Zhenjun Jiao

Manipulation of a droplet in a planar channel by periodic thermocapillary actuation

Thermocapillary manipulation of a droplet in a planar microchannel with periodic actuations has been demonstrated by both theoretical simulation and experimental characterization. The driving temperature gradients are provided by four micro heaters embedded in the boundaries of the planar channel. The temperature distributions corresponding to the periodic actuations are calculated, and are coupled to the droplet through the surface tensions which drive the droplet. The results show that the droplet will be driven to move along closed loops whose patterns can be designed and controlled by the periodic heating schemes and actuation frequencies. Qualitative agreement between the simulation and experimental observation, in terms of the temperature distributions and droplet moving tracks, has been obtained.

Thermocapillary actuation of a water droplet encapsulated in an oil plug

This paper reports the theoretical and experimental results of thermocapillary actuation of silicon oil plugs with encapsulated water droplet inside a capillary tube. The paper first describes the analytical models for this actuation concept. The model considers periodic actuation based on heat transfer in the capillary wall and the surface tension driven motion of the plug. In the experiments, both temperature field and plug motion were measured and evaluated. While the temperature field was measured by an infrared thermal tracer camera, the position of the plugs was automatically captured and evaluated with a CCD camera. Several phenomena resulting from the interaction between surfactant modified silicone oil and water were observed. Different cases were observed and investigated: the water droplet partially wets the wall, the water droplet completely wets the wall and the water droplet is encapsulated.

Scattering and attenuation of surface acoustic waves in droplet actuation

This paper presents an analytical model for surface acoustic waves (SAW) in actuating a single liquid droplet on a piezoelectric substrate. Both scattering waves outside the droplet and attenuation waves beneath the droplet are obtained, and the energy transfer from SAW to droplet in the process of actuation is calculated. The results from this analytical model can provide qualitative explanations to some experimental observations, such as the weak actuation behind the droplet and flow patterns inside the droplet. It is found that effective actuation wavelength is around 1/5 of the droplet radius, at which the droplet absorbs the maximum incident SAW energy.

Chaotic motion of microplugs under high-frequency thermocapillary actuation

This paper reports experimental results on chaotic motion of liquid plugs under high-frequency thermocapillary actuation. In the experiments, two heaters were alternately activated. The liquid plug was positioned in a glass capillary between these two heaters. The periodic temperature gradients generated by the two heaters made the liquid plug move back and forth. The images of the plug were captured by a camera. Plug position by a customized image-processing programme. The results were recorded as time series for further analysis. The nonlinear motion of the liquid plug in a glass capillary was systematically investigated with tools coming from chaotic dynamics. Phase-space portraits are reconstructed from the time series of the plug position obtained in the experiments. The results show that the limit sets formed by the phase-space trajectories are attractors. The largest Lyapunov exponent and correlation dimension of these attractors are calculated and indicate that these attractors are chaotic. A chaotic plug motion has potential applications in fluid mixing inside liquid plugs of capillary-based microfluidics. External chaotic motion could improve mixing in liquid plugs of capillary-based lab-on-a-chip platforms.

Nonlinear standing waves in a resonator with feedback control.

An experimental study is presented to demonstrate that nonlinear effect on standing waves in a resonator can be reduced by a feedback loop responding to the second harmonic. The resonator was a cylindrical tube sealed at one end and driven by a horn driver unit at another end. The feedback control loop consisted of a pressure sensor, a frequency filter, a phase shifter, and an actuator. The results show that the waveform distortions can be eliminated and large amplitude sinusoidal pressure oscillations are obtained. A simple model is proposed for a qualitative discussion on the control mechanism, which shows that the feedback loop alters the imaginary part of the complex mode frequency so as to suppress (or enhance) the second harmonic.

Study on the correlation between Solid Oxide Fuel Cell Ni-YSZ anode performance and reduction temperature

The influences of reduction temperature on the initial performance and shorttime durability of nickel-yttria-stabilized zirconia composite solid oxide fuel cell anode were investigated. Anode microstructures before and after operation were quantitatively analyzed by three-dimensional reconstruction based on focused ion beam-scanning electron microscopy technique. The anode reduced at 500 oC showed the worst initial performance and stability in operation and the anode reduced at 800 oC showed the smallest polarization resistance. The anode reduced at 1000 oC showed the most stable performance with polarization resistance enhanced with operation. It is found that higher reduction temperature leads to dense nickel and enhances nickel-yttria-stabilized-zirconia interfacial bonding, which can inhibit nickel sintering and improve the composite anode stability in long-time operation.

Study on Degradation of Solid Oxide Fuel Cell with Pure Ni Anode

Zhenjun Jiao, Naoki Shikazono and Nobuhide Kasagi 1 IIS, the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo. 153-8505, Japan 2 Department of Mechanical Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan Abstract Current popular SOFC anode is made by composite cermet of Ni and yttria stabilized zirconia (YSZ). The interaction between Ni and YSZ at triple phase boundary (TPB) must be studied intensively to investigate anode degradation mechanism. In this study, the degradation phenomena at the interface between Ni and YSZ have been investigated by using screen-printed pure Ni anode sintered on YSZ pellet. The time depended microstructural evolution of Ni-YSZ interface in long time galvanostatic polarization is studied in hydrogen with different humidities. Compared with normal composite cermet anode experiments, this method provides easy observation of the Ni-YSZ interface where degradation takes place. The microstructural changes were characterized by scanning electron microscopy (SEM). The influence of humidity on sintered pure Ni anode is investigated and the degradation mechanism at Ni-YSZ interface is discussed.

Thermocapillary Actuation and Cycling of Liquid Plugs

With the aim toward realizing polymerase chain reaction (PCR) of deoxyribonucleic acid (DNA) in plug-based capillary platforms, this paper reports the theoretical and experimental results of thermocapillary actuation for temperature cycling with an arbitrary ramping function. Two concepts were investigated: (a) actuation and spatial temperature cycling with three heaters and (b) actuation and temporal cycling with two heaters. The paper first describes the analytical models of both concepts. The model considers both the transient and coupling effect between heat transfer in the capillary wall and the surface tension driven movement of the plug. In the experiments, both temperature field and plug motion were measured and evaluated. The temperature field were captured by an infrared thermo tracer camera. The position of the plugs was automatically captured and evaluated with a CCD camera. Finally, analytical and experimental results are compared and discussed.Copyright

Programmable Manipulation of a Droplet in a Planar Microchannel

Programmable thermocapillary manipulation of liquid droplet in a planar microchannel has been carried out by both theoretical modeling and experimental characterization in this paper. The driving temperature gradients are provided by four micro-heaters at the channel boundaries. In the modeling, the temperature distributions corresponding to both transient and periodic actuation are calculated, and are coupled to the droplet motion through the surface tensions which drives the droplet to move inside the channel. The droplet trajectories and final positions are simulted, and compared with the experimental results, in which a silicon oil droplet was actuated inside a 10 mm×10 mm planar channel with four heater fabricated on the substrate plate. The results show that the droplet can be positioned anywhere in the channel, determined by a heating code related to the heating strengths. Qualitative agreement between the modeling results and experimental data, in terms of temperature distributions, droplet positions and trajectories, has been obtained.Copyright

Thermal Fluid Interaction in a Periodically Heated Capillary

This paper reports an experimental study on reciprocating thermocapillary motion of a liquid plug in an externally heated glass capillary. The results were qualitatively compared with that from an analytical modeling. In the experiments, a liquid plug in a glass capillary was positioned between two heaters which were activated alternatively. The liquid plug was driven by the surface tension difference generated by the temperature gradients. The periodic temperature gradients generated by the two heaters made the liquid plug to move back and fort. This method has a potential in manipulating not only the plug motion but also the flow field inside the plug. The position of the plugs was captured and evaluated using a CCD camera. The plug position and maximum traveling distance were measured under various switching frequencies, and the results were recorded as time series for the dynamic analysis. The temperature variation between the heaters depended on the heating process, also depended on the liquid plug motions. An infra thermal camera was used to observe and record the capillary surface temperature when the liquid plug oscillated. A simple model was established for the liquid plug oscillation in the capillary under the periodical heating. The measured liquid plug motion and the temperature variations were compared with predicted results from the model.Copyright