270 research outputs found
Automated tracking of colloidal clusters with sub-pixel accuracy and precision
Quantitative tracking of features from video images is a basic technique
employed in many areas of science. Here, we present a method for the tracking
of features that partially overlap, in order to be able to track so-called
colloidal molecules. Our approach implements two improvements into existing
particle tracking algorithms. Firstly, we use the history of previously
identified feature locations to successfully find their positions in
consecutive frames. Secondly, we present a framework for non-linear
least-squares fitting to summed radial model functions and analyze the accuracy
(bias) and precision (random error) of the method on artificial data. We find
that our tracking algorithm correctly identifies overlapping features with an
accuracy below 0.2% of the feature radius and a precision of 0.1 to 0.01 pixels
for a typical image of a colloidal cluster. Finally, we use our method to
extract the three-dimensional diffusion tensor from the Brownian motion of
colloidal dimers.Comment: 20 pages, 8 figures. Non-revised preprint version, please refer to
http://dx.doi.org/10.1088/1361-648X/29/4/04400
Microparticle assembly pathways on lipid membranes
Understanding interactions between microparticles and lipid membranes is of
increasing importance, especially for unraveling the influence of microplastics
on our health and environment. Here, we study how a short-ranged adhesive force
between microparticles and model lipid membranes causes membrane-mediated
particle assembly. Using confocal microscopy, we observe the initial particle
attachment to the membrane, then particle wrapping, and in rare cases
spontaneous membrane tubulation. In the attached state, we measure that the
particle mobility decreases by 26%. If multiple particles adhere to the same
vesicle, their initial single-particle state determines their interactions and
subsequent assembly pathways: 1) attached particles only aggregate when small
adhesive vesicles are present in solution, 2) wrapped particles reversibly
attract one another by membrane deformation, and 3) a combination of wrapped
and attached particles form membrane-mediated dimers, which further assemble
into a variety of complex structures. The experimental observation of distinct
assembly pathways induced only by a short ranged membrane-particle adhesion,
shows that a cellular cytoskeleton or other active components are not required
for microparticle aggregation. We suggest that this membrane-mediated
microparticle aggregation is a reason behind reported long retention times of
polymer microparticles in organisms.Comment: 20 pages, 11 figures (including supporting material
Diffusion-based height analysis reveals robust microswimmer-wall separation
Microswimmers typically move near walls, which can strongly influence their
motion. However, direct experimental measurements of swimmer-wall separation
remain elusive to date. Here, we determine this separation for model catalytic
microswimmers from the height dependence of the passive component of their
mean-squared displacement. We find that swimmers exhibit "ypsotaxis", a
tendency to assume a fixed height above the wall for a range of salt
concentrations, swimmer surface charges, and swimmer sizes. Our findings
indicate that ypsotaxis is activity-induced, posing restrictions on future
modeling of their still debated propulsion mechanism
Soft and stiff normal modes in floppy colloidal square lattices
Floppy microscale spring networks are widely studied in theory and
simulations, but no well-controlled experimental system currently exists. Here,
we show that square lattices consisting of colloid-supported lipid bilayers
functionalized with DNA linkers act as microscale floppy spring networks. We
extract their normal modes by inverting the particle displacement correlation
matrix, showing the emergence of a spectrum of soft modes with low effective
stiffness in addition to stiff modes that derive from linker interactions.
Evaluation of the softest mode, a uniform shear mode, reveals that shear
stiffness decreases with lattice size. Experiments match well with Brownian
particle simulations and we develop a theoretical description based on mapping
interactions onto linear response to describe the modes. Our results reveal the
importance of entropic steric effects, and can be used for developing
reconfigurable materials at the colloidal length scale
Thermodynamic equilibrium of binary mixtures on curved surfaces
We study the global influence of curvature on the free energy landscape of
two-dimensional binary mixtures confined on closed surfaces. Starting from a
generic effective free energy, constructed on the basis of symmetry
considerations and conservation laws, we identify several model-independent
phenomena, such as a curvature-dependent line tension and local shifts in the
binodal concentrations. To shed light on the origin of the phenomenological
parameters appearing in the effective free energy, we further construct a
lattice-gas model of binary mixtures on non-trivial substrates, based on the
curved-space generalization of the two-dimensional Ising model. This allows us
to decompose the interaction between the local concentration of the mixture and
the substrate curvature into four distinct contributions, as a result of which
the phase diagram splits into critical sub-diagrams. The resulting free energy
landscape can admit, as stable equilibria, strongly inhomogeneous mixed phases,
which we refer to as antimixed states below the critical temperature. We
corroborate our semi-analytical findings with phase-field numerical simulations
on realistic curved lattices. Despite this work being primarily motivated by
recent experimental observations of multi-component lipid vesicles supported by
colloidal scaffolds, our results are applicable to any binary mixture confined
on closed surfaces of arbitrary geometry.Comment: 20 Pages, 7 Figures; comments and references added, typos correcte
Interface geometry of binary mixtures on curved substrates
Motivated by recent experimental work on multicomponent lipid membranes
supported by colloidal scaffolds, we report an exhaustive theoretical
investigation of the equilibrium configurations of binary mixtures on curved
substrates. Starting from the J\"ulicher-Lipowsky generalization of the
Canham-Helfrich free energy to multicomponent membranes, we derive a number of
exact relations governing the structure of an interface separating two lipid
phases on arbitrarily shaped substrates and its stability. We then restrict our
analysis to four classes of surfaces of both applied and conceptual interest:
the sphere, axisymmetric surfaces, minimal surfaces and developable surfaces.
For each class we investigate how the structure of the geometry and topology of
the interface is affected by the shape of the substrate and we make various
testable predictions. Our work sheds light on the subtle interaction mechanism
between membrane shape and its chemical composition and provides a solid
framework for interpreting results from experiments on supported lipid
bilayers.Comment: 26 pages, 10 figure
Brownian motion of flexibly-linked colloidal rings
Ring, or cyclic, polymers have unique properties compared to linear polymers,
due to their topologically closed structure that has no beginning or end.
Experimental measurements on molecular ring polymers are challenging due to
their polydispersity in molecular weight and the presence of undesired side
products such as chains. Here, we study an experimental model system for cyclic
polymers, that consists of rings of flexibly-linked micron-sized colloids with
=4..8 segments. We characterize the conformations of these flexible
colloidal rings and find that they are freely-jointed up to steric
restrictions. We measure their diffusive behavior and compare it to
hydrodynamic simulations. Interestingly, flexible colloidal rings have a larger
translational and rotational diffusion coefficient compared to colloidal
chains. In contrast to chains, their internal deformation mode shows slower
fluctuations for and saturates for higher values of . We show
that constraints stemming from the ring structure cause this decrease in
flexibility for small and infer the expected scaling of the flexibility as
function of ring size. Our findings could have implications for the behavior of
both synthetic and biological ring polymers, as well as for the dynamic modes
of floppy colloidal materials
Power-law intermittency in the gradient-induced self-propulsion of colloidal swimmers
Active colloidal microswimmers serve as archetypical active fluid systems,
and as models for biological swimmers. Here, by studying in detail their
velocity traces, we find robust power-law intermittency with system-dependent
exponential cut off. We model the motion by an interplay of the field
gradient-dependent active force and the locally fluctuating hydrodynamic drag,
set by the wetting properties of the substrate. The model closely describes the
velocity distributions of two disparate swimmer systems: AC field activated and
catalytic swimmers. The generality is highlighted by the collapse of all data
in a single master curve, suggesting the applicability to further systems, both
synthetic and biological.Comment: 5 pages, 3figure
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