Dynamics of self-assembly of flower-shaped magnetic colloidal clusters.

In a static magnetic field paramagnetic and nonmagnetic colloids immersed in a ferrofluid self-assemble into fluctuating colloidal flowers. Adsorption and desorption of nonmagnetic petals to larger paramagnetic cores and changes in the petal conformation around the paramagnetic core induce a fluctuating dynamics. We track the motion of colloidal petals on the paramagnetic core. Adsorption and desorption of petals occur on a larger time scale than the rotational diffusion of the petals. Magnetic dipole interactions split the motion of the petals into different modes of rotational diffusion. Modes of rotational diffusion that change the petal conformation are suppressed compared to the conformation invariant rotational diffusion of all petals. The suppression of higher modes of rotational diffusion results in a subdiffusive dynamics of the individual petals.

Colloidal optomagnetic dimmer.

We demonstrate a colloidal optomagnetic dimmer based on the interaction between micrometer-sized paramagnetic colloidal spheres and a magnetic film. The colloidal particles undergo Brownian motion, which when exposed to light results in characteristic intensity fluctuations, and we demonstrate that weak magnetic fields that are typically 200 A/m (2.5 G) can be used to control both the average intensity and the intensity fluctuations. The system can be used as a colloidal optical dimmer in microfluidic systems.

Physical mechanisms of rehydration in Polypodium polypodioides, a resurrection plant.

Resurrection plants have an amazing ability to withstand water drought. Here we investigate experimentally the rapidity of such revivals using the resurrection fern (Polypodium polypodioides) as a model example. Upon drying, the leaves of the resurrection fern fold into a thin cylindrical shell, thus protecting the photosynthetic area from light. In the dry state the fern looks dead, but will quickly come back once exposed to water by unfolding the cylindrical shell into a nearly planar sheet. We investigate here the mass and radius of curvature of the cylindrical shell as a function of time after rehydration and develop a phenomenological model to describe the observed phenomena. In particular, we demonstrate that the mass of the rehydrating plant follows a simple kinetic relationship, whereas the unfolding is governed by a more complex nonlinear constitutive relationship between the water uptake and the induced strain.

Monolayer to bilayer transition in a dipolar system.

We study the transition from a one-dimensional magnetic dipolar monolayer to a bilayer as it is compressed beyond the close-packed condition. The pressure in a close-packed monolayer is found to be nearly independent of the number of dipoles. In the case of weak dipolar interactions, our experimental results indicate that the bilayer formation is governed by short-range steric and electrostatic repulsion, whereas for strong dipolar interactions the bilayer formation is governed by long-range dipolar repulsion.

Colloidal rings in a liquid mixture.

We investigate the self-assembly of colloidal particles on microscopic decane droplets in water and show that, by use of paramagnetic colloids, it is possible to assemble ringlike structures that can be controlled with a magnetic field. Moreover, the use of paramagnetic colloids allows us to determine the attractive forces between the colloids located at the three-phase contact line between decane, water, and air. The attractive force is in the femtonewton range and is attributed to capillary interactions due to interface deformations. When the liquid emulsion dries on a glass slide, we observe solid deposits in the form of microscopic rings of varying diameters.

Colloidal crystallization and transport in stripes and mazes.

We study guided crystallization and transport of paramagnetic spheres on top of a magnetic film that arranges its domains into stripes or mazes. In the absence of liquid flow, the paramagnetic spheres are confined within the magnetic domains, and it is shown how the particles self-assemble into several interesting phases depending on the complexity of the domain patterns. We also find that colloids guided through a complex maze exhibit structured patterns that can be controlled by an external magnetic field. The results presented here could help us understand both static and dynamic properties of pattern formation in confined geometries of tunable complexity.

Compactification of a myelin mimetic Langmuir monolayer upon adsorption and unfolding of myelin basic protein.

The surface shear viscosity of a myelin mimetic Langmuir monolayer is investigated upon adsorption of myelin basic protein (MBP). We measure an increase of the surface shear viscosity at picomolar concentrations of the protein, suggesting that the globular conformation of MBP changes upon adsorption at the monolayer. The conformational change enables hydrodynamic interactions of the proteins, with a typical separation of hundreds of nanometers. This unfolding is essential for the compactification of the myelin sheath, serving an enhanced saltatory signal transduction in vertebrates. The viscometry used extends the sensitivity of standard surface viscometers toward lower viscosities.

Pressure versus length isotherms of homogenous and mixed one-dimensional dipolar monolayers.

We demonstrate a novel method for compressing and expanding microscopic one-dimensional monolayers consisting of a finite number of aligned magnetic dipoles using a pair of microscopic magnetic barriers. By measuring the interaction between the beads and the barriers, we are able to determine the pressure of the dipolar monolayers. Our sensor can measure one-dimensional pressure in the femto and piconewton regime and is used to probe both homogeneous and mixed monolayers consisting of magnetic beads with diameters 1.0 microm and 2.8 microm. The larger beads appear to be well-described by a formalism taking into account magnetic dipolar interactions, whereas for smaller beads, such a simple picture does not hold. Upon compressing the monolayer above a certain density, it forms a bilayer. This process is governed by steric interactions or dipolar interactions, depending on the applied magnetic field. We also found odd-even effects, where the number of beads in the monolayer determines the initial structure of the bilayer.

Assembling and manipulating two-dimensional colloidal crystals with movable nanomagnets.

We study crystallization of paramagnetic beads in a magnetic field gradient generated by one-dimensional nanomagnets. The pressure in such a system depends on both the magnetic forces and the hydrodynamic flow, and we estimate the flow threshold for disassembling the crystal near the magnetic potential barrier. A number of different defects have been observed which fluctuate in shape or propagate along the crystal, and it is found that the defect density increases away from the nanomagnet. We also study the melting of the crystal/fluid system after removal of the nanomagnet and demonstrate that the bond-oriental order parameter decreases with time. The nanomagnet can be moved in a controlled manner by a weak external magnetic field, and at sufficiently large driving velocities we observe self-healing crack formation characterized by a roughening of the lattice as well as gap formation. Finally, when confined between two oscillating nanomagnets, the colloidal crystal is shown to break up and form dipolar chains above a certain oscillation frequency.

Fundamental limits of optical microrheology.

We estimate the fundamental limits of different microrheological techniques based on optical detection. It is suggested that particle tracking systems using nondifferential detection have a minimum detectable displacement given by 0.2(lambda0/NA)(1/square root of (SNR)), where lambda0 is the wavelength, NA is the numerical aperture of the focusing objective, and SNR is the signal-to-noise ratio of the system. This limit has important consequences in microrheology, since the noise contributes with an apparent diffusion constant of Dl approximately 0.02(lambda0/NA)2(B/SNR), where B is the bandwidth of the detection unit. As the SNR of ordinary microscopes is limited, one should be extra careful when probing soft materials with low diffusion constants. On the other hand, in differential systems based on laser detection, the SNR is considerably increased due to reduced laser noise, and the minimum detectable displacement is given by 0.4(lambda0/NA)(1/SNR). One may therefore expect to measure the diffusion constant with higher accuracy if the SNR is large. Finally, we find that total internal reflection microscopy (TIRM) has a minimum detectable displacement given by 0.1lambda0/SNR.

Crystallization and chain formation in liquid drops.

Colloidal crystals are easily formed in liquid drops and thin films upon evaporation. In this study we use spherical paramagnetic beads, which make it possible to manipulate them by an external magnetic field. We show that the hydrophilic beads position themselves at a distance from the contact line so that they barely touch the water-air interface. Upon applying a magnetic field, the magnetic beads can either arrange themselves in a two-dimensional repulsive lattice or form attractive vertical chains, depending on the contact angle of the drop. We also demonstrate that the vertical chains' position from the contact line is quantized and depends on the number of beads in the chain.

Particle interactions near the contact line in liquid drops.

Micrometer-sized particles distributed in a drop of water are drawn toward the contact line upon evaporation, where they arrange themselves into a crystal structure. Here we report experiments with paramagnetic particles, which make it possible to measure the particle interactions by tuning an external magnetic field. By aligning the magnetic dipoles, we are also able to redistribute the particles back into the bulk of the droplet. This method allows us to assemble and disassemble colloidal crystals simply by applying a magnetic field.

Domain wall tip for manipulation of magnetic particles.

We demonstrate a method for manipulation of single magnetic microparticles based on a domain wall tip displaced in a controlled manner. By applying an external magnetic field, the tip can either drag or push magnetic particles. This kind of tweezers has potential applications in probing and manipulating colloidal systems.

Adsorption and diffusion in a one-dimensional potential well.

We investigate the adsorption and diffusion of colloidal particles at an interface containing a one-dimensional potential well. It is observed how the adsorption kinetics onto the wall is altered with time, and one observes the formation of a particle chain. We find that the time it takes for a bead to penetrate into the chain depends strongly on the particle density, and beyond a critical value this time diverges. We also study diffusion within the well, and find that at low particle densities the short time behavior is governed by normal Fickian diffusion.

Paramagnetic beads surfing on domain walls.

Paramagnetic beads electrostatically stabilized in aqueous solution are attracted toward domain walls in magnetic films. The position above the domain wall can be destabilized by realigning the beads magnetic moment with an external magnetic field. The destabilization may result in a steady state dissipative mode, where the beads surf on the slope of the moving domain wall. The technique could be an alternative route to probe electrostatic and hydrodynamic interactions between particles and interfaces, and could also serve as a model system for studying motion in a one-dimensional potential.

Combined effects of intrinsic facilitation and modulatory inhibition of identified interneurons in the siphon withdrawal circuitry of Aplysia.

Synaptic plasticity can be induced through mechanisms intrinsic to a synapse or through extrinsic modulatory mechanisms. In this study, we investigated the relationship between these two forms of plasticity at the excitatory synapse between L29 interneurons and siphon motor neurons (MNs) in Aplysia. Using isolated ganglia, we confirmed that the L29-MN synapses exhibit a form of intrinsic facilitation: post-tetanic potentiation (PTP). We also found that L29-MN synapses are modulated by exogenous application of 5-HT: they are depressed after 5-HT exposure. We next investigated the functional relationship between an intrinsic facilitatory process (PTP) and extrinsic inhibitory modulation (5-HT-induced depression). First, we found that application of 5-HT just before L29 activation results in a reduction of PTP. Second, using semi-intact preparations, we found that tail shock (TS) mimics the effect of 5-HT by both depressing L29 synaptic transmission and by reducing L29 PTP. Third, we observed a significant correlation between L29 activity during TS and subsequent synaptic change: low-responding L29s showed synaptic depression after TS, whereas high-responding L29s showed synaptic facilitation. Finally, we found that we could directly manipulate the sign and magnitude of TS-induced synaptic plasticity by controlling L29 activity during TS. Collectively, our results show that the L29-MN synapses exhibit intrinsic facilitation and extrinsic modulation and that the sign and magnitude of L29-MN plasticity induced by TS is governed by the combined effects of these two processes. This circuit architecture, which combines network inhibition with cell-specific facilitation, can enhance the signal value of a specific stimulus within a neural network.

Contribution of postsynaptic Ca2+ to the induction of post-tetanic potentiation in the neural circuit for siphon withdrawal in Aplysia.

Recent studies in Aplysia have revealed a novel postsynaptic Ca(2+) component to posttetanic potentiation (PTP) at the siphon sensory to motor neuron (SN-MN) synapse. Here we asked whether the postsynaptic Ca(2+) component of PTP was a special feature of the SN-MN synapse, and if so, whether it reflected a unique property of the SN or the MN. We examined whether postsynaptic injection of BAPTA reduced PTP at SN synapses onto different postsynaptic targets by comparing PTP at SN-MN and SN-interneuron (L29) synapses. We also examined PTP at L29-MN synapses. Postsynaptic BAPTA reduced PTP only at the SN-MN synapse; it did not affect PTP at either the SN-L29 or the L29-MN synapse, indicating that the SN and the MN do not require postsynaptic Ca(2+) for PTP with all other synaptic partners. The postsynaptic Ca(2+) component of PTP is present at other Aplysia SN-MN synapses; tail SN-MN synapses also showed reduced PTP when the MN was injected with BAPTA. Surprisingly, in both tail and siphon SN-MN synapses, there was an inverse relationship between the initial size of the EPSP and the postsynaptic component to PTP; only the initially weak SN-MN synapses showed a BAPTA-sensitive component. Homosynaptic depression of initially strong SN-MN synapses into the size range of initially weak synapses did not confer postsynaptic Ca(2+) sensitivity to PTP. Finally, the postsynaptic Ca(2+) component of PTP could be induced in the presence of APV, indicating that it is not mediated by NMDA receptors. These results suggest a dual model for PTP at the SN-MN synapse, in which a postsynaptic Ca(2+) contribution summates with the conventional presynaptic mechanisms to yield an enhanced form of PTP.

Dynamic regulation of the siphon withdrawal reflex of Aplysia californica in response to changes in the ambient tactile environment.

The state of an animal's environment can be viewed as a source of information that can be used to regulate both ongoing and future behavior. The present work examined how the ambient environment can regulate the Aplysia siphon withdrawal reflex (SWR) by changing the environment between calm and turbulent. Results indicate that the SWR is dynamically regulated on the basis of variations in external conditions, so that responsiveness (measured as both reflex duration and threshold) is matched to the state of the environment. Prior exposure to a noxious stimulus (tailshock) has selective effects on this regulation, suggesting the existence of multiple regulatory mechanisms. Further, neurophysiological correlates to behavioral observations were measured in sensory and motor neurons. This will allow for a detailed cellular analysis of environmental information-processing in this system.

Activity-dependent potentiation of synaptic transmission from L30 inhibitory interneurons of aplysia depends on residual presynaptic Ca2+ but not on postsynaptic Ca2+.

Activity-induced short-term synaptic enhancement (STE) is a common property of neurons, one that can endow neural circuits with the capacity for rapid and flexible information processing. Evidence from a variety of systems indicates that the expression of STE depends largely on the action of residual Ca2+, which enters the presynaptic terminal during activity. We have shown previously that a Ca2+-dependent STE in the inhibitory synapse between interneurons L30 and L29 in the abdominal ganglion of Aplysia californica has a functional role in regulating the gain of the siphon withdrawal circuit through facilitated recurrent inhibition onto the L29s. In the present paper, we further explore the role of Ca2+ in L30 STE by examining two basic issues: 1) What is the role of residual presynaptic Ca2+ in the maintenance of L30 STE? We examine this question by first inducing STE in the L30s then rapidly buffering presynaptic free calcium through the use of the photoactivated Ca2+ chelator diazo-4, which was preloaded into the L30 neurons. Three forms of STE in the L30s were examined: frequency facilitation (FF), augmentation (AUG), and posttetanic potentiation (PTP). In each case, the activation-induced enhancement of the L30 to L29 synapse was reduced to preactivation levels at the first test pulse following photolysis of diazo-4. 2) What is the role of postsynaptic Ca2+ in the induction of L30 STE? We examine whether there is a postsynaptic requirement of elevated Ca2+ for the induction of L30 STE by first injecting the calcium chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the postsynaptic cell L29 (at levels sufficient to block transmitter release from the L29s), to prevent any increase in postsynaptic intracellular Ca2+ that may occur during L30 (presynaptic) activation. We found that BAPTA injection did not effect either the induction or the time course of FF, AUG, or PTP in the L30s. Taken collectively, our data indicate that all forms of STE in the L30s depend on presynaptic free cytosolic Ca2+ for their maintenance but do not require the elevation of postsynaptic Ca2+ for their induction.

Multiple overlapping processes underlying short-term synaptic enhancement.

Recently there have been exciting advances in understanding the mechanisms and functional roles of a form of short-term synaptic enhancement (STE) that results from an activity-dependent accumulation of Ca2+ in the presynaptic terminal. This form of STE is composed of at least four processes: fast-decaying facilitation (FI), slow-decaying facilitation (F2), augmentation (AUG) and post-tetanic potentiation (PTP). Recent results suggest that these processes can now be distinguished mechanistically by the site of their induction within the presynaptic terminal: FI and F2 appear to be induced by a rapid, high concentration of Ca2+ at or near the site of exocytosis, whereas AUG and PTP seem to be induced by lower levels of Ca2+ with slower kinetics, possibly within the core of the terminal. STE is highly conserved across diverse species, and appears to serve as a flexible mechanism for temporal information processing in systems ranging from peripheral motor control to higher cortical integration.