18 research outputs found
Electrotaxis of self-propelling artificial swimmers in microchannels
Ciliated microswimmers and flagellated bacteria alter their swimming trajectories to follow the direction of an applied electric field exhibiting electrotaxis. Both for matters of application and physical modelling, it is instructive to study such behaviour in synthetic swimmers. We show here that under an external electric field, self-propelling active droplets autonomously modify their swimming trajectories in microchannels, even undergoing `U-turns', to exhibit robust electrotaxis. Depending on the relative initial orientations of the microswimmer and the external electric field, the active droplet can also navigate upstream of an external flow following a centre-line motion, instead of the oscillatory upstream trajectory observed in absence of electric field. Using a hydrodynamic theory model, we show that the electrically induced angular velocity and electrophoretic effects, along with the microswimmer motility and its hydrodynamic interactions with the microchannel walls, play crucial roles in dictating the electrotactic trajectories and dynamics. Specifically, the transformation in the trajectories during upstream swimming against an external flow under an electric field can be understood as a reverse Hopf bifurcation for a dynamical system. Our study provides a simple methodology and a systematic understanding of manoeuvring active droplets in microconfinements for micro-robotic applications especially in biotechnology
Arrested on heating: controlling the motility of active droplets by temperature
One of the challenges in tailoring the dynamics of active, self-propelling
agents lies in arresting and releasing these agents at will. Here, we present
an experimental system of active droplets with thermally controllable and
reversible states of motion, from unsteady over meandering to persistent to
arrested motion. These states depend on the P\'eclet number of the chemical
reaction driving the motion, which we can tune by using a temperature sensitive
mixture of surfactants as a fuel medium. We quantify the droplet dynamics by
analysing flow and chemical fields for the individual states, comparing them to
canonical models for autophoretic particles. In the context of these models, we
are able to observe in situ the fundamental first transition between the
isotropic, immotile base state and self-propelled motility
Quantitative characterization of chemorepulsive alignment-induced interactions in active emulsions
The constituent elements of active matter in nature often communicate with
their counterparts or the environment by chemical signaling which is central to
many biological processes. Examples range from bacteria or sperm that bias
their motion in response to an external chemical gradient, to collective cell
migration in response to a self-generated gradient. Here, in a purely
physicochemical system based on self-propelling oil droplets, we report a novel
mechanism of dynamical arrest in active emulsions: swimmers are caged between
each other's trails of secreted chemicals. We explore this mechanism
quantitatively both on the scale of individual agent-trail collisions as well
as on the collective scale where the transition to caging happens as a result
of autochemotactic interactions
Oscillatory rheotaxis of active droplets in microchannels
Biological microswimmers are known to navigate upstream of an external flow
(positive rheotaxis) in trajectories ranging from linear, spiral to
oscillatory. Such rheotaxis stems from the interplay between the motion and
complex shapes of the microswimmers, e.g. the chirality of the rotating
flagella, the shear flow characteristics, and the hydrodynamic interaction with
a confining surface. Here, we show that an isotropic, active droplet
microswimmer exhibits a unique oscillatory rheotaxis in a microchannel despite
its simple spherical geometry. The swimming velocity, orientation, and the
chemical wake of the active droplet undergo periodic variations between the
confining walls during the oscillatory navigation. Using a hydrodynamic model
and concepts of dynamical systems, we demonstrate that the oscillatory
rheotaxis of the active droplet emerges primarily from the interplay between
the hydrodynamic interaction of the finite-sized microswimmer with all the
microchannel walls, and the shear flow characteristics. Such oscillatory
rheotactic behavior is different from the directed motion near a planar wall
observed previously for artificial microswimmers in shear flows. Our results
provide a realistic understanding of the behaviour of active particles in
confined microflows, as will be encountered in majority of the applications
like targeted drug delivery
Collective Entrainment and Confinement Amplify Transport by Schooling Microswimmers
Microswimmers can serve as cargo carriers that move deep inside complex flow networks. When a school collectively entrains the surrounding fluid, their transport capacity can be enhanced. This effect is quantified with good agreement between experiments with self-propelled droplets and a confined Brinkman squirmer model. The volume of liquid entrained can be much larger than the droplet itself, amplifying the effective cargo capacity over an order of magnitude, even for dilute schools. Hence, biological and engineered swimmers can efficiently transport materials into confined environments
Spontaneously rotating clusters of active droplets
We report on the emergence of spontaneously rotating clusters in active emulsions. Ensembles of self-propelling droplets sediment and then self-organise into planar, hexagonally ordered clusters which hover over the container bottom while spinning around the plane normal. This effect exists for symmetric and asymmetric arrangements of isotropic droplets and is therefore not caused by torques due to geometric asymmetries. We found, however, that individual droplets exhibit a helical swimming mode in a small window of intermediate activity in a force-free bulk medium. We show that by forming an ordered cluster, the droplets cooperatively suppress their chaotic dynamics and turn the transient instability into a steady rotational state. We analyse the collective rotational dynamics as a function of droplet activity and cluster size and further propose that the stable collective rotation in the cluster is caused by a cooperative coupling between the rotational modes of individual droplets in the cluster
Chemotactic self-caging in active emulsions
A common feature of biological self-organization is how active agents communicate with each other or their environment via chemical signaling. Such communications, mediated by self-generated chemical gradients, have consequences for both individual motility strategies and collective migration patterns. Here, in a purely physicochemical system, we use self-propelling droplets as a model for chemically active particles that modify their environment by leaving chemical footprints, which act as chemorepulsive signals to other droplets. We analyze this communication mechanism quantitatively both on the scale of individual agent-trail collisions as well as on the collective scale where droplets actively remodel their environment while adapting their dynamics to that evolving chemical landscape. We show in experiment and simulation how these interactions cause a transient dynamical arrest in active emulsions where swimmers are caged between each other's trails of secreted chemicals. Our findings provide insight into the collective dynamics of chemically active particles and yield principles for predicting how negative autochemotaxis shapes their navigation strategy