The principles of molecular gel formation

Molecular gels are associated with the formation of strongly anisotropic structures at low volume fractions (less than 1 wt%) that induce solid-like mechanical properties. Low molecular weight gelators based on aromatic short peptide derivatives have been shown to self-assemble into fibrous networks featuring highly ordered molecular packing. The building units of these structures are individual molecules experiencing hydrogen bonding and π-π stacking interactions, making them distinct from typical gels formed by aggregation of colloidal particles or crosslinking of polymer chains. The remarkable structural and mechanical properties of these materials offer a wide range of potential applications. Despite a surge in scientific publications on a variety of molecular gelators over the past decade, the mechanism and fundamental thermodynamic principles of molecular gel formation remain poorly understood. The aim of this thesis is to address these issues by a thorough and systematic experimental characterization of a model molecular gelator, fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF). The nature of the gel transition, as well as the relationship between composition, dynamics, structural and mechanical properties are discussed within the framework of current soft matter theories. The experimental observations reveal that the formation of the gel is a result of the system undergoing an equilibrium first order phase transition and a generalized phase diagram is developed

Field responsive mechanical metamaterials.

Typically, mechanical metamaterial properties are programmed and set when the architecture is designed and constructed, and do not change in response to shifting environmental conditions or application requirements. We present a new class of architected materials called field responsive mechanical metamaterials (FRMMs) that exhibit dynamic control and on-the-fly tunability enabled by careful design and selection of both material composition and architecture. To demonstrate the FRMM concept, we print complex structures composed of polymeric tubes infilled with magnetorheological fluid suspensions. Modulating remotely applied magnetic fields results in rapid, reversible, and sizable changes of the effective stiffness of our metamaterial motifs

Formation and mechanics of self-assembled molecular gels

Molecular gels are associated with the formation of space spanning structures produced by aggregation of low molecular weight molecules that associate through hydrogen bonding, π-stacking, acid base or van der Waals interactions. One specific type of gel forming molecule is based on a hydrophobic peptide – fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF). This thesis explores gels formed when water is added to Fmoc-FF dissolved in dimethyl sulfoxide (DMSO). At high water concentrations, gels are formed at concentrations as low as 0.001%. We establish the gel line defining the Fmoc-FF and water concentrations where gels are formed. At fixed water concentration, over a narrow range of Fmoc-FF concentrations, solutions pass from being low viscosity liquids to a rigid material with elastic moduli G’ > 10^5 Pa. Here we characterize the kinetics of gelation and demonstrate that these gels are reversible in the sense that they can be disrupted mechanically and will rebuild strength over time. We attempt to understand the gelation process as arising from increasing strength of attraction between Fmoc-FF molecules with increasing water concentration. Furthermore, an effort is made to describe the underlying changes in strength of attraction leading to gelation and the mechanical behavior of the resulting gels using a dynamic localization theory

Dynamic holdup in a countercurrent gas - flowing solids - packed bed contactors

Equations for the prediction of the holdup of dynamic solids in countercurrent gas flowing solids packed bed contactors are presented in this paper. The correlations do not require the use of parameters that need to be determined by experimental measurements in the actual system of interest. They could be used for a wide range of operational conditions, different packing types and a variety of flowing solids materials. The equations are compared with all available experimental data from the literature

Exchange between the stagnant and flowing zone in gas-flowing solids-fixed bed contactors

In countercurrent gas flowing solids fixed bed contactors, a fraction of the flowing solids is in motion (dynamic holdup), while the other fraction is resting on the fixed bed elements. In this study it was experimentally proved that the stagnant zone should not be considered as a dead part of the column, but that there is a dynamic exchange between these two portions of flowing solids particles. Combining a mathematical model with tracer experiments, the rate of exchange was determined and it was shown that only a small part (ca. 20 %) of the stagnant region should be considered as a dead one

3D Printing of High Viscosity Reinforced Silicone Elastomers

Recent advances in additive manufacturing, specifically direct ink writing (DIW) and ink-jetting, have enabled the production of elastomeric silicone parts with deterministic control over the structure, shape, and mechanical properties. These new technologies offer rapid prototyping advantages and find applications in various fields, including biomedical devices, prosthetics, metamaterials, and soft robotics. Stereolithography (SLA) is a complementary approach with the ability to print with finer features and potentially higher throughput. However, all high-performance silicone elastomers are composites of polysiloxane networks reinforced with particulate filler, and consequently, silicone resins tend to have high viscosities (gel- or paste-like), which complicates or completely inhibits the layer-by-layer recoating process central to most SLA technologies. Herein, the design and build of a digital light projection SLA printer suitable for handling high-viscosity resins is demonstrated. Further, a series of UV-curable silicone resins with thiol-ene crosslinking and reinforced by a combination of fumed silica and MQ resins are also described. The resulting silicone elastomers are shown to have tunable mechanical properties, with 100–350% elongation and ultimate tensile strength from 1 to 2.5 MPa. Three-dimensional printed features of 0.4 mm were achieved, and complexity is demonstrated by octet-truss lattices that display negative stiffness

Predicting Nanoparticle Suspension Viscoelasticity for Multimaterial 3D Printing of Silica–Titania Glass

A lack of predictive methodology is frequently a major bottleneck in materials development for additive manufacturing. Hence, exploration of new printable materials often relies on the serendipity of trial and error approaches, which is time-consuming, labor-intensive, and costly. We present an approach to overcome these issues by quantifying and controlling the viscoelasticity of inks for multimaterial 3D printing of silica–titania glass using direct ink writing (DIW). We formulate simple silica and silica–titania inks from a suspension of fumed silica nanoparticles in an organic solvent with a dissolved molecular titania precursor. We use a small set of experimental rheological data and estimates of interaction potentials from colloidal theory to develop a predictive tool that allows us to design and obtain compatible inks that are matched both in desired rheological properties (viscosity profiles and elastic modulus) as well as solids loading. The model incorporates silica particle volume fraction, particle size, particle size distribution, and titania precursor concentration, and captures the effects of all formulation parameters on the measured viscoelasticity in a single curve. We validate the ink formulations predicted by the model and find that the materials can be very well matched in rheological properties as desired for 3D printing. Using the DIW and heat treatment methods we have reported previously, we use these inks to print and process a fully transparent glass with spatial change in dopant composition and refractive index. We believe that this approach can be extended to other colloidal systems and allow predictive ink formulation design for desired printability in direct ink write manufacturing

Predicting Nanoparticle Suspension Viscoelasticity for Multimaterial 3D Printing of Silica–Titania Glass

A lack of predictive methodology is frequently a major bottleneck in materials development for additive manufacturing. Hence, exploration of new printable materials often relies on the serendipity of trial and error approaches, which is time-consuming, labor-intensive, and costly. We present an approach to overcome these issues by quantifying and controlling the viscoelasticity of inks for multimaterial 3D printing of silica–titania glass using direct ink writing (DIW). We formulate simple silica and silica–titania inks from a suspension of fumed silica nanoparticles in an organic solvent with a dissolved molecular titania precursor. We use a small set of experimental rheological data and estimates of interaction potentials from colloidal theory to develop a predictive tool that allows us to design and obtain compatible inks that are matched both in desired rheological properties (viscosity profiles and elastic modulus) as well as solids loading. The model incorporates silica particle volume fraction, particle size, particle size distribution, and titania precursor concentration, and captures the effects of all formulation parameters on the measured viscoelasticity in a single curve. We validate the ink formulations predicted by the model and find that the materials can be very well matched in rheological properties as desired for 3D printing. Using the DIW and heat treatment methods we have reported previously, we use these inks to print and process a fully transparent glass with spatial change in dopant composition and refractive index. We believe that this approach can be extended to other colloidal systems and allow predictive ink formulation design for desired printability in direct ink write manufacturing