike helmets, compared to expanded polystyrene (EPS) foam padding in the event of double or multiple impacts occurring around the same representative area, due to its high elastic recoil after deformation. However, its performance is clearly inferior in the first impact and this is why cork micro-agglomerates cannot  
Figure 4: (**a**) Components of a viscoelastic damper; (**b**) some installation examples to piping systems; (**c**) and (**d**) pictures from real installations of viscoelastic dampers. Courtesy of VICODA GmbH. get approval from regulatory bodies and are not yet included in the European Standard for helmets (ECE-R.22/05), see Figure 5b[179]. Thus, although cork is a promising alternative natural material for the development of composites dedicated to energy dissipation applications, it is still far from fulfilling stringent regulatory requirements. CorkSTFfluidics composites offer a way of overcoming this limitation by reinforcing micro-agglomerate cork with a complex fluid having engineered properties. The advantage of this design is two fold: first, it allows incorporating STFs in closed-cell cellular materials; second, the microfluidic pattern can be numerically optimized to provide the best performance of the sandwich for any external mechanical load, either impacts or vibrations, on each single point of the composite Figure 5c. Moreover, controlling the dimensions and shape of the microfluidic pattern allow a better control over the local shear rate activating the shear thickening fluid than it does the cell size in an open cell foam, which is non-uniform across the volume of the foam and cannot be well controlled. Besides, the density of the final pad can also be controlled, as the total amount of incorporated fluid is well defined. In their patent, [PERSON] and [PERSON][180] disclosed that their sandwich structures are also suitable to work with VEFs instead of STFs, just by optimizing accordingly the microfluidic pattern. All these approaches, although rather simple and low cost, they lack of adaptability.