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Motivated by applications to fluid flows with conjugate heat transfer and electrokinetic effects, we propose a direct forcing immersed boundary method for simulating general, discontinuous, Dirichlet and Robin conditions at the interface between two materials. In comparison to existing methods, our approach uses smaller stencils and accommodates complex geometries with sharp corners. The method is built on the concept of a “forcing pair,” defined as two grid points that are adjacent to each other, but on opposite sides of an interface. For 2D problems this approach can simultaneously enforce discontinuous Dirichlet and Robin conditions using a six-point stencil at one of the forcing points, and a 12-point stencil at the other. In comparison, prior work requires up to 14-point stencils at both points. We also propose two methods of accommodating surfaces with sharp corners. The first locally reduces stencils in sharp corners. The second uses the signed distance function to globally smooth all corners on a surface. The smoothing is defined to recover the actual corners as the grid is refined. We verify second-order spatial accuracy of our proposed methods by comparing to manufactured solutions to the Poisson equation with challenging dis- continuous fields across immersed surfaces. Next, to explore the performance of our method for simulating fluid flows with conjugate heat transport, we couple our method to the incompressible Navier–Stokes and continuity equations using a finite-volume projection method. We verify the spatial-temporal accuracy of the solver using manufactured solutions and an analytical solution for circular Couette flow with conjugate heat transfer. Finally, to demonstrate that our method can model moving surfaces, we simulate fluid flow and conjugate heat transport between a stationary cylinder and a rotating ellipse or square. 

Colloids can be used either as model systems for directed assembly or as the necessary building blocks for making functional materials. Previous work primarily focused on assembling colloids under a single external field, where controlling particle−particle interactions is limited. This work presents results under a combination of electric and magnetic fields. When these two fields are orthogonally applied, we can independently tune the magnitude and direction of the dipolar attraction and repulsion between the particles. As a result, we obtain well-aligned, highly dense, but individually separated linear chains at intermediate particle concentrations. Both the inter- and intrachain spacings can be tuned by adjusting the particle concentration and relative strengths of both fields. At high particle concentrations and by tuning the electric field frequency, the individual microspheres can assemble into colloidal oligomers such as trimers, tetramers, heptamers, and nonamers in response to the electric field due to the synergy between dipolar and electrohydrodynamic interactions. These oligomers, in turn, serve as building blocks for making hierarchical structures with finer architectures upon superimposing a one-dimensional (1D) magnetic field. In addition to experiments, Monte Carlo (MC) simulations have been performed on colloids confined near the electrode, interacting through a Stockmayer-like potential. They faithfully reproduce key observations in the experiments. Our work demonstrates the potential of using orthogonal electric and magnetic fields to assemble diversified types of highly aligned structures for applications in high-strength composites, optical materials, or structured battery electrodes. 

Coxiella burnetii is an obligate intracellular bacterium that lives in a modified lysosome termed the Coxiella containing vacuole (CCV). C. burnetii is the causative agent of the zoonotic illness Q Fever, which primarily infects ruminant livestock and can spread to humans via inhalation. Acute Q fever is characterized by a flu like illness, while chronic disease is associated with more severe symptoms including endocarditis and chronic fatigue syndrome. C. burnetii has a biphasic life cycle consisting of a small cell variant (SCV) and large cell variant (LCV). The SCV is environmentally stable and can cause infection via inhalation of 1 - 10 infectious particles. Once taken up by the host cell, C. burnetii must manipulate the signaling pathways of the host cell to form the CCV where it switches to the LCV, the metabolic and replicative form of the bacterium. It primarily accomplishes this goal by utilizing a Type IV B Secretion System (T4BSS), which is unique to C. burnetii and Legionella pneumophila. The T4BSS, along with some other secretion mechanisms, secretes effector proteins into the host cell. These proteins then interfere with or modulate the host cell to recruit vacuoles, evade detection by the immune system, and prevent the host cell from initiating apoptosis. After about 6 days, the LCV will convert to SCV and then initiate host cell lysis to spread infection. This review looked at many eukaryotic cells signaling pathways and the interactions between C. burnetii and host proteins. These interactions are responsible for the modulation of host cell pathways necessary for CCV formation and C. burnetii survival. Understanding these interactions better will help with future treatments for C. burnetii infection. Further discoveries in these interactions are crucial for the future of C. burnetii research. 

Soft microbots based on magnetic Pickering emulsions exhibit tractions higher than their rigid counterparts. 

To overcome the reversible nature of low-Reynolds-number flow, a variety of biomimetic microrobotic propulsion schemes and devices capable of rapid transport have been developed. However, these approaches have been typically optimized for a specific function or environment and do not have the flexibility that many real organisms exhibit to thrive in complex microenvironments. Here, inspired by adaptable microbes and using a combination of experiment and simulation, we demonstrate that one-dimensional colloidal chains can fold into geometrically complex morphologies, including helices, plectonemes, lassos, and coils, and translate via multiple mechanisms that can be varied with applied magnetic field. With chains of multiblock asymmetry, the propulsion mode can be switched from bulk to surface-enabled, mimicking the swimming of microorganisms such as flagella-rotating bacteria and tail-whipping sperm and the surface-enabled motion of arching and stretching inchworms and sidewinding snakes. We also demonstrate that reconfigurability enables navigation through three-dimensional and narrow channels simulating capillary blood vessels. Our results show that flexible microdevices based on simple chains can transform both shape and motility under varying magnetic fields, a capability we expect will be particularly beneficial in complex in vivo microenvironments. 

Abstract A backbone engineering strategy is developed to tune the mechanical and electrical properties of conjugated polymer semiconductors. Four Donor–Acceptor (D–A) polymers, named PTDPPSe, PTDPPTT, PTDPPBT, and PTDPPTVT, are synthesized using selenophene (Se), thienothiophene (TT), bithiophene (BT), and thienylenevinylenethiophene (TVT) as the donors and siloxane side chain modified diketopyrrolopyrrole (DPP) as acceptor. The influences of the donor structure on the polymer energy level, film morphology, molecular stacking, carrier transport properties, and tensile properties are all examined. The films of PTDPPSe show the best stretchability with crack‐onset‐strain greater than 100%, but the worst electrical properties with a mobility of only 0.54 cm² V⁻¹ s⁻¹. The replacement of the Se donor with larger conjugated donors, that is, TT, BT, and TVT, significantly improves the mobility of conjugated polymers but also leads to reduced stretchability. Remarkably, PTDPPBT exhibits moderate stretchability with crack‐onset‐strain ≈50% and excellent electrical properties. At 50% strain, it has a mobility of 2.37 cm²V⁻¹ s⁻¹parallel to the stretched direction, which is higher than the mobility of most stretchable conjugated polymers in this stretching state.