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Friday, December 19, 2014

Trapping of nanoparticles in a liquid by laser-induced microbubbles

V I Yusupov, S I Tsypina and V N Bagratashvili

Nanoparticles a few nanometers in diameter contained in a colloidal solution can be captured by the surface of a microbubble produced by a well-focused beam of a continuous wave visible laser. The concentration of the nanoparticles on the surface of the microbubble is found to gradually increase during the course of irradiation. The mechanism of such a trapping is associated with the laser-induced Marangoni convection developing in the vicinity of the microbubble as a result of the temperature gradient obtained at its surface.

DOI

Organic Component Vapour Pressures and Hygroscopicities of Aqueous Aerosol Measured by Optical Tweezers

Chen Cai, David Stewart, Jonathan Philip Reid, Yun-Hong Zhang, Peter Ohm, Cari S. Dutcher, and Simon Leslie Clegg

Measurements of the hygroscopic response of aerosol and the particle-to-gas partitioning of semi-volatile organic compounds are crucial for providing more accurate descriptions of the compositional and size distributions of atmospheric aerosol. Concurrent measurements of particle size and composition (inferred from refractive index) are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes. The measurements are shown to allow accurate retrievals of component vapour pressures and hygroscopic response through examining correlated variations in size and composition for binary droplets containing water and a single organic component. Measurements are reported for an homologous series of dicarboxylic acids, maleic acid, citric acid, glycerol or 1,2,6-hexanetriol. An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach. We also show that the method of molar refraction provides an accurate characterisation of the compositional dependence of the refractive index of the solutions. In this method, the density of the pure liquid solute is the largest uncertainty and must be either known or inferred from subsaturated measurements with an error of <± 2.5 % to discriminate between different thermodynamic treatments.

DOI

The electromagnetic force in the terahertz band generated by a cross-shaped absorber

Jiahui Fu, Wan Chen, Bo Lv, Lei Zhu, Qun Wu

A 2D periodic force generator in terahertz band is presented. A multi-layered structure is designed, which consists of two adjacent dielectric layers and a cross-shaped metal patch. Once the structure is resonant, strong coupling effect takes place between the two dielectric layers. Due to the coupling effect, the electromagnetic field is enhanced, which leads to a large electromagnetic force. The force is calculated by the Maxwell Stress Tensor. The magnitude of the force is three orders higher than the force generated in the optical band presented in other papers. The power density of the incident light is studied for the demonstration of the usefulness of the force in terahertz band generated by the structure. Dielectric loss is also taken into consideration. The result shows that the magnitude of the force is still enough to offset the gravity of the unit cell even with a high dielectric loss.

DOI

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

Oliver R. Hunt, Andrew D. Ward and Martin D. King

The oxidation of nitrite anion within an aqueous atmospheric droplet may be a sink for HONO in the lower atmosphere. An optical trap with Raman spectroscopy is used to demonstrate that the oxidation of aqueous nitrite anion in levitated, micron sized, aqueous droplets by gas-phase ozone is consistent with bulk aqueous-phase kinetics and diffusion. There is no evidence of an enhanced or retarded reaction at the droplet surface at the concentrations used in the experiment or likely to be found in the atmosphere. The oxidation of nitrite in an aqueous droplet by gas-phase ozone does not cause the droplet to hydrodynamically change in size and demonstrates use of an optical trap as a wall-less reactor to measuring aqueous-phase rate coefficients.

DOI

Thursday, December 18, 2014

Toward optical-tweezers-based force microscopy for airborne microparticles

Rory M. Power, Daniel R. Burnham, and Jonathan P. Reid

Optical tweezers have found widespread application in biological and colloidal physics for the measurement of pN forces over nanometer to micrometer length scales. Similar aerosol-phase measurements of interparticle force have not been reported in spite of the potential to better resolve particle coagulation kinetics. Various refractive index mismatches in the beam path as well as the need to explicitly account for gravity and inertial particle motion provide a number of challenges that must be overcome to make such measurements tractable. In this regard, we demonstrate schemes by which the particle position and trap stiffness may be unambiguously measured using bright-field microscopy with resolution comparable with analogous condensed-phase measurements. Moreover, some of the challenges of working with highly dynamic aqueous particles are introduced and exploited to observe size-dependent phenomena in aerosol optical tweezers. Notably, when combined with cavity-enhanced Raman spectroscopy, this provides a unique opportunity to explore trapping forces over a continuum of particle size and refractive index. It is expected that the methods developed will provide a basis for the measurement of pairwise interaction forces in aerosol optical tweezers while providing a probe of fundamental airborne particle trapping dynamics.

DOI

Stretching of red blood cells using an electro-optics trap

Md. Mozzammel Haque, Mihaela G. Moisescu, Sándor Valkai, András Dér, and Tudor Savopol

The stretching stiffness of Red Blood Cells (RBCs) was investigated using a combination of an AC dielectrophoretic apparatus and a single-beam optical tweezer. The experiments were performed at 10 MHz, a frequency high enough to avoid conductivity losses, but below the second turnover point between positive and negative dielectrophoresis. By measuring the geometrical parameters of single healthy human RBCs as a function of the applied voltage, the elastic modulus of RBCs was determined (µ = 1.80 ± 0.5 µN/m) and compared with similar values of the literature got by other techniques. The method is expected to be an easy-to-use, alternative tool to determine the mechano-elastic properties of living cells, and, on this basis, to distinguish healthy and diseased cells.

DOI

Evaluating the toxic effect of an antimicrobial agent on single bacterial cells with optical tweezers

Akbar Samadi, Chensong Zhang, Joseph Chen, S. N. S. Reihani, and Zhigang Chen

We implement an optical tweezers technique to assess the effects of chemical agents on single bacterial cells. As a proof of principle, the viability of a trapped Escherichia coli bacterium is determined by monitoring its flagellar motility in the presence of varying concentrations of ethyl alcohol. We show that the “killing time” of the bacterium can be effectively identified from the correlation statistics of the positional time series recorded from the trap, while direct quantification from the time series or associated power spectra is intractable. Our results, which minimize the lethal effects of bacterial photodamage, are consistent with previous reports of ethanol toxicity that used conventional culture-based methods. This approach can be adapted to study other pairwise combinations of drugs and motile bacteria, especially to measure the response times of single cells with better precision.

DOI

Wednesday, December 17, 2014

Measurements of liposome biomechanical properties by combining line optical tweezers and dielectrophoresis

Ellas Spyratou, Efrosini Cunaj, George Tsigaridas, Elena A. Mourelatou, Costas Demetzos, Alexander A. Serafetinides, and Mersini Makropoulou

Liposomes are well-known cell simulators and are currently studied as drug delivery systems, for a targeted delivery of higher drug concentrations, in specific cells. Novel biophotonic techniques for manipulation and characterization of liposomes have been developed; among which are optical tweezers. In our work, we demonstrate a novel use of line optical tweezers to manipulate and cause liposome deformations. Optical forces induce tension on liposomes, which are stretched along the line optical trap. The method of dielectrophoresis, combined with optical tweezers, was used to measure the exerted optical forces. As a consequence, in the case of reversible liposome deformations, the value of the shear and bending moduli of liposomes was calculated. We anticipate that the selective manipulation of liposomes will help us toward a better understanding of the cellular–liposome interactions. Studying the biomechanical properties of liposomes will provide an insight into the mechanical behavior of individual living cells, which have recently been implicated in many aspects of human physiology and patho-physiology. The biomechanical properties of cells (i.e. deformability, stiffness and elasticity) can be useful biomarkers for various disease processes and changes of the cell state.

DOI

In situ seriate droplet coalescence under an optical force

Jin Ho Jung, Kyung Heon Lee, Ghulam Destgeer, Kang Soo Lee, Hyunjun Cho, Byung Hang Ha, Hyung Jin Sung

We demonstrated the induced coalescence of droplets under a highly accurate optical force control. Optical scattering and gradient forces were used to push and trap the droplets prior to coalescence within a microfluidic channel. The behavior of the droplets under the influence of an optical force was predicted using an analytical model that agreed well with the experimental data. The optical gradient force accelerated and decelerated the droplet within the laser beam region, and the drag force acting on the droplet was thoroughly characterized. A description of the optical trap was presented in terms of the momentum transfer from the photons to the droplet, effectively restricting droplet motion inside the microfluidic channel prior to coalescence. A phase diagram was plotted to distinguish between the three regimes of droplet coalescence, including the absence of coalescence, coalescence, and multiple coalescence events. The phase diagram permitted the laser power input and the net flow rate in the microfluidic channel to be estimated. This technique was applied to the synthesis of biodegradable gel microparticles.

DOI

Dynamic orientation of azopyridine units within the shell of vesicles PNIPAM-b-PAzPyn copolymers

Yingjue Wang, Guangyong Shen, Jiangang Gao, Gang Zou andQijin Zhang

Vesicles with hydrophobic shells have been self-assembled through three kinds of amphiphilic block copolymers containing pendent azopyridine groups with different spacers, namely PNIPAM-b-PAzPyn (n = 0, 2, 6), respectively. By polarization laser-trapping Raman spectroscopy, the photoinduced orientation behaviors of azopyridine groups within the vesicle shells have been investigated and it is found that spacer lengths affect the orientation of the azopyridine groups and the morphologic structure of the vesicle shells. The exact experimental results show that the orientation is dynamic for the pendent azopyridine groups with connecting spacers of 2 or 6 methylene units rather than those without spacers, so the vesicles of PNIPAM-b-PAzPy6 can be changed to show a typical “soft” character compared with its solid films when irradiated with a relatively weak polarized UV light of 190 µW/cm2. However, the vesicles of PNIPAM-b-PAzPy0 without spacers do not change even though the azopyridine units can be oriented. By quantitative Raman spectral analysis, it is found that the isomerization degree of azopyridine units is 70% for PNIPAM-b-PAzPy6 yet it is 10% for PNIPAM-b-PAzPy0, which shows a close relationship between aggregation and isomerization of azopyridine units under a weak UV light.

DOI