The Neural Circuitry Of Social Behavior In C. elegans

Most animal species, from simple invertebrates to complex mammals, require behavioral mechanisms to communicate with and respond to conspecifics, whether to mate, to assess predatory danger, or evaluate the nutritional quality of the surrounding environment. Understanding the molecular and cellular underpinnings of these social behaviors remains a central challenge in neurobiology. I used the nematode C. elegans as a model system to study the genetics and neural circuitry that underlie social behavior. First, I evaluated the behavioral responses of C. elegans to a nematode extract (deathmone), which served as a model for alarm pheromones in other animal species (chapter 2). Worms showed acute avoidance of deathmone, and reduced their exploration when cultivated on it, a behavior termed “dwelling.†I combined chemical analysis, laser ablation studies, and genetic studies to identify the sensory neurons and molecular signaling pathways that promote dwelling in response to deathmone. Second, I investigated the neuronal substrates responsible for social feeding, a behavior in which certain strains of C. elegans display high lomocotory speeds, accumulate on the border of bacterial food lawns, and aggregate into groups. A low activity or null allele of the neuropeptide y receptor homologue npr-1 promotes social feeding, while a high activity form—which is found in the wild-type N2 strain—promotes solitary behavior1. Expression of a high-activity npr-1 cDNA specifically in the interneuron RMG converted npr-1 loss-of-function mutants from social feeders into solitary ones. The RMG neurons are gap junctional hubs that electrically couple the sensory neurons URX, ASH, and ADL—all previously implicated in social feeding—and the pheromone-sensing neuron ASK, suggesting that social feeding and pheromone responses may be related. Indeed, npr-1 social feeders are attracted to ascarosides, while N2 solitary feeders are repelled, a behavioral difference that is dependent on RMG function. Calcium imaging of ASK and its postsynaptic partner AIA demonstrated that RMG promotes signaling from ASK to AIA. Taken together, these data provide a common neural circuitry for social behaviors in C. elegans, and offer some insights into the molecular mechanisms of their regulation

Decoupling the effects of shear and extensional flows on the alignment of colloidal rods

Cellulose nanocrystals (CNC) can be considered as model colloidal rods and have practical applications in the formation of soft materials with tailored anisotropy. Here, we employ two contrasting microfluidic devices to quantitatively elucidate the role of shearing and extensional flows on the alignment of a dilute CNC dispersion. Characterization of the flow field by micro-particle image velocimetry is coupled to flow-induced birefringence analysis to quantify the deformation rate--alignment relationship. The deformation rate required for CNC alignment is 4$\times$ smaller in extension than in shear. Alignment in extension is independent of the deformation rate magnitude, but is either 0$^\circ$ or 90$^\circ$ to the flow, depending on its sign. In shear flow the colloidal rods orientate progressively towards 0$^\circ$ as the deformation rate magnitude increases. Our results decouple the effects of shearing and extensional kinematics at aligning colloidal rods, establishing coherent guidelines for the manufacture of structured soft materials

Viscoelastic shear banding in foam

Shear banding is an important feature of flow in complex fluids. Essentially, shear bands refer to the coexistence of flowing and non-flowing regions in driven material. Understanding the possible sources of shear banding has important implications for a wide range of flow applications. In this regard, quasi-two dimensional flow offers a unique opportunity to study competing factors that result in shear bands. One proposal is the competition between intrinsic dissipation and an external source of dissipation. In this paper, we report on the experimental observation of the transition between different classes of shear-bands that have been predicted to exist in cylindrical geometry as the result of this competition [R. J. Clancy, E. Janiaud, D. Weaire, and S. Hutzlet, Eur. J. Phys. E, {\bf 21}, 123 (2006)]

Impact of boundaries on velocity profiles in bubble rafts

Under conditions of sufficiently slow flow, foams, colloids, granular matter, and various pastes have been observed to exhibit shear localization, i.e. regions of flow coexisting with regions of solid-like behavior. The details of such shear localization can vary depending on the system being studied. A number of the systems of interest are confined so as to be quasi-two dimensional, and an important issue in these systems is the role of the confining boundaries. For foams, three basic systems have been studied with very different boundary conditions: Hele-Shaw cells (bubbles confined between two solid plates); bubble rafts (a single layer of bubbles freely floating on a surface of water); and confined bubble rafts (bubbles confined between the surface of water below and a glass plate on top). Often, it is assumed that the impact of the boundaries is not significant in the ``quasi-static limit'', i.e. when externally imposed rates of strain are sufficiently smaller than internal kinematic relaxation times. In this paper, we directly test this assumption for rates of strain ranging from $10^{-3}$ to $10^{-2} {\rm s^{-1}}$. This corresponds to the quoted quasi-static limit in a number of previous experiments. It is found that the top plate dramatically alters both the velocity profile and the distribution of nonlinear rearrangements, even at these slow rates of strain.Comment: New figures added, revised version accepted for publication in Phys. Rev.

Flow transitions in two-dimensional foams

For sufficiently slow rates of strain, flowing foam can exhibit inhomogeneous flows. The nature of these flows is an area of active study in both two-dimensional model foams and three dimensional foam. Recent work in three-dimensional foam has identified three distinct regimes of flow [S. Rodts, J. C. Baudez, and P. Coussot, Europhys. Lett. {\bf 69}, 636 (2005)]. Two of these regimes are identified with continuum behavior (full flow and shear-banding), and the third regime is identified as a discrete regime exhibiting extreme localization. In this paper, the discrete regime is studied in more detail using a model two dimensional foam: a bubble raft. We characterize the behavior of the bubble raft subjected to a constant rate of strain as a function of time, system size, and applied rate of strain. We observe localized flow that is consistent with the coexistence of a power-law fluid with rigid body rotation. As a function of applied rate of strain, there is a transition from a continuum description of the flow to discrete flow when the thickness of the flow region is approximately 10 bubbles. This occurs at an applied rotation rate of approximately $0.07 {\rm s^{-1}}$

A compact 90 kilowatt electric heat source for heating inert gases to 1700 F

Design and fabrication of compact electric heat source for heating inert gase

Statistics of Bubble Rearrangements in a Slowly Sheared Two-dimensional Foam

Many physical systems exhibit plastic flow when subjected to slow steady shear. A unified picture of plastic flow is still lacking; however, there is an emerging theoretical understanding of such flows based on irreversible motions of the constituent ``particles'' of the material. Depending on the specific system, various irreversible events have been studied, such as T1 events in foam and shear transformation zones (STZ's) in amorphous solids. This paper presents an experimental study of the T1 events in a model, two-dimensional foam: bubble rafts. In particular, I report on the connection between the distribution of T1 events and the behavior of the average stress and average velocity profiles during both the initial elastic response of the bubble raft and the subsequent plastic flow at sufficiently high strains

The electrorheology of suspensions consisting of Na-Fluorohectorite synthetic clay particles in silicon oil

Under application of an electric field greater than a triggering electric field $E_c \sim 0.4$ kV/mm, suspensions obtained by dispersing particles of the synthetic clay fluoro-hectorite in a silicon oil, aggregate into chain- and/or column-like structures parallel to the applied electric field. This micro-structuring results in a transition in the suspensions' rheological behavior, from a Newtonian-like behavior to a shear-thinning rheology with a significant yield stress. This behavior is studied as a function of particle volume fraction and strength of the applied electric field, $E$. The steady shear flow curves are observed to scale onto a master curve with respect to $E$, in a manner similar to what was recently found for suspensions of laponite clay [42]. In the case of Na-fluorohectorite, the corresponding dynamic yield stress is demonstrated to scale with respect to $E$ as a power law with an exponent $\alpha \sim 1.93$, while the static yield stress inferred from constant shear stress tests exhibits a similar behavior with $\alpha \sim 1.58$. The suspensions are also studied in the framework of thixotropic fluids: the bifurcation in the rheology behavior when letting the system flow and evolve under a constant applied shear stress is characterized, and a bifurcation yield stress, estimated as the applied shear stress at which viscosity bifurcation occurs, is measured to scale as $E^\alpha$ with $\alpha \sim 0.5$ to 0.6. All measured yield stresses increase with the particle fraction $\Phi$ of the suspension. For the static yield stress, a scaling law $\Phi^\beta$, with $\beta = 0.54$, is found. The results are found to be reasonably consistent with each other. Their similarities with-, and discrepancies to- results obtained on laponite-oil suspensions are discussed

Velocity Profiles in Slowly Sheared Bubble Rafts

Measurements of average velocity profiles in a bubble raft subjected to slow, steady-shear demonstrate the coexistence between a flowing state and a jammed state similar to that observed for three-dimensional foams and emulsions [Coussot {\it et al,}, Phys. Rev. Lett. {\bf 88}, 218301 (2002)]. For sufficiently slow shear, the flow is generated by nonlinear topological rearrangements. We report on the connection between this short-time motion of the bubbles and the long-time averages. We find that velocity profiles for individual rearrangement events fluctuate, but a smooth, average velocity is reached after averaging over only a relatively few events.Comment: typos corrected, figures revised for clarit