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A popular principle in designing chemical micromachines is to take advantage of asymmetric chemical reactions such as the catalytic decomposition of H2O2. Contrary to intuition, we use Janus micromotors half-coated with platinum (Pt) or catalase as an example to show that this ingredient is not sufficient in powering a micromotor into self-propulsion. In particular, by annealing a thin Pt film on a SiO2 microsphere, the resulting microsphere half-decorated with discrete Pt nanoparticles swims ∼80% more slowly than its unannealed counterpart in H2O2, even though they both catalytically produce comparable amounts of oxygen. Similarly, SiO2 microspheres half-functionalized with the enzyme catalase show negligible self-propulsion despite high catalytic activity toward decomposing H2O2. In addition to highlighting how surface morphology of a catalytic cap enables/disables a chemical micromotor, this study offers a refreshed perspective in understanding how chemistry powers nano- and microscopic objects (or not): our results are consistent with a self-electrophoresis mechanism that emphasizes the electrochemical decomposition of H2O2 over nonelectrochemical pathways. More broadly, our finding is a critical piece of the puzzle in understanding and designing nano- and micromachines, in developing capable model systems of active colloids, and in relating enzymes to active matter.

The emerging field of active matter, and its subset active colloid, is in great need of good model systems consisting of moving entities that are uniform and highly tunable. In this article, we address this challenge by introducing core–shell SiO2–TiO2 microspheres, prepared by chemically coating a thin layer of TiO2 on an inert core, that are highly monodisperse in size (polydispersity 4.1%) and regular in shape (circularity 0.93). Compared with similar samples prepared by the classic sol–gel method, Janus TiO2–Pt active colloids prepared with core–shell TiO2 spheres move faster and boast a much clearer Janus interface. Moreover, a unique feature of these core–shell TiO2 microspheres is their great tunability in the colloid size, shell thickness, and even the type of the core particle. These advantages are highlighted in two examples, one demonstrating a TiO2–Pt active colloid with a magnetic core that enables magnetic manipulation, and the other demonstrating the collective expansion and contraction of a uniform cluster of core–shell TiO2 colloids under UV light illumination. We believe that TiO2 microspheres produced by this core–shell technique compare favorably with many other types of active colloids being employed as model systems, and thus open up many research possibilities.

Distinguishing the operating mechanisms of nano- and micromotors powered by chemical gradients, i.e. "autophoresis", holds the key for fundamental and applied reasons. In this article, we propose and experimentally confirm that the speeds of a self-diffusiophoretic colloidal motor scale inversely to its population density but not for self-electrophoretic motors, because the former is an ion source and thus increases the solution ionic strength over time while the latter does not. They also form clusters in visually distinguishable and quantifiable ways. This pair of rules is simple, powerful, and insensitive to the specific material composition, shape or size of a colloidal motor, and does not require any measurement beyond typical microscopy. These rules are not only useful in clarifying the operating mechanisms of typical autophoretic micromotors, but also in predicting the dynamics of unconventional ones that are yet to be experimentally realized, even those involving enzymes.

Efficient fuels are critical for designing photocatalytic micromotors with high performance. We discover that 0.5 mM of triethanolamine can power TiO2–Pt motors at 35 μm s−1 without producing bubbles, a significant improvement over conventional fuels such as water, H2O2 or hydroquinone. The effectiveness of hole scavengers such as triethanolamine can be generalized to other photocatalytic micromotors containing a heterojunction with an n-type (but not a p-type) semiconductor.

The emerging field of active matter, and its subset active colloid, is in great need of good model systems consisting of moving entities that are uniform and highly tunable. In this article, we address this challenge by introducing core-shell SiO2-TiO2 microspheres, prepared by chemically coating a thin layer of TiO2 on an inert core, that are highly monodisperse in size (polydispersity 4.1%) and regular in shape (circularity 0.93). Compared with similar samples prepared by the classic sol-gel method, Janus TiO2-Pt active colloids prepared with core-shell TiO2 spheres move faster and boast a much clearer Janus interface. Moreover, a unique feature of these core-shell TiO2 microspheres is their great tunability in the colloid size, shell thickness, and even the type of the core particle. These advantages are highlighted in two examples, one demonstrating a TiO2-Pt active colloid with a magnetic core that enables magnetic manipulation, and the other demonstrating the collective expansion and contraction of a uniform cluster of core-shell TiO2 colloids under UV light illumination. We believe that TiO2 microspheres produced by this core-shell technique compare favorably with many other types of active colloids being employed as model systems, and thus open up many research possibilities.

The traditional inertial impact switch based on experience design often appears in the problem of early closure within the threshold. To solve this problem, a new method of inertial switch design is presented in this paper. Firstly, the force analysis of inertia switch is carried out and a mechanical model is established. Through the model, it is concluded that the closing threshold of inertia switch is related to the spring resistance, the mass and center of mass of contact pole, the spring radius of contact pole and the horizontal projection of the minimum distance between the spring and the reference point. After that, the mechanical model is verified by simulation, and the correctness of the mechanical model is proved. The experimental results show that the proposed model can effectively solve the problem of premature closing of the inertial switch within the threshold and improve the pass rate of the product.

Based on mooring observations carried out in the southern Yellow Sea during late summer 2017, this study focuses on the characteristics of the internal tide-induced turbulent mixing and the influences on the vertical transport of nitrates. We observe clear semidiurnal pycnocline displacements with an amplitude of ~3.5 m during 25 h. The baroclinic velocity and velocity shear are mainly controlled by the low-mode semidiurnal internal tide. The turbulence profiling measurements reveal that the measured turbulent kinetic energy dissipation rates e are largely enhanced at the thermocline ( e ~2.0 × 10−8 Wkg−1) where the maximum displacements occur. The depth of the enhanced e and velocity shear coincide with each other which all follow the displacements of isopycnals induced by the internal tide, indicating the role of local velocity shear in generating the strong turbulence. Further analysis shows that the observed interior e is in good agreement with the MacKinnon-Gregg parametrization model which is formulated based on the wave-wave interaction theory. The strong turbulence has induced large vertical turbulent diffusivity ( K ρ ~1.0 × 10−5 m2s−1) in the nitracline. The estimated vertical nitrate fluxes across the nitracline are 0.07–0.44 mmolNm−2d−1. Considering the fact that the temporal variation of vertical nitrate fluxes is mainly determined by the trend of K ρ , our results suggest that the internal tides which have generated large nitracline K ρ may play an important role in transporting nitrates upward, supporting the growth of phytoplankton in the summer subsurface chlorophyll maximum zone at least near our measurement location where internal tides may prevail.