Microstructured Fibers for the Production of Food

Food engineering faces the difficult challenge of combining taste, i.e., tailoring texture and rheology of food matrices with the balanced intake of healthy nutrients. In materials science, fiber suspensions and composites have been developed as a versatile and successful approach to tailor rheology while imparting materials with added functionalities. Structures based on such types of physical (micro)fibers are however rare in food production mainly due to a lack of food‐grade materials and processes allowing for the fabrication of fibers with controlled sizes and microstructures. Here, the controlled fabrication of multi‐material microstructured edible fibers is demonstrated using a food compatible process based on preform‐to‐fiber thermal drawing. It is shown that different material systems based on gelatin or casein, with plasticizers such as glycerol, can be thermally drawn into fibers with various geometries and cross‐sectional structures. It is demonstrated that fibers can exhibit tailored mechanical properties post‐drawing, and can encapsulate nutrients to control their release. The versatility of fiber materials is also exploited to demonstrate the fabrication of food‐grade fabrics and scaffolds for food growth. The end results establish a new field in food production that relies on fiber‐based simple and eco‐friendly processes to realize enjoyable yet healthy and nutritious products.

Shear Thickening Fluid Impregnated Foam Composites for Vibration Control and Impact Protection

Mathieu Soutrenon

Highly concentrated suspensions of silica particles present shear thickening properties, shifting from a low to a high viscosity state, instantly and reversibly at a critical shear rate. Their viscosity increases by up to 3 orders of magnitude, changing from a honey-like liquid to a brittle solid material. This sharp rise in viscosity is associated with a large absorption of energy that can be used in damping applications. These shear thickening fluids (STF) are among the few passive materials that can simultaneously bring both an increase in damping and in stiffness properties. In mechanical design, this leads potentially to a weight reduction of the overall system. This thesis work is aimed to develop and test a composite material solution for vibration control and impact protection, using STF as the damping element. A main challenge was to overcome the fact that STF are liquid at rest, and very sensitive to their environment. The proposed damping material was therefore constituted of foam impregnated with STF and encapsulated with silicone (STF/foam). A comparison of properties with other damping materials such as Smactane®was emphasized along the thesis. Storage and processing conditions of STF constituted of highly concentrated suspensions were shown to affect their rheological properties similarly to other sensitive parameters such as particle concentration. Rigorous processing methods were thus required to produce STF with defined and stable rheological properties. The use of sonication was preferred to disperse the aggregates of particles in concentrated colloidal suspensions due to the packing of the dry powder. Contact with air or humidity deteriorates the shear thickening properties of STF. Storage of STF in a freezer at −24 ◦C or under nitrogen prevents this. The use of a physical barrier to air and humidity such as silicone to protect STF is also shown to greatly reduce the ageing problem. The energy dissipated during a cyclic strain cycle was used as a measure of the damping property of the STF, and compared to that of other materials. It was measured with dynamic rheological measurements performed on suspensions with variations of particle concentration, size and test frequency. STF were shown to dissipate a large amount of energy during their transition, which increases with the increase of frequency and particle concentration and decrease with the increase of particle size. In specific cases of large strain amplitude, they potentially dissipate more energy than viscoelastic materials such as rubber. An open cell melamine resin foam was used as a lightweight scaffold to provide a three dimensional structure to the STF. The foam distributes the load and locally shears the STF. The shape of the foam is easily adjustable. To impregnate the foam with the STF, a system similar to vacuum infusion was used. The foam was placed under vacuum in a mold and the STF heated at 70 ◦C to reduce its viscosity and shear thickening properties was sucked in the foam with the depressurization. The STF/foam systems were then encapsulated in silicone to protect the STF/foam damping pads from contamination and avoid outgassing. During compression tests at low frequencies and large deformations, we demonstrated that STF could be activated in this type of structure, with mechanical properties comparable to those of a viscous honey-like fluid at rest and silicone when the STF is in the solid state. The same behavior was observed during impact solicitations. STF/foam presents a very low coefficient of restitution and absorbs the shock wave associated with impacts. Shock tests done separately confirm the shock absorption properties of STF/foam even if the STF is not activated. The combination of mechanical and damping properties of the foam/STF systems remain lower than for very high performance damping materials like Smactane®. The reasons are their low mechanical compression and shear properties at rest and the need to overcome a threshold strain or stress to activate the STF. However, the large dissimilarity between the states before and after the transition makes these materials interesting for impact or large strain amplitude applications such as sole for running shoes or several space structures.

EPFL2013

Nanocomposites based on luminescent colloidal nanocrystals in polymers: new opportunities for micro and nano fabrication

Jürgen Brugger, Vahid Fakhfouri

In recent years a wide range of materials including metals, semiconductors and metal oxides have been synthesized with nanometer dimensions. In particular II–VI semiconductor nanocrystals (NCs) are receiving considerable attention for fundamental studies as zerodimensional quantum confined material and for their potential application in optoelectronics and photonics. The colour tunability of the emission of semiconductor NCs as a function of size is one of their most attractive properties. A current bottle-neck of these materials is their poor compatibility with micropatterning methods in general and MEMS processes in particular. Inorganic colloidal nanoparticles encapsulated in polymer matrix may lead to versatile hybrid micro- and nanoscale devices made by simple techniques. Colloidal methods provide effective routes to prepare semiconductor NCs that are soluble in organic solvents with a narrow size distribution. Moreover the ability to manipulate their surface chemistry is a powerful tool to place the NCs in almost any chemical environment, due to the possibility of properly engineering their surface by ligand exchange techniques. Recent efforts have been directed towards the fabrication of polymer nanocomposites containing nanometer sized inorganic semiconductor. Such interest is due to the combination the processability and the structural properties of the polymeric matrix, i.e. thermoplastic or photosensitive materials, and the unique optoelectronic characteristics of the inorganic moiety, that can be thus used as emitters for many purposes such as optical switches, sensors, electro-luminescent devices, lasers and biomedical tags. In particular, microelectromechanical systems (MEMS) that are currently built mainly from Silicon, dielectrics, polymers and some selected metals will benefit from new functional polymeric nanocomposit materials, which combine simple UV processing and superior material properties such as biocompatibility, chemical stability, luminescence, photochromism and more. This paper describes the combination of synthesis and functionalization of luminescent colloidal NCs for polymer modification, with micro- and nanofabrication methods to form basic elements of advanced polymer MEMS/NEMS with increased inherent functionality. We prepared nanocomposites based on highly luminescent II-VI semiconductors colloidal CdS, CdSe, and CdSe@ZnS, core shell type. The NCs were incorporated in poly-methyl methacrylate (PMMA) a thermoplastic material, and SU-8, a photosensitive polymer with superior lithographic properties, respectively. Spectroscopical (UV-vis absorption and emission), structural (TEM) and morphological analysis were performed on both types of nanocomposite solutions as well as the thin films spin coated onto silicon and quartz substrates. The effect of NC composition, concentration, size, and surface chemistry was evaluated in order to understand the role played by these factors on the optical properties of the investigated nanocomposites. The presence of organic ligand shell at colloidal NC surface shows to be critical for their incorporation either into SU8 and PMMA. Incorporation of the different types of luminescent NCs in PMMA and SU-8 did not show any relevant change of the optical properties of the NCs. The excitonic peaks as well as the emission properties were retained. Preliminary spinning, UV exposure and development experiments were performed on the SU-8 based nanocomposites. The obtained structures were inspected by optical microscope and confirmed that the overall UVstructuring capability of the modified polymer was not macroscopically affected by the NC incorporation in the polymer.

2005

Shear Thickening Fluid Impregnated Foam Composites for Vibration Control and Impact Protection

Mathieu Soutrenon

EPFL2013

Nanocomposites based on luminescent colloidal nanocrystals in polymers: new opportunities for micro and nano fabrication

Jürgen Brugger, Vahid Fakhfouri

2005