Tess J. Moon

The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells

When a dielectric object is placed between two opposed, nonfocused laser beams, the total force acting on the object is zero but the surface forces are additive, thus leading to a stretching of the object along the axis of the beams. Using this principle, we have constructed a device, called an optical stretcher, that can be used to measure the viscoelastic properties of dielectric materials, including biologic materials such as cells, with the sensitivity necessary to distinguish even between different individual cytoskeletal phenotypes. We have successfully used the optical stretcher to deform human erythrocytes and mouse fibroblasts. In the optical stretcher, no focusing is required, thus radiation damage is minimized and the surface forces are not limited by the light power. The magnitude of the deforming forces in the optical stretcher thus bridges the gap between optical tweezers and atomic force microscopy for the study of biologic materials.

Cure Kinetic Model, Heat of Reaction, and Glass Transition Temperature of AS4/3501-6 Graphite–Epoxy Prepregs

A new isothermally-based, cure kinetic model for the carbon graphite–epoxy AS4/3501-6 prepreg is presented using an industrially supplied prepreg rather than the neat epoxy resin. A two-stage model is used: first, rate-controlled (autocatalytic); then diffusion-controlled. A Differential Scanning Calorimeter (DSC) is employed to investigate the prepreg’s cure kinetics using both isothermal and dynamic DSC scans, although only isothermal DSC scan data is used to fit the model’s parameters. A Thermogravimetric Analyzer (TGA) is used to determine the mass fraction of the epoxy resin in the prepreg. The coefficients are provided for a prepreg having 37.4% resin mass fraction; they must be re-evaluated for prepregs having significantly different resin mass fractions. The ultimate heat of reaction of the resin incorporated within the prepreg is measured as 433.7 J/g (based upon a linear baseline) or 422.7 J/g (based upon a specific-heat-corrected baseline) using dynamic DSC scans at 20°C/min. This value is 8–17% less than the reported 472–508 J/g for the neat 3501-6 epoxy also measured using dynamic DSC scans at 20°C/min. Important to manufacturing applications, dynamic DSC scans reveal that the prepreg’s ultimate heat of reaction is sensitive to scan rate. Finally, the prepreg’s carbon fiber and/or sizing appears to suppress the curing reaction and decrease the ultimate heat of reaction over that for the neat epoxy resin. The glass transition temperature T g (°C) of partially-cured AS4/3501-6 prepregs is measured using DSC and is expressed as a function of degree of cure: T g = 22.9 exp(2.16α) – markedly similar to 3501-6 neat resin T g data measured earlier using a dynamic mechanical analyzer. The small specimen DSC thermal scans provide an effective means to measure the T g of completely- or partially-cured prepregs. The T g of the 100%-cured prepreg was experimentally determined to be 199°C, slightly higher than the 193°C reported for the 100%-cured neat epoxy resin. The AS4 fibers play little or no role in the degree-of-cure dependence of the AS4/3501-6 prepreg’s T g. Recommended for general purpose structural applications, the AS4/3501-6 prepreg is composed of continuous, PAN-based, AS4C carbon (graphite) fibers and an amine-cured epoxy resin system. The epoxy resin, 3501-6, is a tetraglycidyl diamino diphenyl methane (TGDDM) with diaminodiphenylsulfone (DDS) catalyzed with boron trifluoride monoethylamine. 3501-6 was developed to operate in atemperature environment of up to 350°F (177°C).

Identification of the most significant processing parameters on the development of fiber waviness in thin laminates

This paper identifies the material and processing parameters which most significantly influence the development of in-plane waviness in laminates. Thin laminates of unidirectional, T300 carbon-fiber/polysulfone matrix prepreg were processed in an autoclave and a custom-made water-cooled chamber, which allowed fast cooling rates. Multivariate regression analysis of process-induced waviness was performed for combinations of the select process variables and their interactions to identify those factors responsible for waviness development. Of the eight parameters investigated – hold temperature, hold time, pressure, length, width, thickness, cooling rate, and tool plate material – only three affected the development of fiber waviness: length, cooling rate, and tool plate material. Length affects not only the number of wrinkles and wrinkle distribution, but also the average amplitude of the waviness. Cooling rate affects the wavelength and amplitude of the waviness, as well as the number of wrinkles. Tool plate material primarily affects the number of wrinkles, without showing a significant effect on the average wave geometry. There is also an interaction between tool plate material and cooling rate in producing fiber waviness. For the three relevant parameters, the possible waviness-inducing mechanisms are tool plate/part coefficient of thermal expansion (CTE) mismatch, temporal temperature gradients (or cooling rates), and spatial temperature gradients. The tool plate/part CTE mismatch proved to be the most important mechanism driving fiber waviness in plates, although changes in cooling rates also dramatically affected the quantity of waviness which developed. Spatial temperature gradients were negligible for this study. The tool plate/part CTE mismatch-driven axial buckling loads on the fibers were substantial in the outermost laminate plies, or skin, but negligible in the laminate core. Waviness was limited to the surface or skin plies, even in identically-processed thick laminates. This study confirmed that if the fibers experience axial loads – albeit a small fraction of their Young’s modulus – while the matrix is unable to provide some level of transverse fiber support, the fibers will microbuckle resulting in waviness (in-plane or out-of-plane depending upon the laminate constraint).

Flame Retardant Polyamide 11 and 12 Nanocomposites: Thermal and Flammability Properties

Polyamide (nylon) 11 (PA11) and 12 (PA12) were melt-blended, dispersing low concentrations of nanoparticles, namely nanoclays (NCs), carbon nanofibers (CNFs), and nanosilicas (NSs) via twin-screw extrusion. To enhance their thermal and flame-retardant (FR) properties, an intumescent FR additive was added to the mechanically superior NC and CNF PA11 formulations. For neat and nanoparticle-reinforced PA11 and PA12, as well as for PA11 reinforced by both intumescent FR and select nanoparticles (NC or CNF), decomposition and heat deflection temperatures were measured, as were the peak heat release rates while burning the composites. All PA11 polymer systems infused with both nanoparticles and FR additive had higher decomposition temperatures than those infused with solely FR additive. For the PA11/FR/NC polymer blends, only the 20 wt% FR and 7.5 wt% clay formulation passed the UL 94 V-0 requirement, while all PA11/FR/ CNF formulations passed UL 94 V-0 requirement.

Flame-retardant polyamide 11 nanocomposites: further thermal and flammability studies

Polyamide (nylon) 11 (PA11) were melt-blended by dispersing low concentrations of nanoparticles (NPs), namely nanoclays (NCs) and carbon nanofibers (CNFs) via twin-screw extrusion. To enhance their thermal and flame retardant (FR) properties, an intumescent FR additive was added to the mechanically superior NC and CNF PA11 formulations. For neat and NP-reinforced PA11 as well as for PA11 reinforced by both intumescent FR and select NPs (NC or CNF), decomposition temperatures by TGA, flammability properties by UL 94, and cone calorimetry values were measured. All PA11 polymer systems infused with both NPs and FR additive had higher decomposition temperatures than those infused with solely FR additive. For the PA11/FR/NC polymer blends, Exolit® OP 1312 (FR2) is the preferred FR additive to pass the UL 94 V-0 requirement with 20 wt%. For the PA11/FR/CNF formulations, all Exolit® OP 1311 (FR1), OP 1312 (FR2), and OP 1230 (FR3) FR additives passed the UL 94 V-0 requirement with 20 wt%.

Linear stability analysis for high-velocity boundary layers in liquid-metal magnetohydrodynamic flows

Abstract This paper presents a linear stability analysis for the fully developed liquid-metal now in a constant-area rectangular duct with thin metal walls and with a strong, uniform, transverse magnetic field. For the steady flow, there are large velocities inside the boundary layers adjacent to the sides which are parallel to the applied magnetic field. There are two independent eigenvalue problems for the linear stability of the high-velocity side layers. The first problem involves disturbance vorticity which is perpendicular to the magnetic field, and these disturbances decay for all wavelengths and Reynolds numbers. The second problem involves disturbance vorticity which is parallel to the magnetic field, and the critical Reynolds number for these disturbances is 313. The critical disturbance involves a short axial scale and a high velocity in the direction perpendicular to the side. Both of these characteristics have positive implications for the heat transfer through this boundary layer. This heat transfer is important in liquid-lithium cooling systems or “self-cooled blankets” for magnetic confinement fusion reactors. In such blankets, a high-velocity boundary layer occurs adjacent to the “first wall”, which faces the fusing plasma.

Flame-retardant Polyamide 11 and 12 Nanocomposites: Processing, Morphology, and Mechanical Properties

The objective of this study is to develop improved polyamide (nylon) 11 (PA11) and 12 (PA12) polymers with enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering rapid manufacturing. PA11 and PA12 were melt-blended, dispersing low concentrations of nanoparticles, namely nanoclays (NCs), carbon nanofibers (CNFs), and nanosilicas (NSs) via twin-screw extrusion. To enhance their thermal and flammability properties, an intumescent flame retardant (FR) was added to the mechanically superior NC and CNF PA11 formulations. NC or CNF additions to either PA11 or PA12 generally increased its tensile strength and modulus, but sharply reduced its elongation at rupture. FR additives reduced PA11’s properties considerably. This substitution, however, only exacerbated the already steep drop in elongation at rupture due to FR additives alone; while elongation dropped 58% with the addition of 30 wt% FR, it dropped 98% with the addition of 25 wt% FR/5 wt% CNF.

Catch bond-like kinetics of helix cracking: network analysis by molecular dynamics and milestoning.

The first events of unfolding of secondary structure under load are considered with Molecular Dynamics simulations and Milestoning analysis of a long helix (126 amino acids). The Mean First Passage Time is a non-monotonic function of the applied load with a maximum of 3.6 ns at about 20 pN. Network analysis of the reaction space illustrates the opening and closing of an off-pathway trap that slows unfolding at intermediate load levels. It is illustrated that the nature of the reaction networks changes as a function of load, demonstrating that the process is far from one-dimensional.

Transient thermal modeling of in-situ curing during tape winding of composite cylinders

Fully-transient, two-dimensional, heat transfer analysis for the simultaneous tape winding and in-situ curing of composite cylinders is presented. During processing, the orthotropic composites are continuously wound onto an isotropic mandrel and cured simultaneously by infrared (IR) heating. To most efficiently and effectively consider the continual accretion of composite, the model is formulated within a Lagrangian reference frame in which the heating source rotates while the coordinate system and composite are stationary. This enables prediction of composite temperature and degree-of-cure history from the first to last layer. Separate heat conduction equations are formulated for both the mandrel and composite cylinder The composite cylinder s outer surface is modeled as a moving boundary due to the accumulated layers. Exothermic heat generation due to the epoxy resin s chemical reaction is included as a function of temperature and degree of cure. Numerical simulations using a control-volume-based finite difference method are run for a common graphite/epoxy (AS4/3501-6) composite. The Lagrangian approach was found to more accurately predict the in-situ curing temperature and degree-of-cure histories than the previously used, quasi-steady-state Eulerian approaches, which underpredict thermal losses. The model and its computational implementation were verified using analytical solutions and actual experiments. During winding, the top layer maximum temperature increases with total number of layers wound, demonstrating that a given incoming prepreg tapes temperature history evolves with time. Moreover, with appropriate mandrel preheating, the inner layers can reach a very high degree of cure by the end of the winding process, revealing that the mandrels initial temperature has a significant effect on the composites temperature and degree-of-cure history. Substantial increases in the winding speed have little or no effect on the composites temperature history, but can significantly reduce the corresponding degree-of-cure. The development of structurally debilitating residual stresses are an important concern in selecting process parameters, such as winding speed and heating power. Taking advantage of the strong correlation between winding speed and IR heat flux, process windows can be used to guide the selection of manufacturing process parameters. These definitively show that there are thermodynamically imposed limits on how fast the cylinders may be wound and radiatively cured.

On-line processing of unidirectional fiber composites using radiative heating: I. Model

Thermal radiation is a clean, flexible, efficient and effective means to supply energy to process a composite on-line. Radiative transfer in high-fiberdensity, unidirectional composites is complex, if analyzed completely. A detailed thermal model for on-line processing of unidirectional fiber composites by surface or volumetric radiative heating is presented. The physical geometry and imposed thermal and radiative boundary conditions correspond to a unidirectional, hoop-wound cylinder. In-situ (or continuous) curing of thermosets or on-line consolidation of thermoplastics is represented by the inclusion or omission, respectively, of the exothermic, chemical energy release term in the energy equation. Numerical results are presented for the temperature and degree of cure of graphite/epoxy and glass/epoxy cylinders. The effects of: surface or volumetric radiative heat flux; radiant-source emissive power level and efficiency; radiative emission from the composite; exothermic, chemical energy release; radiation’s angle of incidence; and independent and dependent scattering in the composite’s interior are presented. Validation of the model is presented in Part II of this paper, as well as recommended manufacturing process windows for process parameters, such as radiant-source emissive power level and winding speed. Part II also includes surface and volumetric radiative properties of unidirectional graphite/epoxy and glass/epoxy composites that were used in the numerical simulations.