William B. Krantz

A mechanism for differential frost heave and its implications for patterned-ground formation

The genesis of some types of patterned ground, including hummocks, frost boils and sorted stone circles, has been attributed to differential frost heave (DFH). However, atheoretical modelthat adequatelydescribes DFHhasyettobe developedand validated. In this paper, we present a mathematical model for the initiation of DFH, and discuss how variations inphysical (i.e. soil/vegetationproperties) andenvironmental (i.e. ground/air temperatures) properties affect its occurrence and length scale. Using the Fowler and Krantz multidimensional frost-heave equations, a linear stability analysis anda quasi-steady-statereal-timeanalysisareperformed.Resultsindicatethatthefollow- ing conditions positively affect the spontaneous initiationof DFH: silty soil, smallYoungs modulus, small non-uniform surface heat transfer or cold uniform surface temperatures, and small freezing depths.The initiating mechanism for DFH is multidimensional heat transfer within the freezing soil. Numerical integration of the linear growth rates indi- catesthatexpressionof surfacepatternscanbecomeevidentonthe10^100yeartime-scale.

Flow-visualization during macrovoid pore formation in dry-cast cellulose acetate membranes

Video-microscopy flow-visualization (VMFV) is adapted to study the development of macrovoid (MV) pores in the dry-casting of cellulose acetate (CA)/acetone/water solutions. Particle tracer velocities provide the first direct evidence for the presence of solutocapillary-driven convection that can enhance mass-transfer to a MV. Three phases of MV development are observed: fast initial growth, slow growth, and collapse. During the latter, MVs were observed on occasion to initiate far from the demixing front. These studies have led to a significantly modified hypothesis for MV development. Extremely rapid initial MV growth is thought to occur owing to coalescence of dispersed phase microdroplets. To ensure net mass-transfer to a growing MV, it is postulated that a homogeneous supersaturated solution layer must exist between the demixed fluid layer and the homogeneous stable solution layer. Fast growth also involves convective mass-transfer to the MV whose surface is initially entirely immersed in this homogeneous supersaturated solution layer. Slow growth involves net transport that results from both convective mass-transfer to the MV across the portion of its surface in contact with the homogeneous supersaturated solution layer, and convective mass-transfer from the portion of its surface that extends into the homogeneous stable solution layer. Active collapse is thought to occur owing to skin formation at the MV surface. Passive collapse occurs when the convective mass-transfer from the MV in the homogeneous stable solution layer exceeds that entering the MV in the homogeneous supersaturated solution layer.

Fabrication of poly (ECTFE) membranes via thermally induced phase separation

Abstract Poly (ethylene chlorotrifluoroethylene) (ECTFE) is a 1:1 alternating copolymer of ethylene and chlorotrifluoroethylene with an excellent combination of chemical, thermal and mechanical properties. Although the lack of a suitable solvent for ECTFE precludes membrane casting by conventional room-temperature processes of immersion precipitation or evaporative casting, thermally induced phase separation provides an attractive pathway for membrane fabrication. We have successfully identified three possible latent solvents for ECTFE and have studied the ECTFE-dibutyl phthalate (DBP) polymer-diluent system. ECTFE membranes were fabricated using a TIPS-casting apparatus that allowed control of several important process parameters. ECTFE membranes were characterized via scanning electron microscopy (SEM), porometry and permeation techniques. Results indicated that membranes with continuous pores having diameters in the microfiltration range (0.1–0.5xa0μm) could be reliably fabricated.

Macrovoid pore formation in dry-cast cellulose acetate membranes: buoyancy studies

Experiments were conducted onboard a NASA KC-135 aircraft in order to assess the validity of two hypotheses proposed for the growth of macrovoid (MV) pores formed during the dry-casting of cellulose acetate (CA)/acetone/water casting solutions. The KC-135 aircraft provides the capability for greatly reducing the effective gravitational body forces that influence the buoyancy force on MVs. Buoyancy should have no effect on MV growth as proposed in the purely diffusive growth hypothesis but should influence MV growth via the solutocapillary convection hypothesis since the latter involves a balance between Marangoni, viscous drag, and buoyancy forces. CA membranes were cast in low-gravity (low-g) (KC-135) and normal-gravity (1-g) (ground-based control) from CA/acetone/water solutions as a function of the solvent/non-solvent (S/NS) ratio. Morphological analysis indicated that MV growth was enhanced in low-g only for the case in which the S/NS ratio = 2.0; no effect was observed for higher values of the S/NS ratio. These studies provide support for the solutocapillary convection hypothesis; however, the present data do not unambiguously demonstrate the occurrence of solutocapillary convection. Further research is required to provide such proof.

Sensitivity analysis of the rapid decomposition of methane in an aerosol flow reactor

A one-dimensional, nonisothermal model is developed to describe the thermal dissociation of methane to hydrogen and carbon black occurring in a fluid-wall aerosol flow reactor. The model expressions are scaled and nondimensionalized to determine the minimum parametric representation of the system. The sensitivity of this thermal dissociation to three parameters (flow rate of carbon particles fed to initiate the reaction, carbon particle radius, and reactor wall temperature) is studied. The results of the study indicate that in order to achieve nearly complete conversion, high reactor wall temperatures must be maintained. In addition, micron-sized carbon black particles must be fed into the reactor to enhance the heat transfer to the gas phase. The actual flow rate of particles fed is not critical, as long as some flow rate of fine particles is maintained.

Macrovoid growth during polymer membrane casting

The Solutocapillary Convection (SC) hypothesis contends that macrovoid (MV) growth in dry-cast membranes is governed by a solutal-Marangoni convection-induced force caused by the rapid evaporation of volatile solvent from the liquid/gas interface, a viscous drag force, and a gravity-induced buoyancy force. Two different sets of experiments using the cellulose acetate-acetone-water system were conducted to test the SC hypothesis. Membranes were cast aboard a KC-135 aircraft that enabled short periods of microgravity (∼0-g) as well as 2-g conditions. The studied process variables included the solvent/non-solvent (S:NS) ratio, surface tension, and the magnitude of the body force (buoyancy). SEM analysis of the resulting membrane morphologies indicated that the MV morphology was strongly influenced by the S:NS ratio. However, dependence of MV size and number density on the buoyancy force could not be established. In the second set of experiments, videomicroscopy flow-visualization (VMFV) was utilized to measure fluid velocities at the MV/casting-solution interface and in the bulk solution. The magnitude of the solutocapillary convection was controlled via surfactant additions. A comparison of the ratio of the edge to the bulk velocity for MVs made from surfactant-free and surfactant-containing casting solutions did not provide evidence of a statistically significant surfactant effect. However, the presence of the surfactant did affect the MV number density. In addition, the presence of tracer particles inside the MVs indicated that a convective flow enables their transfer from the bulk to the interior of the MV.

Chemical Modification of Cellulose Acetate with Titanium Isopropoxide

This study describes the chemical modification of cellulose acetate (CA) using titanium isopropoxide (TiP) in a sol-gel process for the formation of an organic/inorganic hybrid (OIH) material. The hydrolysis and condensation reactions that characterize this process result in CA cross-linking and formation of inorganic oxide particles. TiP-modified CA gels and membrane materials are characterized by solubility and swelling measurements, Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy, thermogravimetric analysis, differential scanning calorimetry (DSC) and ultrasonic time-domain reflectometry. Whereas the solubility of the CA significantly decreased with increasing TiP exposure consistent with high levels of cross-linking, unambiguous spectroscopic evidence for cross-linking could not be determined. In addition, DSC measurements indicated no significant change in CA glass-transition temperature as a function of TiP exposure. On the other hand, TiP treatment dramatically improved the creep behavior of treated porous CA membranes, whereby the total compressive strain decreases by as much as 70% relative to the untreated materials. Overall, the results suggest that TiP treatment can be used as a post-fabrication processing step to create OIH-modified CA membranes with improved chemical and mechanical stability.

Development of A Technique for the In-Situ Measurement of the Mechanical Properties of Ultra-Thin Interfacially Polymerized Films

Crosslinked polyamide barrier layers made by a self-limiting interfacial polymerization (IP) process are extremely thin (= 1000 A), relatively defect-free and possess excellent perm selective properties which enable their commercial application in thin film composite (TFC) membranes. Little information has been reported in the open literature concerning the physical as well as mechanical properties of these polyamide IP films due to the substantial difficulties in making direct measurements on such thin crosslinked materials. Consequently, the development of IP-TFC membranes for commercial separations has been largely a trial-and-error process. We are developing a novel experimental technique, Pendant Drop Mechanical Analysis (PDMA) that utilizes the capabilities of the pendant-drop tensiometer for in-situ measurements of the mechanical behavior of the IP films. The current effort focuses on relating the mechanical behavior of the IP films to their structure, which in turn will be related to the permselective performance. Preliminary PDMA results indicate significant differences in the mechanical behavior of the IP films as a function of composition and contact time. These differences can be related to changes in the network characteristics.

Analysis of the rapid carbothermal reduction synthesis of ultra-fine silicon carbide powders

A nondimensionalized and scaled nonisothermal model is developed for the rapid carbothermal reduction synthesis of sub-micron silicon carbide particles in an aerosol flow reactor to determine the minimum parametric representation of the system. Seven dimensionless groups are needed to completely describe the system, and these dimensionless groups are varied to determine the effects of the furnace wall temperature, inlet carbon particle size, carrier gas flow rate, and solids feed rate on final product quality. Analysis shows that radiation dominates the heating process, sintering dominates the primary particle growth, and conversion is controlled with precursor carbon particle size, wall temperature, and carrier gas flow rate.

Sliding‐Cavity Fluid Contactors in Low‐Gravity Fluids, Materials, and Biotechnology Research

Abstract: The well‐known method of sliding‐cavity fluid contactors used by Gosting for diffusion measurements and by Tiselius in electrophoresis has found considerable use in low‐gravity research. To date, sliding‐cavity contactors have been used in liquid diffusion experiments, interfacial transport experiments, biomolecular crystal growth, biphasic extraction, multistage extraction, microencapsulation, seed germination, invertebrate development, and thin‐film casting. Sliding‐cavity technology has several advantages for spaceflight: it is simple, it accommodates small samples, samples can be fully enclosed, phases can be combined, multiple samples can be processed at high sample density, real‐time observations can be made, and mixed and diffused samples can be compared. An analysis of the transport phenomena that govern the sliding‐cavity method is offered. During sliding of one liquid over another flow rates between 0.001 and 0.1m/sec are developed, giving Reynolds numbers in the range 0.1‐100. Assuming no slip at liquid‐solid boundaries shear rates are of the order 1sec−1. The measured consequence is the transfer of 2‐5% of the content of a cavity to the opposite cavity. In the absence of gravity, buoyancy‐driven transport is assumed absent. Transport processes are limited to (1) molecular diffusion, in which reactants diffuse toward one another at rates that depend on their diffusion coefficient and concentration gradient (Ficks second law), (2) solutocapillary (Marangoni) flow driven by surface‐tension gradients, (3) capillary flow (drop spreading) at liquid‐solid three‐phase lines leading to immiscible phase demixing, and (4) vapor‐phase diffusive mass transfer in evaporative processes. Quantitative treatment of these phenomena has been accomplished over the past few years in low‐gravity research in space and on aircraft.