19 research outputs found
Rational design and dynamics of self-propelled colloidal bead chains: from rotators to flagella
The quest for designing new self-propelled colloids is fuelled by the demand
for simple experimental models to study the collective behaviour of their more
complex natural counterparts. Most synthetic self-propelled particles move by
converting the input energy into translational motion. In this work we address
the question if simple self-propelled spheres can assemble into more complex
structures that exhibit rotational motion, possibly coupled with translational
motion as in flagella. We exploit a combination of induced dipolar interactions
and a bonding step to create permanent linear bead chains, composed of
self-propelled Janus spheres, with a well-controlled internal structure. Next,
we study how flexibility between individual swimmers in a chain can affect its
swimming behaviour. Permanent rigid chains showed only active rotational or
spinning motion, whereas longer semi-flexible chains showed both translational
and rotational motion resembling flagella like-motion, in the presence of the
fuel. Moreover, we are able to reproduce our experimental results using
numerical calculations with a minimal model, which includes full hydrodynamic
interactions with the fluid. Our method is general and opens a new way to
design novel self-propelled colloids with complex swimming behaviours, using
different complex starting building blocks in combination with the flexibility
between them.Comment: 27 pages, 10 figure
Light-switchable propulsion of active particles with reversible interactions
Abstract: Active systems such as microorganisms and self-propelled particles show a plethora of collective phenomena, including swarming, clustering, and phase separation. Control over the propulsion direction and switchability of the interactions between the individual self-propelled units may open new avenues in designing of materials from within. Here, we present a self-propelled particle system, consisting of half-gold-coated titania (TiO2) particles, in which we can quickly and on-demand reverse the propulsion direction, by exploiting the different photocatalytic activities on both sides. We demonstrate that the reversal in propulsion direction changes the nature of the hydrodynamic interaction from attractive to repulsive and can drive the particle assemblies to undergo both fusion and fission transitions. Moreover, we show these active colloids can act as nucleation sites, and switch rapidly the interactions between active and passive particles, leading to reconfigurable assembly and disassembly. Our experiments are qualitatively described by a minimal hydrodynamic model
A colloidal viewpoint on the finite sphere packing problem: the sausage catastrophe
It is commonly believed that the most efficient way to pack a finite number
of equal-sized spheres is by arranging them tightly in a cluster. However,
mathematicians have conjectured that a linear arrangement may actually result
in the densest packing. Here, our combined experimental and simulation study
provides a realization of the finite sphere packing problem by studying
non-close-packed arrangements of colloids in a flaccid lipid vesicle. We map
out a state diagram displaying linear, planar and cluster conformations of
spheres, as well as bistable states which alternate between cluster-plate and
plate-linear conformations due to membrane fluctuations. Finally, by
systematically analyzing truncated polyhedral packings, we identify clusters of
spheres, excluding and 63, that pack more efficiently
than linear arrangements
Rotational averaging-out gravitational sedimentation of colloidal dispersions and phenomena
We report on the differences between colloidal systems left to evolve in the
earth's gravitational field and the same systems for which a slow continuous
rotation averaged out the effects of particle sedimentation on a distance scale
small compared to the particle size. Several systems of micron-sized colloidal
particles were studied: a hard sphere fluid, colloids interacting via
long-range electrostatic repulsions above the freezing volume fraction, an
oppositely charged colloidal system close to either gelation and/or
crystallization, colloids with a competing short-range depletion attraction and
a long-range electrostatic repulsion, colloidal dipolar chains, and colloidal
gold platelets under conditions where they formed stacks. Important differences
in the structure formation were observed between the experiments where the
particles were allowed to sediment and those where sedimentation was averaged
out. For instance, in the case of colloids interacting via long-range
electrostatic repulsions, an unusual sequence of
dilute-Fluid/dilute-Crystal/dense-Fluid/dense-Crystal phases was observed
throughout the suspension under the effect of gravity, related to the volume
fraction dependence of the colloidal interactions, whereas the system stayed
homogeneously crystallized with rotation. For the oppositely charged colloids,
a gel-like structure was found to collapse under the influence of gravity with
a few crystalline layers grown on top of the sediment, whereas when the
colloidal sedimentation was averaged out, the gel completely transformed into
crystallites that were oriented randomly throughout the sample. Rotational
averaging out gravitational sedimentation is an effective and cheap way to
estimate the importance of gravity for colloidal self-assembly processes.Comment: 13 pages, 13 figure
Colloidal Switches by Electric and Magnetic Fields
External electric and magnetic fields
have already been proven to be a versatile tool to control the particle
assembly; however, the degree of control of the dynamics and versatility
of the produced structures is expected to increase if both can be
implemented simultaneously. For example, while micromagnets can rapidly
assemble superparamagnetic particles, repeated, rapid disassembly
or reassembly is not trivial because of the remanence and coercivity
of metals used in such applications. Here, an interdigitated design
of micromagnet and microfabricated electrodes enables rapid switching
of colloids between their magnetic and electric potential minima.
Active control over colloids between two such adjacent potential minima
enables a fast on/off mechanism, which is potentially important for
optical switches or display technologies. Moreover, we demonstrate
that the response time of the colloids between these states is on
the order of tens of milliseconds, which is tunable by electric field
strength. By carefully designing the electrode pattern, our strategy
enables the switchable assembly of single particles down to few microns
and also hierarchical assemblies containing many particles. Our work
on precise dynamic control over the particle position would open new
avenues to find potential applications in optical switches and display
technologies
Colloidal Switches by Electric and Magnetic Fields
External electric and magnetic fields
have already been proven to be a versatile tool to control the particle
assembly; however, the degree of control of the dynamics and versatility
of the produced structures is expected to increase if both can be
implemented simultaneously. For example, while micromagnets can rapidly
assemble superparamagnetic particles, repeated, rapid disassembly
or reassembly is not trivial because of the remanence and coercivity
of metals used in such applications. Here, an interdigitated design
of micromagnet and microfabricated electrodes enables rapid switching
of colloids between their magnetic and electric potential minima.
Active control over colloids between two such adjacent potential minima
enables a fast on/off mechanism, which is potentially important for
optical switches or display technologies. Moreover, we demonstrate
that the response time of the colloids between these states is on
the order of tens of milliseconds, which is tunable by electric field
strength. By carefully designing the electrode pattern, our strategy
enables the switchable assembly of single particles down to few microns
and also hierarchical assemblies containing many particles. Our work
on precise dynamic control over the particle position would open new
avenues to find potential applications in optical switches and display
technologies
Colloidal Switches by Electric and Magnetic Fields
External electric and magnetic fields
have already been proven to be a versatile tool to control the particle
assembly; however, the degree of control of the dynamics and versatility
of the produced structures is expected to increase if both can be
implemented simultaneously. For example, while micromagnets can rapidly
assemble superparamagnetic particles, repeated, rapid disassembly
or reassembly is not trivial because of the remanence and coercivity
of metals used in such applications. Here, an interdigitated design
of micromagnet and microfabricated electrodes enables rapid switching
of colloids between their magnetic and electric potential minima.
Active control over colloids between two such adjacent potential minima
enables a fast on/off mechanism, which is potentially important for
optical switches or display technologies. Moreover, we demonstrate
that the response time of the colloids between these states is on
the order of tens of milliseconds, which is tunable by electric field
strength. By carefully designing the electrode pattern, our strategy
enables the switchable assembly of single particles down to few microns
and also hierarchical assemblies containing many particles. Our work
on precise dynamic control over the particle position would open new
avenues to find potential applications in optical switches and display
technologies
Colloidal Switches by Electric and Magnetic Fields
External electric and magnetic fields
have already been proven to be a versatile tool to control the particle
assembly; however, the degree of control of the dynamics and versatility
of the produced structures is expected to increase if both can be
implemented simultaneously. For example, while micromagnets can rapidly
assemble superparamagnetic particles, repeated, rapid disassembly
or reassembly is not trivial because of the remanence and coercivity
of metals used in such applications. Here, an interdigitated design
of micromagnet and microfabricated electrodes enables rapid switching
of colloids between their magnetic and electric potential minima.
Active control over colloids between two such adjacent potential minima
enables a fast on/off mechanism, which is potentially important for
optical switches or display technologies. Moreover, we demonstrate
that the response time of the colloids between these states is on
the order of tens of milliseconds, which is tunable by electric field
strength. By carefully designing the electrode pattern, our strategy
enables the switchable assembly of single particles down to few microns
and also hierarchical assemblies containing many particles. Our work
on precise dynamic control over the particle position would open new
avenues to find potential applications in optical switches and display
technologies