23 research outputs found
Dynamics of Vesicle Formation from Lipid Droplet: Mechanism and Controllability
A coarse-grained model developed by Marrink et al. [J. Phys. Chem. B 111,
7812 (2007)] is applied to investigate vesiculation of lipid
[dipalmitoylphosphatidylcholine (DPPC)] droplets in water. Three kinds of
morphologies of micelles are found with increasing lipid droplet size. When the
initial lipid droplet is smaller, the equilibrium structure of the droplet is a
spherical micelle. When the initial lipid droplet is larger, the lipid ball
starts to transform into a disk micelle or vesicle. The mechanism of vesicle
formation from a lipid ball is analyzed from the self-assembly of DPPC on the
molecular level, and the morphological transition from disk to vesicle with
increasing droplet size is demonstrated. Importantly, we discover that the
transition point is not very sharp, and for a fixed-size lipid ball, the disk
and vesicle appear with certain probabilities. The splitting phenomenon, i.e.,
the formation of a disk/vesicle structure from a lipid droplet, is explained by
applying a hybrid model of the Helfrich membrane theory. The elastic module of
the DPPC bilayer and the smallest size of a lipid droplet for certain formation
of a vesicle are successfully predicted.Comment: 22 pages, 11 figures Submitted to J. Chem. Phy
Spontaneous formation of complex micelles from homogeneous solution
We present an extensive computer simulation study of structure formation in
amphiphilic block copolymer solutions after a quench from a homogeneous state.
By using a mesoscopic field-based simulation method, we are able to access time
scales in the range of a second. A phase diagram of final structures is mapped
out as a function of the concentration and solvent-philicity of the copolymers.
A rich spectrum of structures is observed, ranging from spherical and rodlike
micelles and vesicles to toroidal and net-cage micelles. The dynamical pathways
leading to these structures are analyzed in detail, and possible ways to
control the structures are discussed briefly.Comment: PRL (in press
Phase transition of a single star polymer: a Wang-Landau sampling study
Star polymer is a typical nonlinear macromolecule possessing special
thermodynamic behaviors for the existence of a jointing point. The
thermodynamic transitions of a single star polymer are systematically studied
with bond fluctuation model using Wang-Landau sampling technique. A new
analysis method applying the shape factor is proposed to determine coil-globule
(CG) and liquid-crystal (LC) transitions, which shows higher efficiency and
precision than canonical specific heat function. It is found that the LC
transition of star polymer at lower temperature obeys the identical scaling law
as linear polymer. With the increase of the arm density of star polymer,
however, the CG transition point, corresponding to {\theta} temperature, shifts
towards the LC transition and the reason comes from the high density arms of
star polymer, which requires the lower temperature for attracting force to
overcome the volume excluding effects of chain. This work clearly demonstrates
that the distinction of linear and star polymers in structures only affects CG
transition and has no influence on LC transition.Comment: 30 pages, 10 figures, submit to JC
Effect of Tacticity Sequence of the Poly(<i>N</i>‑isopropylacrylamide) Oligomer on Phase Transition Behavior in Aqueous Solution
The
tacticity of poly(N-isopropylacrylamide) (PNIPAM)
has a strong impact on the lower critical solution temperature (LCST)
in aqueous solution. The sequence of meso diads (m) and racemo diads (r) further contributes to such
an effect. In this work, the phase transition behaviors of poly(N-isopropylacrylamide) pentamers with four kinds of sequences,
i.e., rrmm, rmmr, mrrm, and rmrm, in water were studied applying replica
exchange molecular dynamics with a modified OPLS/AA force field. The
difference in local component concentration in the system was used
as an order parameter to quantitatively describe the phase separation
extent. It was found that the phase separation degree of rrmm and rmmr is higher than that of mrrm and rmrm at the same temperature. The LCSTs of rrmm and rmmr are lower than those of mrrm and rmrm. The radial distribution
function and hydrogen bond analysis revealed that the average values
of hydrogen bonds between pentamers for rrmm and rmmr are greater than those of mrrm and rmrm, whereas the average values of hydrogen bonds between
pentamers and water for rrmm and rmmr are less than those of mrrm and rmrm. It was demonstrated that the isotactic triad (mm) plays an important role in the thermosensitive behaviors of the
PNIPAM pentamer. The increase of isotactic triad (mm) content in the PNIPAM chain promotes the formation of intermolecular
hydrogen bonds between amide and amide and leads to a higher aggregation
of the pentamer with the sequence of rrmm or rmmr. Finally, the effect of the isotactic triad was qualitatively
explained with the mean-field theory
Influence of Temperature-Dependent Tensile Strength on Gun Barrel Life Prediction
With comprehensive application of the theories of coating shear failure mechanism and fatigue cumulative damage, a life prediction method of a gun barrel is proposed based on the shear fatigue damage accumulation at the coating-substrate interface. The life of a small-caliber gun barrel is predicted by use of constant tensile strength at normal temperature and temperature-dependent tensile strength, respectively. The influence of the two kinds of tensile strength on barrel life prediction results is analyzed. Life test proves that the prediction method proposed here is credible and practical. The research results show that the reduction of interface tensile strength due to temperature rise in the firing process is an important inducement of interface damage and gun barrel failure. When the temperature-dependent tensile strength is considered in life prediction model, the prediction results are smaller than that predicted by use of constant tensile strength and well matched with the life test results. Therefore, the temperature-dependent tensile strength should be incorporated in the model of gun barrel life prediction
Kinetics of a Multilamellar Lipid Vesicle Ripening: Simulation and Theory
Lipid vesicle ripening via unimolecular
diffusion and exchange
greatly influences the evolution of complex vesicle structure. However,
this behavior is difficult to capture using conventional experimental
technology and molecular simulation. In the present work, the ripening
of a multilamellar lipid vesicle (MLV) is effectively explored using
a mesoscale coarse-grained molecular model. The simulation reveals
that a small MLV evolves into a unilamellar vesicle over a very long
time period. In this process, only the outermost bilayer inflates,
and the inner bilayers shrink. With increasing MLV size, the ripening
process becomes complex and depends on competition between a series
of adjacent bilayers in the MLV. To understand the diffusion behavior
of the unimolecule, the potentials of mean force (PMFs) of a single
lipid molecule across unilamellar vesicles with different sizes are
calculated. It is found that the PMF of lipid dissociation from the
inner layer is different than that of the outer layer, and the dissociation
energy barrier sensitively depends on the curvature of the bilayer.
A kinetics theoretical model of MLV ripening that considers the lipid
dissociation energy for curved bilayers is proposed. The model successfully
interprets the MLV ripening process with various numbers of bilayers
and shows potential to predict the ripening kinetics of complex lipid
vesicles
Molecular Progress in Research on Fruit Astringency
Astringency is one of the most important components of fruit oral sensory quality. Astringency mainly comes from tannins and other polyphenolic compounds and causes the drying, roughening and puckering of the mouth epithelia attributed to the interaction between tannins and salivary proteins. There is growing interest in the study of fruit astringency because of the healthy properties of astringent substances found in fruit, including antibacterial, antiviral, anti-inflammatory, antioxidant, anticarcinogenic, antiallergenic, hepatoprotective, vasodilating and antithrombotic activities. This review will focus mainly on the relationship between tannin structure and the astringency sensation as well as the biosynthetic pathways of astringent substances in fruit and their regulatory mechanisms
Ultrasensitive Gas Refractometer Using Capillary-Based Mach–Zehnder Interferometer
In this paper, we report a capillary-based Mach–Zehnder (M–Z) interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The sensing mechanism is quite straightforward. Cladding and core modes of a capillary are simultaneously excited by coupling coherent laser beams to the capillary cladding and core, respectively. An interferogram would be generated as the light transmitted from the core interferes with the light transmitted from the cladding. Variations in the refractive index of the air filling the core lead to variations in the phase difference between the core and cladding modes, thus shifting the interference fringes. Using a photodiode together with a narrow slit, we could interrogate the fringe shifts. The resolution of the sensor was found to be ~5.7 × 10−8 RIU (refractive index unit), which is comparable to the highest resolution obtained by other interferometric sensors reported in previous studies. Finally, we also analyze the temperature cross sensitivity of the sensor. The main goal of this paper is to demonstrate that the ultra-sensitive sensing of gas refractive index could be realized by simply using a single capillary fiber rather than some complex fiber-optic devices such as photonic crystal fibers or other fiber-optic devices fabricated via tricky fiber processing techniques. This capillary sensor, while featuring an ultrahigh resolution, has many other advantages such as simple structure, ease of fabrication, straightforward sensing principle, and low cost