13 research outputs found
Spiral diffusion of rotating self-propellers with stochastic perturbation
Translationally diffusive behavior arising from the combination of
orientational diffusion and powered motion at microscopic scales is a known
phenomenon, but the peculiarities of the evolution of expected position
conditioned on initial position and orientation have been neglected. A theory
is given of the spiral motion of the mean trajectory depending upon propulsion
speed, angular velocity, orientational diffusion and rate of random chirality
reversal. We demonstrate the experimental accessibility of this effect using
both tadpole-like and Janus sphere dimer rotating motors. Sensitivity of the
mean trajectory to the kinematic parameters suggest that it may be a useful way
to determine those parameters
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
Mode of lysozyme protein adsorption at end-tethered polyethylene oxide brushes on gold surfaces determined by neutron reflectivity
Gravitaxis in Spherical Janus Swimming Devices
In
this work, we show that the asymmetrical distribution of mass
at the surface of catalytic Janus swimmers results in the devices
preferentially propelling themselves upward in a gravitational field.
We demonstrate the existence of this gravitaxis phenomenon by observing
the trajectories of fueled Janus swimmers, which generate thrust along
a vector pointing away from their metallically coated half. We report
that as the size of the spherical swimmer increases, the propulsive
trajectories are no longer isotropic with respect to gravity, and
they start to show a pronounced tendency to move in an upward direction.
We suggest that this effect is due to the platinum caps asymmetric
mass exerting an increasing influence on the azimuthal angle of the
Janus sphere with size, biasing its orientation toward a configuration
where the heavier propulsion generating surface faces down. This argument
is supported by the good agreement we find between the experimentally
observed azimuthal angle distribution for the Janus swimmers and predictions
made by simple Boltzmann statistics. This gravitaxis phenomenon provides
a mechanism to autonomously control and direct the motion of catalytic
swimming devices and so enable a route to make autonomous transport
devices and develop new separation, sensing, and controlled release
applications
Gravitaxis in Spherical Janus Swimming Devices
In
this work, we show that the asymmetrical distribution of mass
at the surface of catalytic Janus swimmers results in the devices
preferentially propelling themselves upward in a gravitational field.
We demonstrate the existence of this gravitaxis phenomenon by observing
the trajectories of fueled Janus swimmers, which generate thrust along
a vector pointing away from their metallically coated half. We report
that as the size of the spherical swimmer increases, the propulsive
trajectories are no longer isotropic with respect to gravity, and
they start to show a pronounced tendency to move in an upward direction.
We suggest that this effect is due to the platinum caps asymmetric
mass exerting an increasing influence on the azimuthal angle of the
Janus sphere with size, biasing its orientation toward a configuration
where the heavier propulsion generating surface faces down. This argument
is supported by the good agreement we find between the experimentally
observed azimuthal angle distribution for the Janus swimmers and predictions
made by simple Boltzmann statistics. This gravitaxis phenomenon provides
a mechanism to autonomously control and direct the motion of catalytic
swimming devices and so enable a route to make autonomous transport
devices and develop new separation, sensing, and controlled release
applications
Gravitaxis in Spherical Janus Swimming Devices
In
this work, we show that the asymmetrical distribution of mass
at the surface of catalytic Janus swimmers results in the devices
preferentially propelling themselves upward in a gravitational field.
We demonstrate the existence of this gravitaxis phenomenon by observing
the trajectories of fueled Janus swimmers, which generate thrust along
a vector pointing away from their metallically coated half. We report
that as the size of the spherical swimmer increases, the propulsive
trajectories are no longer isotropic with respect to gravity, and
they start to show a pronounced tendency to move in an upward direction.
We suggest that this effect is due to the platinum caps asymmetric
mass exerting an increasing influence on the azimuthal angle of the
Janus sphere with size, biasing its orientation toward a configuration
where the heavier propulsion generating surface faces down. This argument
is supported by the good agreement we find between the experimentally
observed azimuthal angle distribution for the Janus swimmers and predictions
made by simple Boltzmann statistics. This gravitaxis phenomenon provides
a mechanism to autonomously control and direct the motion of catalytic
swimming devices and so enable a route to make autonomous transport
devices and develop new separation, sensing, and controlled release
applications
Effect of Catalyst Distribution on Spherical Bubble Swimmer Trajectories
Spherical colloids decorated with
a surface coating of catalytically
active material are capable of producing autonomous motion in fluids
by decomposing dissolved fuel molecules to generate a gaseous product,
resulting in momentum generation by bubble growth and release. Such
colloids are attractive as they are relatively simple to manufacture
compared to more complex tubular devices and have the potential to
be used for applications such as environmental remediation. However,
despite this interest, little effort has been devoted to understanding
the link between the catalyst distribution at the colloid surface
and the resulting propulsive trajectories. Here we address this by
producing colloids with well-defined distributions of catalytic activity,
which can produce motion without the requirement for the addition
of surfactant, and measure and analyze the resulting trajectories.
By applying analysis including fractal dimension and persistence length
calculations, we show that spatially confining catalytic activity
to one side of the colloid results in a significant increase in directionality,
which could be beneficial for transport applications. Using a simple
stochastic model for bubble propulsion we can reproduce the features
of the experimental data and gain insight into the way in which localizing
catalytic activity can reduce trajectory randomization. However, despite
this route to achieve trajectory control, our analysis makes it clear
that bubble-driven swimmers are subject to very rapid randomization
of direction compared to phoretic catalytic swimming devices with
equivalent geometries
Gravitaxis in Spherical Janus Swimming Devices
In this work, we show that the asymmetrical
distribution of mass at the surface of catalytic Janus swimmers
results in the devices preferentially propelling themselves
upward in a gravitational field. We demonstrate the existence
of this gravitaxis phenomenon by observing the trajectories of
fueled Janus swimmers, which generate thrust along a vector
pointing away from their metallically coated half. We report
that as the size of the spherical swimmer increases, the
propulsive trajectories are no longer isotropic with respect to
gravity, and they start to show a pronounced tendency to
move in an upward direction. We suggest that this effect is due
to the platinum caps asymmetric mass exerting an increasing influence on the azimuthal angle of the Janus sphere with size,
biasing its orientation toward a configuration where the heavier propulsion generating surface faces down. This argument is
supported by the good agreement we find between the experimentally observed azimuthal angle distribution for the Janus
swimmers and predictions made by simple Boltzmann statistics. This gravitaxis phenomenon provides a mechanism to
autonomously control and direct the motion of catalytic swimming devices and so enable a route to make autonomous transport
devices and develop new separation, sensing, and controlled release applications
In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating
Spin coating polymer blend thin films provides a method to produce multiphase
functional layers of high uniformity covering large surface areas. Applications for such layers include
photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase
separation morphology of the spun !lm. Furthermore, at micrometer scales, phase separation
provides a route to produce self-organized structures for templating applications. Understanding the
factors that determine the final phase-separated morphology in these systems is consequently an
important goal. However, it has to date proved problematic to fully test theoretical models for phase
separation during spin coating, due to the high spin speeds, which has limited the spatial resolution
of experimental data obtained during the coating process. Without this fundamental understanding,
production of optimized micro- and nanoscale structures is hampered. Here, we have employed
synchronized stroboscopic illumination together with the high light gathering sensitivity of an
electron-multiplying charge-coupled device camera to optically observe structure evolution in such
blends during spin coating. Furthermore the use of monochromatic illumination has allowed
interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and
phase separates with nanometer precision. We have used this new method to directly observe the
phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing
new insights into the spin-coating process and opening up a route to understand and control phase
separation structures