• Korea-Australia Rheology Journal
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• v.18 no.2
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• pp.67-81
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• 2006

Using a strain-controlled rheometer, the dynamic viscoelastic properties of aqueous xanthan gum solutions with different concentrations were measured over a wide range of strain amplitudes and then the linear viscoelastic behavior in small amplitude oscillatory shear flow fields was investigated over a broad range of angular frequencies. In this article, both the strain amplitude and concentration dependencies of dynamic viscoelastic behavior were reported at full length from the experimental data obtained from strain-sweep tests. In addition, the linear viscoelastic behavior was explained in detail and the effects of angular frequency and concentration on this behavior were discussed using the well-known power-law type equations. Finally, a fractional derivative model originally developed by Ma and Barbosa-Canovas (1996) was employed to make a quantitative description of a linear viscoelastic behavior and then the applicability of this model was examined with a brief comment on its limitations. Main findings obtained from this study can be summarized as follows: (1) At strain amplitude range larger than 10%, the storage modulus shows a nonlinear strain-thinning behavior, indicating a decrease in storage modulus as an increase in strain amplitude. (2) At strain amplitude range larger than 80%, the loss modulus exhibits an exceptional nonlinear strain-overshoot behavior, indicating that the loss modulus is first increased up to a certain strain amplitude(${\gamma}_0{\approx}150%$) beyond which followed by a decrease in loss modulus with an increase in strain amplitude. (3) At sufficiently large strain amplitude range (${\gamma}_0>200%$), a viscous behavior becomes superior to an elastic behavior. (4) An ability to flow without fracture at large strain amplitudes is one of the most important differences between typical strong gel systems and concentrated xanthan gum solutions. (5) The linear viscoelastic behavior of concentrated xanthan gum solutions is dominated by an elastic nature rather than a viscous nature and a gel-like structure is present in these systems. (6) As the polymer concentration is increased, xanthan gum solutions become more elastic and can be characterized by a slower relaxation mechanism. (7) Concentrated xanthan gum solutions do not form a chemically cross-linked stable (strong) gel but exhibit a weak gel-like behavior. (8) A fractional derivative model may be an attractive means for predicting a linear viscoelastic behavior of concentrated xanthan gum solutions but classified as a semi-empirical relationship because there exists no real physical meaning for the model parameters.

• The Korean Journal of Physiology
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• v.28 no.2
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• pp.181-190
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• 1994

Endothelium-derived relaxing factor (EDRF) activates guanylate cyclase which mediates the formation of cGMP from GTP in vascular smooth muscle. It is well known that endothelium-dependent relaxation is impaired in spontaneously hypertensive rats (SHR). However, it is still unknown whether the impaired endothelium-dependent relaxation in SHR results from the reduced release of EDRF or from the decrease of vascular response to EDRF. We investigated the effects of cGMP on the contractility and Ca movement in the aorta of SHR and Wistar-Kyoto rats (WKY). The amplitude of the endothelium-dependent relaxation to actylcholine (ACh) was significantly less in SHR than in WKY. L-arginine $(10^{-3}M)$ did not increase endothelium-dependent relaxation in both strains. Sodium nitroprusside (SNP), an activator of guanylate cyclase, relaxed the 40 mM $K^+-induced$ contraction in a dose-dependent manner $(10^{-10}{\sim}10^{-6}\;M)$ in the endothelium-rubbed aortic strips of both strains. However, there was no significant difference in these relaxations between WKY and SHR. 8-bromo-cyclic guanosine monophosphate (8-Br-cGMP), a cell membrane-permeable derivative of cGMP relaxed the 40 mM $K^+-induced$ contraction in a dose-dependent manner $(10^{-6}{\sim}10^{-4}\;M)$ in the endothelium-rubbed aortic strips of both strains. Also norepinephrine $(10^{-6}\;M)-induced$ contractions in normal and Ca-free Tyrode's solution were suppressed by the pretreatment with 8-Br-cGMP $(10^{-4}\;M)$ in either strain. However, the amplitudes of suppression induced by 8-Br-cGMP were greater in SHR than that in WKY. Basal $^{45}Ca$ uptake and 40mM $K^+-stimulated\;^{45}Ca$ uptake were not suppressed by pretreatment with 8-Br-cGMP $(10^{-4}\;M)$ in single aortic smooth muscle cells of both SHR and WKY. From the above results, it is suggested that cGMP decreases Ca sensitivity in vascular smooth muscle cells and that the impaired endothelium-dependent relaxation in the aortic strips of SHR is not the result of a reduced vascular response to EDRF.

• Korean Journal of Optics and Photonics
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• v.13 no.1
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• pp.58-64
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• 2002

Using Mach-Zehnder type intensity modulator, we stabilized a 10 GHz harmonically mode-locked dispersion-compensated fiber ring laser using a feedback controlling system, and we measured its stability. The laser was stabilized for more than 16 hours by controlling the cavity length to suppress the relaxation oscillation frequency component which had caused the laser output instability. The ms timing jitter and ms amplitude noise were measured to be 260-524 fsec and 4~11.5%, respectively, and BER test measurement showed a value of 10$^{-13}$ .

• The Korean Journal of Physiology
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• v.21 no.2
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• pp.179-189
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• 1987

Since it has been known that ryanodine has a potent negative inotropic effect on the cardiac muscle contractility (Jenden and Fairhurst, 1968), ryanodine has been a subject of intensive research (Frank and Sleator, 1975; Jones et al, 1978; Sutko et al, 1985). However, the underlying mechanism for the ryanodine dependent negative inotropic effect is still uncertain. In this study, the effects of ryanodine on the generation and relaxation of contracture due to Na-withdrawal and on the force-frequency relationship of heart muscles isolated from rats and guinea pigs were measured in an effort to understand the underlying mechanism of the ryanodine-induced negative inotropy. Results are summerized as follows: 1 ) Ryanodine significantly reduced the contractility of heart muscles produced at low frequency of stimulation, but showed a little effect on the contractility at high frequency stimulation. 2) Ryanodine, at the concentrations ranging from $10^{-6}\;M$ to $10^{-8}\;M$, had no significant effect on the Na-dependent relaxation of Na-withdrawl contracture. 3) Ryandoine significantly reduced the amplitude of the Na-withdrawl contracture, and this inhibitory effect was reinforced by procaine, antiagonized by caffeine and high potassium. From these results, it may be concluded that the negative inotropic effect of ryanodine is mainly due to an inhibition of calcium release from sarcoplasmic reticulum.

• Journal of the Korean Graphic Arts Communication Society
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• v.23 no.2
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• pp.1-14
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• 2005

Generally, printing inks are composed of pigment, vehicle and additive. Among others, the vehicle transfers the pigment to substrate and then binds it on the surface. So, rheological properties of the vehicle are an important factor which has influence on printability. Thus, in this study, rheology of the vehicle was investigated by using rotational rheometer according to molecular weight of resin. Also, emlusion rheology of water in oil type and its microstructure were examined with increasing the shear rate. Consequently, the following results were obtained: (1) By viscometric flow test, zero shear viscosity and shear thinning index of vehicle increased with increasing the molecular weight of resin. (2) By relaxation and creep test, relaxation time and retardation time of vehicle increased with increasing the molecular weight of resin. (3) By frequency sweep test, crossover point of vehicle increased with increasing the molecular weight of resin. (4) G' and G" of emlusions increased with increasing the molecular weight by amplitude sweep test. (5) The shape of water drop in emlusions was changed to the capillary tube.

• Biomolecules & Therapeutics
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• v.7 no.1
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• pp.66-78
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• 1999

In order to investigate the pharmacological properties of New Wonbang Woohwangchungsimwon Liquid (NSCL), effects of Wonbang Woohwangchungsimwon Liquid (SCL) and NSCL were compared. In isolated rat aorta, NSCL and SCL showed the relaxation of blood vessels in maximum contractile response to phenylephrine (10$^{-6}$ M) regardless to intact endothelium or denuded rings of the rat aorta. Furthermore, the presences of the inhibitor of NO synthase and guanylate cyclase did not affect the relaxing effect of NSCL and SCL. NSCL and SCL inhibited the vascular contractions induced by acetylcholine, prostaglandin endoperoxide or peroxide in a dose-dependent manner. In conscious spontaneously hypertensive rats (SHRs), NSCL and SCL significantly decreased heart rate. NSCL and SCL, at high doses, had a negative inotropic effect that was a decrease of left ventricular developed pressure and (-dp/dt)/(+dp/dt) in the isolated perfused rat hearts, and also decreased the contractile force and heart rate in the isolated rat right atria. In excised guinea-pig papillary muscle, NSCL and SCL had no effects on parameters of action potential such as resting membrane potential, action potential amplitude, APD$_{90}$ and V$_{max}$ at low doses, whereas inhibited the cardiac contractility at high doses. These results suggested that NSCL and SCL have weak cardiovascular effects with relaxation of blood vessels and decrease of heart rate, and that this effect is no significant differences between cardiovascular effects of two preparations.s.

• Progress in Superconductivity and Cryogenics
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• v.21 no.1
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• pp.1-5
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• 2019

The pulsed field magnetization of the short-circuited soldered double pancake coil made of stabilized commercial high-Tc superconductor (HTS) tape is experimentally studied. The evolution of the shielding current induced by the pulsed field and the trapped field after the pulsed magnetization was measured at 77 K. It is shown that the trapped field in the coil is close to the value reached in the field cooling process and reduces weakly at 5-fold increasing of pulsed field amplitude. The current relaxation at t~2 ms after the pulse is defined by the current sharing between the tape's copper coating and the $YBa_2Cu_3O_{7-d}$ layer. In the intermediate time scale (1 s < t < 100 s) the flux creep in HTS layer dominates. At t > 100 s the current's relaxation is defined by the resistance of soldered joint between tapes.

• Journal of Preventive Medicine and Public Health
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• v.30 no.3 s.58
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• pp.599-608
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• 1997

We studied on change of lens accommodation and pupil light reflex caused by VDT work in six women and compared them with those of other office works. The results were as follows. 1. In VDT work the accommodation contraction velocity decreased during the first 2-hr VDT task, then recovered markedly by the end of the one-hour lunch break, and decreased again by the end of the 2nd 2-hr VDT task. Changes of relaxation velocity showed similar pattern although it was less typical than that of contraction velocity 2. There was no marked change in accommodation contraction velocity and amplitude of accommodation in general office work and near-distance office work, and in the near-distance office work accommodation relaxation velocity decreased according to work load without recovery after lunch break. 3. Initial pupil diameter, initial pupil area of light reflex decreased significantly during VDT task only in VDT work and didn't recover after lunch break. These results suggest possibility that changes of accommodation contraction velocity, accommodation relaxation velocity, near point distance, and pupil diameter and area are useful in evaluating characteristics of VDT work. For this, more research adjusted individual differences, circadian rhythm, emotional stress needs.

• The Journal of the Acoustical Society of Korea
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• v.23 no.7
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• pp.497-502
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• 2004

Both ultrasonic velocity at 3 MHz and absorption coefficient in the frequency range of 0.2-2 MHz were measured for poly (sodium 4-styrenesulfonate) aqueous solution over the concentration range of 5 to 25 % by weight. Pulse echo overlap method was employed to measure the ultrasonic velocity over the temperature range of 10-90 ℃ and the high-a ultrasonic resonator method was used for the absorption coefficient measurement at 20 ℃. The velocity exhibited a maximum value at approximately 55. 59, 63. 67, and 71 ℃ in 25, 20. 15, 10. and 5 wt% solutions, respectively. The velocity increased with poly (sodium 4-styrenesulfonate) concentration at a given temperature. The concentrations dependences of the relaxation frequency and amplitude showed that the relaxation around 200 kHz is related to the structural fluctuations of polymer molecules, such as the segmental motions of the polymer chains and that around 1 MHz resulted from the proton transfer reaction of the oxygen sites of SO₃. Both the absorption and the shear viscosity increase with the Polymer concentration. but decrease with temperature.

• The Korean Journal of Physiology
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• v.24 no.2
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• pp.377-388
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• 1990

Ca movements during the late plateau phase in rabbit atrium implicate Na-Ca exchange. In single atrial cells isolated from the rabbit the properties of the inward current of Na-Ca exchange were investigated using the whole cell voltage clamp technique. The inward currents were recorded during repolarization following brief 2 ms depolarizing pulse to +40 mV from a holding potential of -70 mV. Followings are the results obtained: 1) When stimulated every 30 sec, the inward currents were activated and reached peak values $6{\sim}12\;ms$ after the beginning of depolarizing pulse. The mean current amplitude was 342 pA/cell. 2) The current decayed spontaneously from the peak activation and the timecourse of the relaxation showed two different phases: fast and slow phase. 3) The recovery of the inward current was tested by paired pulse of various interval. The peak current recovered exponentialy with a time course similar to that of Ca current recovery. 4) Relaxation timecourse was also affected by pulse interval and time constant was reduced almost linearly according to the decrease of pulse interval between 30 sec and 1 sec. 5) The peak inward current was increased by long prepulse stimulation, Bay K, isoprenaline or c-AMP. 6) The relaxation time constant of the inward current was prolonged by Bay K or c-AMP, and shortened by isoprenaline. From the above results, it could be concluded that increase of the calcium current potentiates and prolongs intracellular calcium transients, while shortening of the timecourse by isoprenaline or short interval stimulations might be due to the facilitation of Ca uptake by SR.