4,690 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
Self-assembly of colloidal molecules due to self-generated flow
The emergence of structure through aggregation is a fascinating topic and of
both fundamental and practical interest. Here we demonstrate that
self-generated solvent flow can be used to generate long-range attractions on
the colloidal scale, with sub-pico Newton forces extending into the
millimeter-range. We observe a rich dynamic behavior with the formation and
fusion of small clusters resembling molecules, the dynamics of which is
governed by an effective conservative energy that decays as . Breaking the
flow symmetry, these clusters can be made active
Tunable Assembly of Gold Nanorods in Polymer Solutions to Generate Controlled Nanostructured Materials
Gold nanorods grafted with short chain polymers are assembled into controlled
open structures using polymer-induced depletion interactions and structurally
characterized using small angle x-ray scattering. When the nanorod diameter is
smaller than the radius of gyration of the depletant polymer, the depletion
interaction depends solely on the correlation length of the polymer solution
and not directly on the polymer molecular weight. As the polymer concentration
increases, the stronger depletion interactions increasingly compress the
grafted chains and push the gold nanorods closer together. By contrast, other
structural characteristics such as the number of nearest neighbors and fractal
dimension exhibit a non-monotonic dependence on polymer concentration. These
parameters are maximal at intermediate concentrations, which are attributed to
a crossover from reaction-limited to diffusion-limited aggregation. The control
over structural properties of anisotropic nanoscale building blocks
demonstrated here will be beneficial to designing and producing materials
\emph{in situ} with specific direction-dependent nanoscale properties and
provides a crucial route for advances in additive manufacturing
Activity-controlled annealing of colloidal monolayers.
Molecular motors are essential to the living, generating fluctuations that boost transport and assist assembly. Active colloids, that consume energy to move, hold similar potential for man-made materials controlled by forces generated from within. Yet, their use as a powerhouse in materials science lacks. Here we show a massive acceleration of the annealing of a monolayer of passive beads by moderate addition of self-propelled microparticles. We rationalize our observations with a model of collisions that drive active fluctuations and activate the annealing. The experiment is quantitatively compared with Brownian dynamic simulations that further unveil a dynamical transition in the mechanism of annealing. Active dopants travel uniformly in the system or co-localize at the grain boundaries as a result of the persistence of their motion. Our findings uncover the potential of internal activity to control materials and lay the groundwork for the rise of materials science beyond equilibrium
Active colloids in complex fluids
We review recent work on active colloids or swimmers, such as self-propelled
microorganisms, phoretic colloidal particles, and artificial micro-robotic
systems, moving in fluid-like environments. These environments can be
water-like and Newtonian but can frequently contain macromolecules, flexible
polymers, soft cells, or hard particles, which impart complex, nonlinear
rheological features to the fluid. While significant progress has been made on
understanding how active colloids move and interact in Newtonian fluids, little
is known on how active colloids behave in complex and non-Newtonian fluids. An
emerging literature is starting to show how fluid rheology can dramatically
change the gaits and speeds of individual swimmers. Simultaneously, a moving
swimmer induces time dependent, three dimensional fluid flows, that can modify
the medium (fluid) rheological properties. This two-way, non-linear coupling at
microscopic scales has profound implications at meso- and macro-scales: steady
state suspension properties, emergent collective behavior, and transport of
passive tracer particles. Recent exciting theoretical results and current
debate on quantifying these complex active fluids highlight the need for
conceptually simple experiments to guide our understanding.Comment: 6 figure
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