Adding a nonadsorbing polymer to passive colloids induces an attraction between the particles via the "depletion" mechanism. High enough polymer concentrations lead to phase separation. We combine ...experiments, theory, and simulations to demonstrate that using active colloids (such as motile bacteria) dramatically changes the physics of such mixtures. First, significantly stronger interparticle attraction is needed to cause phase separation. Secondly, the finite size aggregates formed at lower interparticle attraction show unidirectional rotation. These micro-rotors demonstrate the self-assembly of functional structures using active particles. The angular speed of the rotating clusters scales approximately as the inverse of their size, which may be understood theoretically by assuming that the torques exerted by the outermost bacteria in a cluster add up randomly. Our simulations suggest that both the suppression of phase separation and the self-assembly of rotors are generic features of aggregating swimmers and should therefore occur in a variety of biological and synthetic active particle systems.
We demonstrate a method for the fast, high-throughput characterization of the dynamics of active particles. Specifically, we measure the swimming speed distribution and motile cell fraction in ...Escherichia coli suspensions. By averaging over ∼10(4) cells, our method is highly accurate compared to conventional tracking, yielding a routine tool for motility characterization. We find that the diffusivity of nonmotile cells is enhanced in proportion to the concentration of motile cells.
We have measured the spatial distribution of motile Escherichia coli inside spherical water droplets emulsified in oil. At low cell concentrations, the cell density peaks at the water-oil interface; ...at increasing concentration, the bulk of each droplet fills up uniformly while the surface peak remains. Simulations and theory show that the bulk density results from a "traffic" of cells leaving the surface layer, increasingly due to cell-cell scattering as the surface coverage rises above ∼10%. Our findings show similarities with the physics of a rarefied gas in a spherical cavity with attractive walls.
The microaerophilic magnetotactic bacterium Magnetospirillum gryphiswaldense swims along magnetic field lines using a single flagellum at each cell pole. It is believed that this magnetotactic ...behavior enables cells to seek optimal oxygen concentration with maximal efficiency. We analyze the trajectories of swimming M. gryphiswaldense cells in external magnetic fields larger than the earth’s field, and show that each cell can switch very rapidly (in <0.2 s) between a fast and a slow swimming mode. Close to a glass surface, a variety of trajectories were observed, from straight swimming that systematically deviates from field lines to various helices. A model in which fast (slow) swimming is solely due to the rotation of the trailing (leading) flagellum can account for these observations. We determined the magnetic moment of this bacterium using a to our knowledge new method, and obtained a value of (2.0±0.6)×10−16 A · m2. This value is found to be consistent with parameters emerging from quantitative fitting of trajectories to our model.
The densities of (octane+benzene, or toluene, or 1,3-xylene, or 1,3,5-trimethylbenzene) were measured at temperatures (298.15, 308.15, 318.15, and 328.15)K by means of a vibrating-tube densimeter. ...The excess molar volumes VmE calculated from the density data provide the temperature dependence of VmE in the temperature range of (298 to 328)K. The VmE results were correlated using the fourth-order Redlich–Kister equation, with the maximum likelihood principle being applied for the determination of the adjustable parameters. It was found that the values of VmE are not very much dependent on temperature and in all cases decrease with the number of methyl groups on benzene ring of the alkylbenzene.
► We measured density and speed of sound at four temperatures within (298.15 to 328.15)
K. ► Excess quantities were calculated and fitted to the Redlich–Kister equation. ► The complete ternary data ...were fitted to the modified Redlich–Kister equation. ► Even for the systems with self-associating alcohol, only one ternary parameter is sufficient.
The densities and speeds of sound of (ethanol
+
isooctane), (ethanol
+
toluene), and (ethanol
+
isooctane
+
toluene) were measured at four temperatures over the range (298.15 to 328.15)
K, and the respective values of excess volumes
V
m
E
and adiabatic compressibility
κ
S
were calculated. The
V
m
E
and
κ
S
values for the binary systems were fitted to the Redlich–Kister equation. The respective ternary data together with corresponding binary data were then fitted to the modified Redlich–Kister equation considering various numbers of ternary constants. It was found that even for the systems containing self-associating alcohol, only one ternary parameter is sufficient to describe well the ternary system.
The densities and speeds of sound of (toluene
+
isooctane), (MTBE
+
toluene), and (MTBE + isooctane) were measured at four temperatures from (298.15 to 328.15)
K, and the respective values of excess ...volumes
V
m
E
and adiabatic compressibility
κ
S were calculated. The
V
m
E
and
κ
S values were fitted to the fourth-order Redlich–Kister equation. The
V
m
E
values for (MTBE
+
toluene) are negative and decreasing with increasing temperature. The other systems show positive
V
m
E
with comparatively small temperature dependence.
The densities of pyridine, 2-picoline, 3-picoline, and 4-picoline were measured at elevated pressures (0.1 to 40)
MPa at four temperatures over the range (298.15 to 328.15)
K with a high-pressure ...apparatus. The high-pressure density data were fitted to the Tait equation and the isothermal compressibility were calculated with a novel computation procedure with the aid of this equation.
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