268 research outputs found
Helical paths, gravitaxis, and separation phenomena for mass-anisotropic self-propelling colloids: experiment versus theory
The self-propulsion mechanism of active colloidal particles often generates not only translational but also rotational motion. For particles with an anisotropic mass density under gravity, the motion is usually influenced by a downwards oriented force and an aligning torque. Here we study the trajectories of self-propelled bottom-heavy Janus particles in three spatial dimensions both in experiments and by theory. For a sufficiently large mass anisotropy, the particles typically move along helical trajectories whose axis is oriented either parallel or antiparallel to the direction of gravity (i.e., they show gravitaxis). In contrast, if the mass anisotropy is small and rotational diffusion is dominant, gravitational alignment of the trajectories is not possible. Furthermore, the trajectories depend on the angular self-propulsion velocity of the particles. If this component of the active motion is strong and rotates the direction of translational self-propulsion of the particles, their trajectories have many loops, whereas elongated swimming paths occur if the angular self-propulsion is weak. We show that the observed gravitational alignment mechanism and the dependence of the trajectory shape on the angular self-propulsion can be used to separate active colloidal particles with respect to their mass anisotropy and angular self-propulsion, respectively
Nonequilibrium dynamics of mixtures of active and passive colloidal particles
We develop a mesoscopic field theory for the collective nonequilibrium
dynamics of multicomponent mixtures of interacting active (i.e., motile) and
passive (i.e., nonmotile) colloidal particles with isometric shape in two
spatial dimensions. By a stability analysis of the field theory, we obtain
equations for the spinodal that describes the onset of a motility-induced
instability leading to cluster formation in such mixtures. The prediction for
the spinodal is found to be in good agreement with particle-resolved computer
simulations. Furthermore, we show that in active-passive mixtures the spinodal
instability can be of two different types. One type is associated with a
stationary bifurcation and occurs also in one-component active systems, whereas
the other type is associated with a Hopf bifurcation and can occur only in
active-passive mixtures. Remarkably, the Hopf bifurcation leads to moving
clusters. This explains recent results from simulations of active-passive
particle mixtures, where moving clusters and interfaces that are not seen in
the corresponding one-component systems have been observed.Comment: 17 pages, 3 figure
How does a flexible chain of active particles swell?
We study the swelling of a flexible linear chain composed of active particles
by analytical theory and computer simulation. Three different situations are
considered: a free chain, a chain confined to an external harmonic trap, and a
chain dragged at one end. First we consider an ideal chain with harmonic
springs and no excluded volume between the monomers. The Rouse model of
polymers is generalized to the case of self-propelled monomers and solved
analytically. The swelling, as characterized by the spatial extension of the
chain, scales with the monomer number defining a Flory exponent which is
in the three different situations. As a result, we find that
activity does not change the Flory exponent but affects the prefactor of the
scaling law. This can be quantitatively understood by mapping the system onto
an equilibrium chain with a higher effective temperature such that the chain
swells under an increase of the self-propulsion strength. We then use computer
simulations to study the effect of self-avoidance on active polymer swelling.
In the three different situations, the Flory exponent is now and again unchanged under self-propulsion. However, the chain extension
behaves non-monotonic in the self-propulsion strength.Comment: (9 pages, 5 figures
Ultra-thin corrugated metamaterial film as large-area transmission dynode
Large-area transmission dynodes were fabricated by depositing an ultra-thin
continuous film on a silicon wafer with a 3-dimensional pattern. After removing
the silicon, a corrugated membrane with enhanced mechanical properties was
formed. Mechanical materials, such as this corrugated membrane, are engineered
to improve its strength and robustness, which allows it to span a larger
surface in comparison to flat membranes while the film thickness remains
constant. The ultra-thin film consists of three layers (AlO
/TiN/AlO) and is deposited by atomic layer deposition (ALD). The
encapsulated TiN layer provides in-plane conductivity, which is needed to
sustain secondary electron emission. Two types of corrugated membranes were
fabricated: a hexagonal honeycomb and an octagonal pattern. The latter was
designed to match the square pitch of a CMOS pixel chip. The transmission
secondary electron yield was determined with a collector-based method using a
scanning electron microscope. The highest transmission electron yield was
measured on a membrane with an octagonal pattern. A yield of 2.15 was achieved
for 3.15 keV incident electrons for an AlO /TiN/AlO tri-layer
film with layer thicknesses of 10/5/15 nm. The variation in yield across the
surface of the corrugated membrane was determined by constructing a yield map.
The active surface for transmission secondary electron emission is near 100%,
i.e. a primary electron generates transmission secondary electrons regardless
of the point of impact on the corrugated membrane
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