Precisely shaped polymeric particles and structures are widely used for applications in photonic materials, MEMS, biomaterials and self-assembly. Current approaches for particle synthesis are either batch processes or flow-through microfluidic schemes that are based on two-phase systems, limiting the throughput, shape and functionality of the particles. We report a one-phase method that combines the advantages of microscope projection photolithography and microfluidics to continuously form morphologically complex or multifunctional particles down to the colloidal length scale. Exploiting the inhibition of free-radical polymerization near PDMS surfaces, we are able to repeatedly pattern and flow rows of particles in less than 0.1 s, affording a throughput of near 100 particles per second using the simplest of device designs. Polymerization was also carried out across laminar, co-flowing streams to generate Janus particles containing different chemistries, whose relative proportions could be easily tuned. This new high-throughput technique offers unprecedented control over particle size, shape and anisotropy.

http://web.mit.edu/doylegroup/pubs/NatMater_Dhananjay_06.pdf

Continuous-flow lithography for high-throughput microparticle synthesis

This study reports the ballistic penetration performance of a composite material composed of woven Kevlar® fabric impregnated with a colloidal shear thickening fluid (silica particles (450 nm) dispersed in ethylene glycol). The impregnated Kevlar fabric yields a flexible, yet penetration resistant composite material. Fragment simulation projectile (FSP) ballistic penetration measurements at ∼244 m/s have been performed to demonstrate the efficacy of the novel composite material. The results demonstrate a significant enhancement in ballistic penetration resistance due to the addition of shear thickening fluid to the fabric, without any loss in material flexibility. Furthermore, under these ballistic test conditions, the impregnated fabric targets perform equivalently to neat fabric targets of equal areal density, while offering significantly less thickness and more material flexibility. The enhancement in ballistic performance is shown to be associated with the shear thickening response, and possible mechanisms of fabric-fluid interaction during ballistic impact are identified.

The ballistic impact characteristics of Kevlar® woven fabrics impregnated with a colloidal shear thickening fluid

1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

Shampoos, paints, cements, and soft body armor that stiffens under impact are just a few of the materials whose rheology is due to the change in viscosity that occurs when colloidal fluids experience shear stress.

https://physicstoday.scitation.org/doi/pdf/10.1063/1.3248476

Shear thickening in colloidal dispersions

Shear thickening is a type of non-Newtonian behavior in which the stress required to shear a fluid increases faster than linearly with shear rate. Many concentrated suspensions of particles exhibit an especially dramatic version, known as Discontinuous Shear Thickening (DST), in which the stress suddenly jumps with increasing shear rate and produces solid-like behavior. The best known example of such counter-intuitive response to applied stresses occurs in mixtures of cornstarch in water. Over the last several years, this shear-induced solid-like behavior together with a variety of other unusual fluid phenomena has generated considerable interest in the physics of densely packed suspensions. In this review, we discuss the common physical properties of systems exhibiting shear thickening, and different mechanisms and models proposed to describe it. We then suggest how these mechanisms may be related and generalized, and propose a general phase diagram for shear thickening systems. We also discuss how recent work has related the physics of shear thickening to that of granular materials and jammed systems. Since DST is described by models that require only simple generic interactions between particles, we outline the broader context of other concentrated many-particle systems such as foams and emulsions, and explain why DST is restricted to the parameter regime of hard-particle suspensions. Finally, we discuss some of the outstanding problems and emerging opportunities.

https://iopscience.iop.org/article/10.1088/0034-4885/77/4/046602/pdf

Shear thickening in concentrated suspensions: phenomenology, mechanisms and relations to jamming

The transient shear rheology (i.e., frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but is limited to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude.

Dynamic properties of shear thickening colloidal suspensions

Yarn pull-out can be an important energy absorption mechanism during the ballistic impact of woven Kevlar® fabric. This study reports the effects of fabric length, number of yarns pulled, arrangement of yarns, and transverse tension on the force-displacement curves for yam pull-out tests on Kevlar® KM-2 fabric under laboratory conditions. A semi-empirical model is presented for predicting the yam pull-out force and energy as a function of pull-out distance, including both yarn uncrimping and subsequent yam translation. This model is found to replicate the experimental data with a high degree of accuracy, and should prove useful for understanding ballistic experiments and improving computational modeling of fabrics.

Yarn Pull-Out as a Mechanism for Dissipating Ballistic Impact Energy in Kevlar® KM-2 Fabric: Part I: Quasi-Static Characterization of Yarn Pull-Out

The addition of colloidal shear thickening fluids (STFs) to Kevlar fabrics has been shown to enhance the ballistic penetration resistance of the fabric, under conditions of low velocities and small target sizes It is believed that this improvement in ballistic properties is related to the resistance of the STF to deformation at high strain rates, since the addition of Newtonian fluids to Kevlar fabric does not improve ballistic performance However, the precise role of the STF, and its rheological properties, in the ballistic defeat process are not known In this study, ballistic and rheological experiments are performed to determine the effect of fluid viscosity, particle loadings, and particle size and shape on the behavior of STF‐Kevlar composites These results will be help to determine which rheological parameters are most critical to achieving enhanced composite properties

The Effect of Rheological Parameters on the Ballistic Properties of Shear Thickening Fluid (STF)-Kevlar Composites

The energy absorbed in ballistic fabrics is modeled by assuming yarn pull-out, including yarn uncrimping and translation, as the primary energy absorption mechanism. Using a semi-empirical model of yarn pull-out based on laboratory tests, predictions of fabric ballistic performance are compared to ballistic test results. The study demonstrates that quasi-static pull-out results can be correlated quantitatively with yarn pull-out during ballistic impact.

Young Sil Lee

Papers

The ballistic impact characteristics of Kevlar® woven fabrics impregnated with a colloidal shear thickening fluid

Dynamic properties of shear thickening colloidal suspensions

Yarn Pull-Out as a Mechanism for Dissipating Ballistic Impact Energy in Kevlar® KM-2 Fabric: Part I: Quasi-Static Characterization of Yarn Pull-Out

The Effect of Rheological Parameters on the Ballistic Properties of Shear Thickening Fluid (STF)-Kevlar Composites

Yarn pull-out as a mechanism for dissipating ballistic impact energy in Kevlar® KM-2 fabric. Part II: Predicting ballistic performance