Active Bath Research Articles

Bacterial ratchet motors

Self-propelling bacteria are a nanotechnology dream. These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment, and look for food, all packaged in a body measuring a few microns. Before such perfect machines can be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for microdevices. Proposed strategies require the careful task of aligning and binding bacterial cells on synthetic surfaces in order to have them work cooperatively. Here we show that asymmetric environments can produce a spontaneous and unidirectional rotation of nanofabricated objects immersed in an active bacterial bath. The propulsion mechanism is provided by the self-assembly of motile Escherichia coli cells along the rotor boundaries. Our results highlight the technological implications of active matter's ability to overcome the restrictions imposed by the second law of thermodynamics on equilibrium passive fluids.

Fluctuations and Rheology in Active Bacterial Suspensions

We probe nonequilibrium properties of an active bacterial bath through measurements of correlations of passive tracer particles and the response function of a driven, optically trapped tracer. These measurements demonstrate violation of the fluctuation-dissipation theorem and enable us to extract the power spectrum of the active stress fluctuations. In some cases, we observe 1/sqrt[omega] scaling in the noise spectrum which we show can be derived from a theoretical model incorporating coupled stress, orientation, and concentration fluctuations of the bacteria.

Zincate-Free, Electroless Nickel Deposition on Aluminum Bond Pads

A zincate-free electroless nickel deposition on aluminum bond pads is investigated. A three-step, etch, rinse, and electroless plating, is demonstrated for deposition on aluminum bond pads patterned with polyimide. The chemicals used are compatible with this dielectric material. The deposition has been achieved with good selectivity, uniformity, and deposition rate at aluminum bond pads. The adhesion and contact resistance were also determined and improved through anneals in the range 200-400°C for 1 h. The optimized condition for adhesion and contact resistance was an anneal at 400°C. The combination of a nickel hypophosphite reducing agent and the additives used leads to an active plating bath in the early stages of deposition, by comparison with commercial solutions, and hence, good coverage of the aluminum bond pad using the simplified process. © 2004 The Electrochemical Society. All rights reserved.

Sub-Doppler, infrared laser spectroscopy of the propyne 2ν1 band: Evidence of z-axis Coriolis dominated intramolecular state mixing in the acetylenic CH stretch overtone

The eigenstate-resolved 2ν1 (acetylenic CH stretch) absorption spectrum of propane has been observed for J′=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full-width at half-maximum resolution of 6×10−4 cm−1 (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm−1 wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν1 to the near resonant bath states with an average matrix element of 〈V2〉1/2=0.002 cm−1 that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum. The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z-axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z-axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z-axis Coriolis coupled to the dark states. Z-axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν1 normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν1 do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.

The isokinetic relationship. X. Experimental examples to the relationship between isokinetic temperatures and active heat bath frequencies

The relationship between the frequency of an ideal heat bath required at the isokinetic temperature (which is defined by a common point of intersection for Arrhenius lines of a series of homologous reactions) and the available frequencies in a real solvent of a reacting system is examined for a large range of chemical reactions carried out in various solvents. The investigation involves isokinetic relationships based on reactant variation in one solvent and such based on solvent variation without changing the reactants (i.e. isosolvent relationships). The influence of both, reactants and solvents, on the value of the isokinetic temperature is analyzed and discussed.

Investigations of spectral broadening mechanisms in biomolecules: Cytochrome-c

We have studied the optical line shape of the intense π–π* ‘‘Soret’’ transition of ferrocytochrome-c as a function of temperature. Use of the linear electron–nuclear coupling strengths found from the transform analysis of the resonance Raman excitation profiles allows us to simulate the measured line shape changes using a multimode time correlator model that is exact at all temperatures. Four major sources of line broadening are considered: (1) Multimode broadening due to the observed Raman (Franck–Condon) active modes; (2) broadening due to a low frequency Franck–Condon active bath that is not observed directly in the resonance Raman spectrum; (3) inhomogeneous broadening, σ, modeled as a Gaussian distribution of 0–0 transition frequencies; (4) homogeneous broadening, Γ, due to exponential population decay of the excited electronic state. We find conclusive evidence that mechanism (2) is not important in determining the Soret band line shape and that mechanism (3) is constrained to an upper limit of σmax=200 cm−1. The results giving the best fits over the entire temperature range yield σ=100 cm−1, Γ=370 cm−1 and show that the thermal properties of the line shape are well described by the observed low frequency Raman modes. The resulting large value of Γ corresponds to ultrafast population decay times (10–20 fs) that are difficult to measure directly. As a result, we have expanded upon a light scattering sum rule that allows direct determination of Γ even in the presence of inhomogeneous broadening. Recent tests of this independent approach are also discussed. Finally, the analysis uncovers strong evidence for a set of weak temperature dependent transitions underlying the Soret band. These transitions are quantified via fitting to Gaussian line shapes and their temperature dependence is explicitly presented.