16 research outputs found
Self-Assembly of Nanorod Motors into Geometrically Regular Multimers and Their Propulsion by Ultrasound
Segmented goldāruthenium nanorods (300 Ā± 30 nm in diameter and 2.0 Ā± 0.2 Ī¼m in length) with thin Ni segments at one end assemble into few-particle, geometrically regular dimers, trimers, and higher multimers while levitated in water by ā¼4 MHz ultrasound at the midpoint of a cylindrical acoustic cell. The assembly of the nanorods into multimers is controlled by interactions between the ferromagnetic Ni segments. These assemblies are propelled autonomously in fluids by excitation with ā¼4 MHz ultrasound and exhibit several distinct modes of motion. Multimer assembly and disassembly are dynamic in the ultrasonic field. The relative numbers of monomers, dimers, trimers, and higher multimers are dependent upon the number density of particles in the fluid and their speed, which is in turn determined by the ultrasonic power applied. The magnetic binding energy of the multimers estimated from their speed-dependent equilibria is in agreement with the calculated strength of the magnetic dipole interactions. These autonomously propelled multimers can also be steered with an external magnetic field and remain intact after removal from the acoustic chamber for SEM imaging
From One to Many: Dynamic Assembly and Collective Behavior of Self-Propelled Colloidal Motors
ConspectusThe assembly of complex structures from simpler, individual units
is a hallmark of biology. Examples include the pairing of DNA strands,
the assembly of protein chains into quaternary structures, the formation
of tissues and organs from cells, and the self-organization of bacterial
colonies, flocks of birds, and human beings in cities. While the individual
behaviors of biomolecules, bacteria, birds, and humans are governed
by relatively simple rules, groups assembled from many individuals
exhibit complex collective behaviors and functions that do not exist
in the absence of the hierarchically organized structure.Self-assembly
is a familiar concept to chemists who study the formation
and properties of monolayers, crystals, and supramolecular structures.
In chemical self-assembly, disorder evolves to order as the system
approaches equilibrium. In contrast, living assemblies are typically
characterized by two additional features: (1) the system constantly
dissipates energy and is not at thermodynamic equilibrium; (2) the
structure is dynamic and can transform or disassemble in response
to stimuli or changing conditions. To distinguish them from equilibrium
self-assembled structures, living (or nonliving) assemblies of objects
with these characteristics are referred to as <i>active matter.</i>In this Account, we focus on the powered assembly and collective
behavior of <i>self-propelled colloids</i>. These nano-
and microparticles, also called <i>nano-</i> and <i>micromotors</i> or <i>microswimmers</i>, autonomously
convert energy available in the environment (in the form of chemical,
electromagnetic, acoustic, or thermal energy) into mechanical motion.
Collections of these colloids are a form of synthetic active matter.
Because of the analogy to living swimmers of similar size such as
bacteria, the dynamic interactions and collective behavior of self-propelled
colloids are interesting in the context of understanding biological
active matter and in the development of new applications. The progression
from individual particle motion to pairwise interactions, and then
to multiparticle behavior, can be studied systematically with colloidal
particles. Colloidal particles are also amenable to designs (in terms
of materials, shapes, and sizes) that are not readily available in,
for example, microbial systems. We review here our efforts and those
of other groups in studying these fundamental interactions and the
collective behavior that emerges from them. Although this field is
still very new, there are already unique and interesting applications
in analysis, diagnostics, separations, and materials science that
derive from our understanding of how powered colloids interact and
assemble
Density and Shape Effects in the Acoustic Propulsion of Bimetallic Nanorod Motors
Bimetallic nanorods are propelled
without chemical fuels in megahertz
(MHz) acoustic fields, and exhibit similar behaviors to single-metal
rods, including autonomous axial propulsion and organization into
spinning chains. Shape asymmetry determines the direction of axial
movement of bimetallic rods when there is a small difference in density
between the two metals. Movement toward the concave end of these rods
is inconsistent with a scattering mechanism that we proposed earlier
for acoustic propulsion, but is consistent with an acoustic streaming
model developed more recently by Nadal and Lauga (Phys. Fluids 2014, 26, 082001). Longer rods were slower at constant power, and their speed was
proportional to the square of the power density, in agreement with
the acoustic streaming model. The streaming model was further supported
by a correlation between the disassembly of spinning chains of rods
and a sharp decrease in the axial speed of autonomously moving motors
within the levitation plane of the cylindrical acoustic cell. However,
with bimetallic rods containing metals of different densities, a consistent
polarity of motion was observed with the lighter metal end leading.
Speed comparisons between single-metal rods of different densities
showed that those of lower density are propelled faster. So far, these
density effects are not explained in the streaming model. The directionality
of bimetallic rods in acoustic fields is intriguing and offers some
new possibilities for designing motors in which shape, material, and
chemical asymmetry might be combined for enhanced functionality
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound
Shape-Directed Microspinners Powered by Ultrasound
The propulsion of
micro- and nanoparticles using ultrasound is
an attractive strategy for the remote manipulation of colloidal matter
using biocompatible energy inputs. However, the physical mechanisms
underlying acoustic propulsion are poorly understood, and our ability
to transduce acoustic energy into different types of particle motions
remains limited. Here, we show that the three-dimensional shape of
a colloidal particle can be rationally engineered to direct desired
particle motions powered by ultrasound. We investigate the dynamics
of gold microplates with twisted star shape (<i>C</i><sub><i>nh</i></sub> symmetry) moving within the nodal plane
of a uniform acoustic field at megahertz frequencies. By systematically
perturbing the parametric shape of these āspinnersā,
we quantify the relationship between the particle shape and its rotational
motion. The experimental observations are reproduced and explained
by hydrodynamic simulations that describe the steady streaming flows
and particle motions induced by ultrasonic actuation. Our results
suggest how particle shape can be used to design colloids capable
of increasingly complex motions powered by ultrasound