Hyuk Kyu Pak

Electrical power generation by mechanically modulating electrical double layers

Since Michael Faraday and Joseph Henry made their great discovery of electromagnetic induction, there have been continuous developments in electrical power generation. Most people today get electricity from thermal, hydroelectric, or nuclear power generation systems, which use this electromagnetic induction phenomenon. Here we propose a new method for electrical power generation, without using electromagnetic induction, by mechanically modulating the electrical double layers at the interfacial areas of a water bridge between two conducting plates. We find that when the height of the water bridge is mechanically modulated, the electrical double layer capacitors formed on the two interfacial areas are continuously charged and discharged at different phases from each other, thus generating an AC electric current across the plates. We use a resistor-capacitor circuit model to explain the results of this experiment. This observation could be useful for constructing a micro-fluidic power generation system in the near future.

Interfacial energy consideration in the organization of a quantum dot-lipid mixed system

We propose a theoretical model for a quantum dot (QD)–lipid mixed system based on a simple geometrical assumption for a single-component lipid (DOPC) monolayer deformation profile. In this system, there are two possible states: a quantum dot–liposome complex (QLC) state where QDs are incorporated into the lipid bilayer of the liposome, and a quantum dot–micelle complex (QMC) state where an individual QD is covered by a lipid monolayer. In this model, the elastic deformation energy of the QLC is smaller (larger) than that of the QMC for the QD size smaller (larger) than a certain critical size. Therefore, the QLC is a more stable state than the QMC for the QD size smaller than a certain critical size. The prediction shown in this model is very consistent with our recent experimental results. To our knowledge, this is the first theoretical model that predicts the size dependence of the stability of the QD in the lipid bilayer.

Incorporation of quantum dots into the lipid bilayer of giant unilamellar vesicles and its stability.

We studied CdSe Quantum dot-Liposome Complexes (QLCs), which are GUVs (Giant Unilamellar Vesicles) incorporated with quantum dots (QDs) loaded into the DOPC lipid bilayer. QLCs were prepared by employing the electroswelling method combined with spin coating techniques. Hexadecylamine (HDA) coated CdSe QDs of five different sizes from blue- (radius ~2.05 nm) to red-emission (~3.5 nm) were used to examine what size of QDs can be loaded into the DOPC lipid bilayer. Blue (radius ~2.05 nm), green (~2.25 nm), and yellow (~2.65 nm)-emission QDs were successfully inserted in the lipid bilayer. However, we did not observe any QLCs for the orange-emission QDs (~3.0-3.15 nm) and red-emission ones (~3.5 nm). This QD size dependence of the incorporation into the lipid bilayer is partly supporting the predictions in our published theoretical work. DOPC lipids showed a much smaller QLC yield than that of asolectin which is a mixture of many different kinds of lipids. Our model explains this large difference in the population qualitatively. The existence of QDs in the lipid bilayer at a nanometer scale was confirmed by employing laser-scanning confocal microscopy, Cryo-TEM, and negative staining and sectioning TEM.

Thermochemical control of oil droplet motion on a solid substrate

Through the thermochemical control of a liquid droplet on a solid substrate, we investigated the motion of the droplet and the underlying mechanism. Depending on the contact angle of the running droplet on the substrate with a chemical gradient coating, we observed two different behaviors for the droplet under local laser heating: reversing for a contact angle larger than a critical value (∼90°) and passing for a smaller value. The motion at the laser heating position was found to be closely related to the contact angle change, indicating that capillary flow plays an important role in the thermally induced motion of the droplet.

2D imaging ellipsometric microscope

A two‐dimensional (2D) imaging ellipsometric microscope (IEM) has been constructed. It overcomes several problems inherent in existing 2D imaging ellipsometers. One can use IEM to measure and map the 2D film thickness profile with high spatial resolution and thickness sensitivity. The performance of the device is demonstrated through the study of the thin‐film profile of a spreading liquid drop on a molecularly smooth silicon wafer surface.

Real-time observations of mechanical stimulus-induced enhancements of mechanical properties in osteoblast cells

Osteoblast, playing a key role in the pathophysiology of osteoporosis, is one of the mechanical stress sensitive cells. The effects of mechanical load-induced changes of mechanical properties in osteoblast cells were studied at real-time. Osteoblasts obtained from young Wistar rats were exposed to mechanical loads in different frequencies and resting intervals generated by atomic force microscopy (AFM) probe tip and simultaneously measured the changes of the mechanical properties by AFM. The enhancement of the mechanical properties was observed and quantified by the increment of the apparent Youngs modulus, E*. The observed mechanical property depended on the frequency of applied tapping loads. For the resting interval is 50s, the mechanical load-induced enhancement of E*-values disappears. It seems that the enhanced mechanical property was recover able under no additional mechanical stimulus.

Jamming transition in a highly dense granular system under vertical vibration

The dynamics of the jamming transition in a three-dimensional granular system under vertical vibration is studied using diffusing-wave spectroscopy. When the maximum acceleration of the external vibration is large, the granular system behaves like a fluid, with the dynamic correlation function G (t) relaxing rapidly. As the acceleration of vibration approaches the gravitational acceleration g , the relaxation of G (t) slows down dramatically, and eventually stops. Thus the system undergoes a phase transition and behaves like a solid. Near the transition point, we find that the structural relaxation shows a stretched exponential behavior. This behavior is analogous to the behavior of supercooled liquids close to the glass transition.

Flow and jam of granular particles in a two-dimensional hopper

The jamming phenomenon of granular flow of mono-disperse disks in a two-dimensional hopper is studied experimentally. When the opening of the hopper d is small, jamming occurs due to formation of an arch at the hopper opening. The jamming probability as a function of the hopper opening width is measured which show a sharp decrease at some range of d. By observing the disk configurations of the arch in the jamming events, the jamming probability can be explained quantitatively using a restricted random walker model. The average horizontal and vertical spans of the jamming arch are calculated from the restricted random walk model which compares favorably with experiments.

Breathing, crawling, budding, and splitting of a liquid droplet under laser heating

The manipulation of droplets with sizes on the millimetre scale and below has attracted considerable attention over the past few decades for applications in microfluidics, biology, and chemistry. In this paper, we report the response of an oil droplet floating in an aqueous solution to local laser heating. Depending on the laser power, distinct dynamic transitions of the shape and motion of the droplet are observed, namely, breathing, crawling, budding, and splitting. We found that the selection of the dynamic modes is determined by dynamic instabilities due to the interplay between the convection flows and capillary effects. Our findings can be useful for constructing microfluidic devices to control the motion and shape of a small droplet by simply altering the laser power, and for understanding thermal convective systems with fully soft boundaries.

Diffusing-wave spectroscopy study of microscopic dynamics of three-dimensional granular systems

Probing micron-scale dynamics is important in understanding the dynamic property of granular systems. Diffusing-wave spectroscopy, which makes use of multiply scattered light in a highly dense solution of small particles, has been used successfully to study various fluidized-granular systems, such as; channel flow, gas-fluidized beds, avalanche flow, and vibro-fluidized beds. This paper summarizes this method and the existing findings, which include the dynamic behaviour on a short time scale and jamming transition on a long time scale. In addition, the analogy between the jamming transition of granular systems and the glass transition of traditional liquid/solid systems is discussed.