Thermotaxis of Janus particles

The interactions of autonomous microswimmers play an important role for the formation of collective states of motile active matter. We study them in detail for the common microswimmer-design of two-faced Janus spheres with hemispheres made from different materials. Their chemical and physical surface properties may be tailored to fine-tune their mutual attractive, repulsive or aligning behavior. To investigate these effects systematically, we monitor the dynamics of a single gold-capped Janus particle in the external temperature field created by an optically heated metal nanoparticle. We quantify the orientation-dependent repulsion and alignment of the Janus particle and explain it in terms of a simple theoretical model for the induced thermoosmotic surface fluxes. The model reveals that the particle’s angular velocity is solely determined by the temperature profile on the equator between the Janus particle’s hemispheres and their phoretic mobility contrast. The distortion of the external temperature field by their heterogeneous heat conductivity is moreover shown to break the apparent symmetry of the problem. Graphic abstract

Frequency Response of Induced-Charge Electrophoretic Metallic Janus Particles

The ability to manipulate and control active microparticles is essential for designing microrobots for applications. This paper describes the use of electric and magnetic fields to control the direction and speed of induced-charge electrophoresis (ICEP) driven metallic Janus microrobots. A direct current (DC) magnetic field applied in the direction perpendicular to the electric field maintains the linear movement of particles in a 2D plane. Phoretic force spectroscopy (PFS), a phase-sensitive detection method to detect the motions of phoretic particles, is used to characterize the frequency-dependent phoretic mobility and drag coefficient of the phoretic force. When the electric field is scanned over a frequency range of 1 kHz–1 MHz, the Janus particles exhibit an ICEP direction reversal at a crossover frequency at ~30 kH., Below this crossover frequency, the particle moves in a direction towards the dielectric side of the particle, and above this frequency, the particle moves towards the metallic side. The ICEP phoretic drag coefficient measured by PFS is found to be similar to that of the Stokes drag. Further investigation is required to study microscopic interpretations of the frequency at which ICEP mobility switched signs and the reason why the magnitudes of the forward and reversed modes of ICEP are so different.

Phoretic interactions and oscillations in active suspensions of growing Escherichia coli

Bioluminescence imaging experiments were carried out to characterize spatio-temporal patterns of bacterial self-organization in active suspensions (cultures) of bioluminescent Escherichia coli and its mutants. An analysis of the effects of mutations shows that spatio-temporal patterns formed in standard microtitre plates are not related to the chemotaxis system of bacteria. In fact, these patterns are strongly dependent on the properties of mutants that characterize them as self-phoretic (non-flagellar) swimmers. In particular, the observed patterns are essentially dependent on the efficiency of proton translocation across membranes and the smoothness of the cell surface. These characteristics can be associated, respectively, with the surface activity and the phoretic mobility of a colloidal swimmer. An analysis of the experimental data together with mathematical modelling of pattern formation suggests the following: (1) pattern-forming processes can be described by Keller–Segel-type models of chemotaxis with logistic cell kinetics; (2) active cells can be seen as biochemical oscillators that exhibit phoretic drift and alignment; and (3) the spatio-temporal patterns in a suspension of growing E. coli form due to phoretic interactions between oscillating cells of high metabolic activity.

Studies On The Effects Of Protamine On The Antithrombin III Heparin Complex

Protamine has been employed as principal neutralizing agent for heparin but there are few reports on the effect of protamine to antithrombin III (AT 3III)-heparin complex, and this interaction was analyzed by means of column chromatography, crossed immunoelectrophoresis, and heparin sepharose affinity chromatography. Sephadex G-200 chromatography of heat defibrinated plasma treated with 3H- heparin revealed radioactivity in all protein fractions and protein free fraction. Immediate antithrombin activity was detected in AT III fractions. When the plasma treated with 3H labeled heparin was fractioned on sephadex G-200 after neutralization by protamine, most of radioactivity was eluted in the void volume and the radioactivity of other fractions was markedly decreased, and the immediate antithrombin activity was lost in any fractions and progressive antithrombin activity was detected in the third protein peak. Electrophoretic mobility of heparin treated AT III was increased depending to the heparin concentrations but when the heparinized plasma was neutralized with protamine, electrophoretic mobility of AT III was recovered. Meanwhile protamine treatment did not affect on the electro phoretic mobility of not heparinized control AT III. When AT III bound heparin sepharose was eluted with protamine solution and eluted fractions were chromatographed on sephadex G-75, AT III was separated from protamine. These results indicate that protamine dissociates AT III from AT III-heparin complex with binding to heparin and that the separated AT III recovers original progressive antithrombin activity.

Factor VIII Related Properties in Platelets of Patients with von Willebrand’s Disease (VWD)

In 10 normal subjects washed human platelets (Pl)contained FVIII related antigen(VIIIR:AG) as measured by immunoradiometric assay (iRMA)and electroimmunodif fusion (EID);and ristocetin co-factor (VIIIR:RCo)as assayed by a washed platelet method. The observed values were: AG(IRMA) 0.11-0.24u/mg Pl protein; VIIIR:AG (EID)0.11-0.30u/mg; VIIIR:RCo 0.06-0.21u/mg, In 10pts with seve re homozygous VWD, VIIIR:AG was unmeasurable in 7 and extremely low (1x10-3-0.6x10-3u/mg) in 3 u-sing the very sensitive IRMA; VIII RCo was always unmeasurable. In 12 pts with “classical” dominant VWD characterized by very low plasma levels of VIIIR:AG(0.03-0.09u/ml)and VIIIR:RCo(<0.03u/ml), FVIII related properties were normal in Pl and the mobility of PI VIIIR:AG on bidimensional immunolectrophoresis was not different from that of normal controls. In 7 pts showing a faster mo bility of plasma VIIIR:AG, the same abnormality was found in Pl.Pl VIIIR:AG level was within the normal range when assayed by EID whereas IRMA gave lower values both in plasma and in PI.PI VIII R:RCo was lower than in normal subjects and pts with “classical”VWD without electrophoretic variant. These findings show that severe VWD is the expression of a marked reduction of VIII synthesis fully expressed both in PI and in plasma. In “classical” VWD the plasma defects are not reflected in PI, which show normal levels of FVIII-related properties accompanied by normal electro, phoretic mobility of VTIIR:AG; this suggests a defective transfer from PI to plasma. Patients with abnormal mobility are the expression of a qualitative alteration of the FVIII molecole functionally defective both in Pl and in plasma.

Studies on Monotreme Proteins. IV Amino Acid Sequence of Haemoglobin-IA of the Echidna; A Comparison of Major Haemoglobins From Two Geographical Groups of Echidnas

The amino acid sequences of the ex-and fJ-chains from a major haemoglobin (Hb-IA) of an echidna geographically isolated from those animals previously studied have been determined. The fJ-chain of Hb-IA was identical in amino acid sequence with the fJ-chain in Hb-IB. The ex-chain of Hb-IA varied in four positions from that in Hb-IB, and had one more acidic group, in line with the higher electro-phoretic mobility of Hb-IA at pH 8� 5. There were no differences in 'contact site' residues in the ex-chains of the two haemoglobins.

Purification and properties of a chlamydial hemagglutinogen

A hemagglutinogen was isolated from the soluble antigens of a 6BC strain of psittacosis grown in cell culture. The antigen, which was purified by chromatography and electrophoresis, has a molecular weight of 100 000 to 120 000 and a density of 1.235 at 4 °C. The serologic activity of the hemagglutinogen was not affected by acid hydrolysis but was decreased by neuraminidase treatment. It reacted more strongly with homologous than with heterologous antiserum, which might suggest the presence of more than one antigen in the preparation not separable on the basis of molecular weight, density, or electro-phoretic mobility in polyacrylamide gel. A possible alternative explanation would be the presence of a group- and type-specific activity on the same molecule.

A Progressive Change in the Electro Phoretic Mobility of Preaggregation Cells of the Slime Mould, Dictyostelium Discoideum

The electrophoretic mobility of slime-mould preaggregation cells has been found to decrease progressively as they near the chemotactic aggregation stage. This appears to be a spontaneous event dependent on some aspect of cell metabolism. Its possible relevance to cell adhesion and morphogenesis is discussed.