Microstructural Origins of Nonlinear Response in Associating Polymers under Oscillatory Shear

Interplay between Shelf Life and Printability of Silica-Filled Suspensions

Although fumed silica/siloxane suspensions are commonly employed in additive manufacturing technology, the interplay between shelf life, storage conditions, and printability has yet to be explored. In this work, direct ink writing (DIW) was used to print unique three-dimensional structures that required suspensions to retain shape and form while being printed onto a substrate. Suspensions containing varying concentrations of hydrophobic and hydrophilic silica were formulated and evaluated over a time span of thirty days. Storage conditions included low (8%) and high (50%) relative humidity and temperatures ranging from 4 °C to 25 °C. The shelf life of the suspensions was examined by comparing the print quality of pristine and aged samples via rheology, optical microscopy, and mechanical testing. Results showed a significant decrease in printability over time for suspensions containing hydrophilic fumed silica, whereas the printability of suspensions containing hydrophobic fumed silica remained largely unchanged after storage. The findings in this work established the following recommendations for extending the shelf life and printability of suspensions commonly used in DIW technology: (1) higher fumed silica concentrations, (2) low humidity and low temperature storage environments, and (3) the use of hydrophobic fumed silica instead of hydrophilic fumed silica.

Mechanical Coupling of Puller and Pusher Active Microswimmers Influences Motility

Active self-propelled colloidal populations induce time-dependent three-dimensional fluid flows, which alter the rheological (viscoelastic) properties of their fluidic media. Researchers have also studied passive colloids mixed with bacterial suspensions to understand the hydrodynamic coupling between active and passive colloids. With recent developments in biological cell-driven biohybrid microswimmers, different type biological microswimmer (e.g., bacteria and algae) populations need to interact fluidically with each other in the same fluidic media, while such interactions have not been studied experimentally yet. Therefore, we report the swimming behavior of two opposite types of biological microswimmer (active colloid) populations: Chlamydomonas reinhardtii (C. reinhardtii) algae (puller-type microswimmers) population in coculture with Escherichia coli (E. coli) bacteria (pusher-type microswimmers) population. We observed noticeable fluidic coupling deviations from the existing understanding of passive colloids mixed with bacterial suspensions previously studied in the literature. The fluidic coupling among puller- and pusher-type microswimmers led to nonequilibrium fluctuations in the fluid flow due to their opposite swimming patterns. Such coupling could be the main reason behind the shift in motility behaviors of these two opposite-type swimmer populations suspended in the same fluidic media.