Peilong Chen

Non-linear rheology of lamellar liquid crystals

Abstract.We measure the non-linear relation between the shear stress and shear rate in the lyotropic lamellar phase of C12E5 /water system. The measured shear thinning exponent changes with the surfactant concentration. A simple rheology theory of a lamellar or smectic phase is proposed with a prediction

Growth of Co clusters on thin films Al2O3∕NiAl(100)

\dot{{\gamma}}

Lift forces of screws in shear flows

∼ σ3/2 , where

Hosted particle positions and dipole moments of protein-filled reverse micelles

\dot{{\gamma}}

A density functional theory of chiral block copolymer melts

is the shear rate and σ is the shear stress. We consider that the shear flow passed through the defect structure causes the main dissipation. As the defect line density varies with the shear rate, the shear thinning arises. The defect density is estimated by the dynamic balance between the production and annihilation processes. The defect production is caused by the shear-induced layer undulation instability. The annihilation occurs through the shear-induced defect collision process. Further flow visualization experiment shows that the defect texture correlates strongly with the shear thinning exponent.

An atomic force microscope study of thermal behavior of phospholipid monolayers on mica

We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al(2)O(3) thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570 K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6 nm and a height of 0.7 nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3-4-nm-wide crystalline Al(2)O(3) strips where the deposited Co atoms are confined, the same features sustain. Only at 570 K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5 nm and height of 1.4 nm) appears for growth at 450 K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300 K and annealed to 750 K, in which the coalescence is eliminated and the two preferable geometries appear and coexist.

A statistical simulation approach for early stage thin-film growth from vapor-deposited atoms

We have measured the forces in the vorticity direction experienced by chiral screws in shear flows, with the Reynolds number at the order of 103. The measured force directions depend on the screw handedness, being in the positive vorticity direction for left-handed screws and opposite for right-handed ones. These directions are the reverse of those predicted in the low Reynolds number (Stokes flow) calculations [M. Makino and M. Doi, Phys. Fluids 17, 103605 (2005)]. The force magnitude scales with the cube of the shear rate and is about 100dyn (on the order of 10% of the drag force) when the shear rate ∼10s−1.

Electrostatic attraction between ionic reverse micelles with dielectric discontinuity

The nonlinear Poisson–Boltzmann equation is solved numerically in reverse water-in-oil droplet microemulsions with globular charged particles located at arbitrary positions inside the droplets. System free energies are calculated for both mono and dipole charged particles which are usually protein molecules. We find that average particle locations inside the droplets are always off center. Droplet dipole moments induced by the off center particles are also computed. Inter-micelle dipole–dipole interaction is estimated to be at the right magnitude to account for some of the percolation threshold decrease observed experimentally.

Smectic Edge Dislocations under Shear

A density functional theory is developed for the diblock copolymer melt, where one block contains the segment orientation dependent chiral interaction. In addition to the standard (scalar) pair interaction between the two types of monomers, the chiral block has the additional pairwise interaction, which is linear in the tangent vectors of the segments. We construct a density functional, which contains both the scalar density field and the vector chain alignment field. The quadratic part of the density functional comes from the mean field theory of the microscopic model, whereas the fourth order terms are introduced phenomenologically in the spatially local form. From the stability analysis of this model, we find that the additional chiral interaction shifts the order-disorder transition, which is consistent with the behavior of experimental system. Further numerical calculation reveals a new metastable chiral helical cylinder structure, which is similar to the one found experimentally. Another similar metastable structure but with zigzag modulation is also observed. As the helical and zigzag structures disappear when the chiral interaction is switched off, we understand that the chiral effect is the driving force for the formation of these exotic metastable structures.

Stable elliptical vortices in a circular disk.

We observed by using atomic force microscope (AFM) phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers on mica being annealed and cooled to a selection of temperatures through steps of 2-4 degrees C/min. The annealed phospholipid monolayers started to disappear at 45-50 degrees C and disappeared completely above 60-63 degrees C under AFM observation. The phospholipid monolayers reformed when the samples were cooled below 60 degrees C and developed from fractal into compact monolayer films with decreasing temperatures. Simultaneously the height of the reformed phospholipid films also increased with decreasing temperatures from 0.4 nm to the value before annealing. The observed thermal features are attributed to a phase-transition process that upon heating to above 45-50 degrees C, the lipids condensed in the monolayers transform into a low-density expanded phase in which the lipids are invisible to AFM, and the transformation continues and completes at 60-63 degrees C. The lipid densities of the expanded phase inferred from the dissociated area of the condensed phase are observed to be a function of the temperature. The behavior contrasts with a conventional first-order phase transition commonly seen in the Langmuir films. The temperature-dependent height and shape of the reformed phospholipid films during cooling are argued to arise from the adjustment of the packing and molecular tilting (with respect to the mica surface) of the phospholipids in order to accommodate more condensed phospholipids.