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Full-Text Articles in Engineering
Relating Domain Size Distribution To Line Tension And Molecular Dipole Density In Model Cytoplasmic Myelin Lipid Monolayers, Younjin Min
Younjin Min
We fit the size distribution of liquid-ordered (Lo) domains measured from fluorescence images of model cytoplasmic myelin monolayers with an equilibrium thermodynamic expression that includes the competing effects of line tension, λ, dipole density difference, Δm, and the mixing entropy. From these fits, we extract the line tension, λ, and dipole density difference, Δm, between the Lo and liquid-disordered (Ld) phases. Both λ and Δm decrease with increasing surface pressure, , although λ/Δm2 remains roughly constant as the monolayer approaches the miscibility surface pressure. As a result, the mean domain size changed little with surface pressure, although the polydispersity increased …
Mechanomutable And Reversibly Swellable Polyelectrolyte Multilayer Thin Films Controlled By Electrochemically Induced Ph Gradients, Younjin Min
Younjin Min
We present a new strategy to electrochemically control the swelling state and mechanical properties of a polyelectrolyte multilayer thin film. While a number of pH-responsive polymer films and hydrogels have been developed, biological systems typically will not tolerate substantial deviations in pH. Therefore, to apply such pH-responsive systems for biomedical or other sensitive applications, we developed an electrochemical approach to alter local pH, while maintaining a constant, mild, bulk pH. The polymer film investigated in this work comprises polyallylamine hydrochloride (PAH) and sulfonated polystyrene (SPS) assembled at high pH (>9.0), which is known to exhibit a large pH-induced swelling …
Recent Advances In The Surface Forces Apparatus (Sfa) Technique, Younjin Min
Recent Advances In The Surface Forces Apparatus (Sfa) Technique, Younjin Min
Younjin Min
The surface forces apparatus (SFA) has been used for many years to measure the physical forces between surfaces, such as van der Waals (including Casimir) and electrostatic forces in vapors and liquids, adhesion and capillary forces, forces due to surface and liquid structure (e.g. solvation and hydration forces), polymer, steric and hydrophobic interactions, bio-specific interactions as well as friction and lubrication forces. Here we describe recent developments in the SFA technique, specifically the SFA 2000, its simplicity of operation and its extension into new areas of measurement of both static and dynamic forces as well as both normal and lateral …
Normal And Shear Forces Generated During The Ordering (Directed Assembly) Of Confined Straight And Curved Nanowires, Younjin Min
Normal And Shear Forces Generated During The Ordering (Directed Assembly) Of Confined Straight And Curved Nanowires, Younjin Min
Younjin Min
The effects of shape on nanowire interactions and shear-induced ordering were studied. Both the normal and lateral forces were sensitive to the particles' curvature. (i) No adhesion was observed between the confining surfaces, and the force profiles were short-ranged and mostly reversible for straight wires but longer-ranged and irreversible for curved wires. (ii) Structural phase transitions can occur in confined nanoparticle films. (iii) During shearing, straight wires order or align more easily than curved wires.
Effect Of Composition And Synthetic Route On The Microstructure Of Biodegradable Diblock Copolymer, Poly(Þâµ-Caprolactone-Co-L-Lactide)-B-Poly(Ethylene Glycol), Younjin Min
Younjin Min
Biodegradable poly(ε-caprolactone-co-L-lactide)-b-poly(ethylene glycol) (PCLA-b-PEG) copolymers were synthesized via solution polymerization by varying the feed composition of ε-caprolactone (ε-CL) and L-lactide (LLA) (ε-CL: LLA=10∶0, 7∶3, 5∶5, 3∶7, 0∶10). The feed ratio based on weight is in accordance with the copolymer composition except for the case of ε-CL: LLA=3:7 (C3L7), which was verified by1H-NMR. Two different approaches were used for the exceptional case, which is an extension of the reaction time or the sequential introduction of the monomer. A copolymer composition of ε-CL: LLA=3:7 could be obtained in either case. The chemical microstructure of PCLA-b-PEG was determined using the13C-NMR spectra and the …
Forces Between Surfaces Across Nanoparticle Solutions:â Role Of Size, Shape, And Concentration, Younjin Min
Forces Between Surfaces Across Nanoparticle Solutions:â Role Of Size, Shape, And Concentration, Younjin Min
Younjin Min
Using a surface force apparatus, we have measured the normal forces between mica surfaces across various types of nanoparticles consisting of ZnS cores coated with a monolayer of physisorbed surfactant, dispersed in organic solvents. We focused on the effects of nanoparticle size, shape, and concentration on the force−distance profiles. Forces were exponentially repulsive when the surfactant layers were strongly bound to the nanoparticles and were roughly linear when there was adhesion between the nanoparticle cores, i.e., when the surfactant layers detached from the nanoparticles. In both cases, the range and magnitude of the forces were dependent upon the particle size, …
High-Speed Friction Measurements Using A Modified Surface Forces Apparatus, Younjin Min
High-Speed Friction Measurements Using A Modified Surface Forces Apparatus, Younjin Min
Younjin Min
Methods of measuring friction forces in the surface forces apparatus (SFA) are presented for sliding velocities from <1 nm/s to >10 m/s. A feed-forward control (FFC) system for the piezoelectric bimorph slider attachment is introduced to allow experiments at velocities up to ~4 mm/s. For still higher speeds, a motor-driven rotating mini-disk setup using a pin-on-disk geometry is presented, with modifications to enable sliding velocities in the ranges 1 cm/s–5 m/s and 1–25 m/s. Example data sets demonstrate the applicability of the approach to modeling important tribological systems including hard-disk drives. We find that mechanical system parameters such as the resonant frequencies and …