Dissertations / Theses on the topic 'Surface freezing'

Thesis (Ph. D.)--University of Washington, 1992.
Vita. Accompanying video is in VHS Format and contains illustrations of observations described in chapter 2. Includes bibliographical references (leaves [222]-232).

Surface freezing transitions in mixed monolayers of a homologous series of cationic surfactants, the alkyltrimethyl ammonium bromides (CnTAB where n = 12, 14, 16, 18), as well as a range of non-ionic, zwitterionic and biological surfactants, have been investigated ellipsometrically with a range of n-alkanes (Cm where m = 12 – 20, 28). Two distinct solid phases are observed depending upon the chain length difference between surfactant and n-alkane. Type I solid phases consist of a surface frozen mixed monolayer and are formed when this difference is small. Type II solid phases are bilayer structures with a frozen layer of neat n-alkane above a liquid-like mixed monolayer. Type II freezing was thought to occur via wetting of surface frozen n-alkane, as previously reported type II transitions took place in the presence of surface frozen n-alkanes. Thermodynamically stable type II solid phases have now been found in the presence of n-alkanes that do not show surface freezing at the air/alkane interface, however, and so this picture is incomplete. In the presence of pentadecane, for example, the biological surfactant lyso-OPC forms a stable type II solid phase 6.5 °C above the n-alkane bulk melting point. Such a large surface freezing range is unprecedented for a type II system. Studies using external reflection FTIR (ER-FTIRS) and vibrational sum-frequency spectroscopies (VSFS) have been used to probe these novel behaviours. Results were fully consistent with the proposed structures of both type I and type II surface frozen layers. 2D correlation analysis of ER-FTIR spectra as a function of temperature showed that type II frozen layer formation does not proceed via a simple wetting transition, with the formation of a transient intermediate implied. Evidence for such an intermediate was provided by dynamic ellipsometry measurements on the type II C18TAB/n-eicosane system.

The need for improved cloud parameterisations in a road surface temperature model is demonstrated. Case studies from early 1994 are used to investigate methods of tracking cloud cover using satellite imagery and upper level geostrophic flow. Two of these studies are included in this thesis. Errors encountered in cloud tracking methods were investigated as well as relationships between cloud height and pixel brightness in satellite imagery. For the first time, a one dimensional energy balance model is developed to investigate the effects of erroneous cloud forecasts on surface temperature. The model is used to determine detailed dependency of surface freezing onset time and minimum temperature on cloud cover. Case studies from the 1995/96 winter in Devon are undertaken to determine effects of differing scenarios of cloud cover change. From each study, an algorithm for predicting road surface temperature is constructed which could be used in future occurrences of the corresponding scenario of the case study. Emphasis is strongly placed on accuracy of predictions of surface freezing onset time and minimum surface temperature. The role o f surface and upper level geostrophic flow, humidity and surface wetness in temperature prediction is also investigated. In selected case studies, mesoscale data are also analysed and compared with observations to determine feasibility of using mesoscale models to predict air temperature. Finally, the algorithms constructed from the 1995/96 studies are tested using case studies from the 1996/97 winter. This winter was significantly different from its preceding one which consequently meant that the algorithm from only one scenario of the 1995/96 winter could be tested. An algorithm is also constructed from a 1996/97 winter case study involving a completely different scenario Recommendations for future research suggest testing of existing algorithms with guidance on additional scenarios.

An experimental study of phase transitions of long-chain n-alkanes induced by the effect of interfaces is described. ¶ The phase behaviour of long-chain n-alkanes (carbon number 14, 16, 17, 18) adsorbed at isolated mica surfaces and confined between two mica surfaces has been studied in the vicinity of and down to several degrees below the bulk melting points, Tm. Using the Surface Force Apparatus we have measured the thickness of alkane films adsorbed from vapour (0.97 [equal to or greater-than] p/p[subscript o] [equal to or greater-than] 0.997), studied capillary condensation transition, subsequent growth of capillary condensates between two surfaces, and phase transitions in both the adsorbed films and the condensates. By measuring the growth rate of the capillary condensates we have identified a transition in the lateral mobility of molecules in the adsorbed films on isolated mica surfaces. This transition to greater mobility occurs slightly above Tm for n-hexadecane, n-heptadecane and n-octadecane but several degrees below Tm for n-tetradecane, and is accompanied by a change in wetting behaviour and a measurable decrease in adsorbed film thickness for n-heptadecane and n-octadecane. Capillary condensates that form below Tm remain liquid, but may freeze if the degree of confinement is reduced by separation of the mica surfaces. An increase in the area of the liquid-vapour interface relative to that of the liquid-mica interface facilitates freezing in the case of the long-chain alkanes, which show surface freezing at the liquid-vapour interface. ¶ Although thermodynamic properties of the surface freezing transition have been rather well documented, the kinetics involved in formation of such ordered monolayers has so far received very little attention. We studied the surface tension of n-octadecane as a function of temperature in the vicinity of Tm, using the static Wilhelmy plate and the dynamic maximum bubble pressure methods. The two methods give different results on cooling paths, where nucleation of the surface ordered phase is involved, but agree on heating paths, where both methods measure properties of the equilibrium surface phase. On cooling paths, the surface of bubbles may supercool below the equilibrium surface freezing temperature. The onset of surface freezing is marked by a sharp drop in the surface tension. The transition is accompanied by an increased stability of the films resulting in longer bubble lifetimes at the liquid surface, which suggests that the mechanical properties of the surfaces change from liquid-like to solid-like. Our results suggest occurrence of supercooling of the monolayer itself.

This thesis presents two different works. The first part introduces a thermal multimeter which measures heat capacity, thermal conductivity and density. The instrument gives continuous measurement data within a temperature range. With some exceptions this also holds for the prototype of a thermal multimeter which is built and tested. The measuring method is constant heating of one side of a slab. The slab is insulated on all other sides. After some time there will be equilibrium where there is a constant temperature difference over the slab. The thermal conductivity can be calculated from this temperature difference. The heat capacity can be calculated from how fast the temperature rises. Measurements of the slab thickness give density as function of temperature.

The second part discusses a practical method for measuring the heat transfer coefficient (α). The method is based on shell freezing of clear jelly which has the same shape as the product of interest. Transparent jelly is transparent before it freezes and white when frozen. If the sample is removed from the freezer and cut through before it is completely frozen thefreezing front is distinct and the thickness of the frozen layer can be measured. By measuring time the jelly sample was in the freezer and thicknessof the frozen layer the heat transfer coefficient can be calculated by using Plank's equation. The method is suitable for measuring local α because it can be shown that tangential heat flow can be neglected when the frozen layer is thin.

Computer simulations, automated data acquisition and data processing are a considerable part of this thesis, even though it is not obvious from the results presented. There are more lines in the data code written to obtain the results presented here then the number of lines in this thesis. The size of selected simulation results and processed data from the measurements are 6.3 GB.

Attachments can be downloaded from http://www.ub.ntnu.no/dravh/Brennvall_attachment.zip (1,33 GB)

Icing severely affects society, especially in the Nordic countries. Iceaccumulation can result in critical performance problems and safetyconcerns for instance in road, air and sea transportation, transmissionlines, marine and offshore structures, wind turbines and heat exchangers.Present active ice-combating approaches possess environmental,efficiency and cost drawbacks. Thus, fabricating icephobic surfaces orcoatings impeding ice formation (anti-icing), but facilitating ice removal(de-icing) is desired. However, different conditions in the environmentduring ice formation and growth add to the complexity of the problem.An icephobic surface that works for a certain application might not be agood candidate for another. These surfaces and the challenges are infocus in this thesis.Wetting properties are important for ice formation on surfaces fromthe liquid phase (often supercooled water), where the water repellency ofthe surfaces could enhance their anti-icing effect. Considering this,different hydrophobic and superhydrophobic surfaces with differentchemistry, morphology and roughness scale were prepared. Since anyinduced wetting state hysteresis on hydrophobic surfaces could influencetheir performance, the wetting stability was investigated. In particulardynamic wetting studies of the hydrophobic surfaces revealed whatsurface characteristics benefit a stable wetting performance. Further, theeffect of temperature, particularly sub-zero temperatures, on the wettingstate of flat and nanostructured hydrophobic surfaces was investigated.This was complemented with studies of the wetting stability of sessilewater droplets on flat to micro- and multi-scale (micro-nano) roughhydrophobic samples in a freeze-thaw cycle. To be consistent with mostapplications, all temperature-controlled experiments were performed inan environmental condition facilitating frost formation. Further, antiicingproperties of hydrophobic surfaces with different topography butsimilar chemistry were studied by freezing delay measurements.A dynamic wetting study using hydrophobic samples with similarchemistry but different topography revealed that multi-scale roughnesscould benefit the wetting stability. However, when these surfaces areutilized at low temperatures the wetting hysteresis observed during acooling/heating cycle is significant. Such a temperature-inducedhysteresis is also significant on superhydrophobic surfaces. I attributethis to condensation followed by frost formation facilitating spreading of  the supercooled water droplet. The freezing delay measurementsdemonstrate no significant effect of surface topography on anti-icingproperties of hydrophobic surfaces, however the flat surfaces showed thelongest delay. These findings are in agreement with heterogeneous icenucleation theory, suggesting preferential ice nucleation in concave sites,provided they are wetted.In the second part of this thesis, I consider the findings from theprevious part illustrating the limitations of (super)hydrophobic surfaces.The de-icing properties of hydrophilic surfaces with a hydration waterlayer, hypothesized to lubricate the interface with ice, were studied. Heretemperature-controlled shear ice adhesion measurements, down to -25oC, were performed on an adsorbed layer of a polymer, either bottle-brushstructured poly(ethylene oxide) or linear poly(ethylene oxide). The iceadhesion strength was reduced significantly on the bottle-brushstructured polymer layer, specifically at temperatures above -15 oC,whereas less adhesion reduction was observed on the layer formed by thelinear polymer. These findings are consistent with differential scanningcalorimetry (DSC) data, demonstrating that the hydration water, boundto the bottle-brush structured polymer, is in the liquid state at thetemperatures where de-icing benefit is observed. Further, continuingwith the hypothesis of the advantage of surfaces with a natural lubricantlayer for de-icing targets, I studied shear ice adhesion on the molecularlyflat basal plane of hydrophilic mica down to -35 oC. Interestingly, ultralowice adhesion strength was measured on this surface. I relate this to theproposed distinct structure of the first ice-like but fluid water layer onmica, with no free OH groups, followed by more bulk liquid-like layers.This combined with the molecularly smooth nature of mica results in aperfect plane for ice sliding.
Isbildning har en stark inverkan på samhället, speciellt i de nordiskaländerna. Isuppbyggnad kan resultera i kritiska prestandaproblem ochsäkerhetsrisker inom t.ex. väg-, luft-, och sjötransport, kraftledningar,marina- och offshorestrukturer, vindkraftverk och värmeväxlare.Nuvarande aktiva isbekämpningsmetoder uppvisar brister i avseende påmiljö, effektivitet och kostnad. Det finns därmed ett behov av attframställa ytor eller ytbeläggningar som förhindrar isbildning (antiisning)eller underlättar borttagandet av redan bildad is (avisning). Dockkompliceras problemet av de många olika förhållanden under vilka is kanbildas. En beläggning som fungerar för en viss tillämpning behöver intenödvändigtvis vara en bra kandidat för en annan. Dessa ytor ochutmaningar relaterade till dem är i fokus i denna avhandling.Vätningsegenskaper är viktiga för isbildning på ytor från vätskefas(ofta underkylt vatten), och det har visats att vattenavstötande ytor i vissasammanhang kan motverka isbildning. Med detta i åtanke framställdesolika hydrofoba och superhydrofoba ytor, med varierande kemi,morfologi och ytråhet. Eftersom en förändring i de hydrofoba ytornasvätningsegenskaper kan påverka deras funktion studerades vätningsstabilitetenför dessa ytor. I synnerhet dynamiska vätningsstudier av dehydrofoba ytorna avslöjade vilka ytegenskaper som är fördelaktiga förvätningsstabiliteten. Vidare studerades hur temperaturen, särskilt undernoll grader, påverkar vätningstillståndet på släta och nanostruktureradehydrofoba ytor. Arbetet kompletterades med studier av vätningsstabilitetenför vattendroppar på släta samt mikro- och multistrukturerade(mikro-nano) hydrofoba ytor under flera frysningsupptiningscykler.För att vara i linje med de flesta tillämpningar, utfördesalla temperaturkontrollerade mätningar i en miljö där frost kunde bildaspå ytorna. Anti-isegenskaperna hos de hydrofoba ytorna med varierandetopografi men samma kemi studerades vidare genom att studera hur långtid det dröjde innan en vattendroppe på ytan fryste vid en visstemperatur.De dynamiska vätningsstudierna på hydrofoba ytor med samma kemimen olika topografi avslöjade att en ytråhet på flera längdskalor kan haen positiv inverkan på vätningsstabiliteten. När dessa ytor är exponeradeför låga temperaturer är dock vätningshysteresen under en nedkylnings-/uppvärmnings-cykel significant. Den temperatur-inducerade hysteresenär också betydande för superhydrofoba ytor. Detta tillskriver jag  kondensation på ytan som följs av frostbildning, vilket i sin tur möjliggörspridning av den underkylda vattendroppen på ytan. Mätning avfördröjningen i frysningsförloppet påvisade ingen betydande effekt avyttopografin för hydrofoba ytor, men släta hydrofoba ytor uppvisade denlängsta fördröjningen. Dessa resultat är i överensstämmelse med rådandeheterogen iskärnbildningsteori, som visar på fördelaktig iskärnbildningpå konkava delar av ytan, förutsatt att dessa väts.I den andra delen av avhandlingen utnyttjar jag observationerna frånden första delen vilka illustrerade begränsningarna för superhydrofobaytor, och söker en annan lösning. Avisningsegenskaper för hydrofilastarkt hydratiserade ytor studerades, med hypotesen att hydratiseringkan smörja gränsskiktet med is. Temperatur-kontrolleradeisadhesionsmätningar ned till -25 °C utfördes på adsorberade skikt av enpolymer med många sidokedjor av polyetylenoxid (”bottle-brush”), såvälsom på ett skikt av linjär polyetylenoxid. Isadhesionen blev kraftigtreducerad på ”bottle-brush”-polymeren, speciellt vid temperaturer högreän -15°C. Däremot kunde knappast ingen minskad isadhesion observerasför den linjära polymeren. Dessa observationer överensstämmer meddifferentialskanningskalorimetri (DSC) data, som visar att dethydratiserade vattenskiktet, vilket är bundet till ”bottle-brush”-polymeren, är i vätskeform vid de temperaturer där avisningsfördelar ärobserverade. För att vidare undersöka hypotesen att det vore fördelaktigtmed ett naturligt smörjande skikt på ytan för att uppnå godaavisningsegenskaper, utförde jag isadhesionsmätningar på molekylärtsläta glimmerytor ner till -35 °C. Intressant nog uppmättes extremt lågisadhesion på denna yta. Detta relaterar jag till den föreslagna utprägladehydratiseringsstrukturen, bestående av ett första is-liknande vattenskiktutan fria OH-grupper, följt av ett mer bulkliknande skikt. Detta ikombination med den molekylärt släta naturen hos glimmer resulterar iett perfekt plan för isen att glida på.

QC 20160504

TopNano

Ice accretion on various surfaces can cause destructive effect of our lives, from cars, aircrafts, to infrastructure, power line, cooling and transportation systems. There are plenty of methods to overcome the icing problems including electrical, thermal and mechanical process to remove already accumulated ice on the surfaces and to reduce the risk of further operation. But all these process required substantial amount of energy and high cost of operation. To save the global energy and to improvement the safety issue in many infrastructure and transportation systems we have to introduce some passive anti-icing coating known as ice-phobic coating to reduce the ice-formation and ice adhesion onto the surface. Ice-phobic coatings mostly devoted to utilizing lotus-leaf-inspired superhydrophobic coatings. These surfaces show promising behavior due to the low contact area between the impacting water droplets and the surface. In this present study we investigate systematically the influence of chemical composition and functionality as well as structure of surfaces on wetting properties and later on icing behavior of surfaces. Robust anti-icing coating has been prepared by using modified silica particles as a particles film. Polymer brushes were synthesized on flat, particle surfaces by using Surface initiated ATRP. We have also investigated the effect of anti-icing behavior on the surfaces by varying surface chemistry and textures by using different sizes of particles. This approach is based on the reducing ice accumulation on the surfaces by reducing contact angle hysteresis. This is achieved by introducing nano to micro structured rough surfaces with varying surface chemistry on different substrates. Freezing and melting dynamics of water has been investigated on different surfaces by water vapour condensation in a high humidity (80%) condition ranging from super hydrophilic to super hydrophobic surfaces below the freezing point of water. Kinetics of frost formation and ice adhesion strength measurements were also performed for all samples. All these experiments were carried out in a custom humidity and temperature controlled chamber. We prepared a superhydrophobic surface by using Poly dimethyl siloxane (PDMS) modified fumed silica which display very low ice-adhesion strength almost 10 times lower than the unmodified surface. Also it has self-cleaning behavior after melting of ice since whole ice layer was folded out from the surface to remove the ice during melting. Systematic investigation of the effect of three parameters as surface energy, surface textures (structure, geometry and roughness) and mechanical properties of polymers (soft and stiff) on icing behavior has also been reported.

Nucleation of ice crystals in the atmosphere often occurs through heterogeneous freezing processes facilitated by an atmospheric aerosol that acts as the ice nuclei (IN). Depending on ambient conditions and aerosol composition, heterogeneous nucleation will occur through one of several mechanisms including the contact and immersion freezing mechanisms. Through a series of contact freezing experiments, we have characterized the ability of aerosols composed of volcanic ash, soot, and peat soil, to act as ice nuclei (IN) as a function of temperature. The immersion freezing ability of the ash particles has also been measured. In these studies, an optical microscope apparatus equipped with a cooling stage and a digital camera was used to observe the freezing events. For each experiment, a particular IN was placed in contact with the surface, or immersed in the bulk, of an ultra pure water droplet. The droplet was then subjected to freezing-melting cycles resulting in 25 independent measurements of the freezing temperature of the droplet. In the volcanic ash experiments, we observed contact freezing at warmer temperatures than immersion freezing. As contact freezing IN, the peat was the most effective with an average contact freezing temperature of -10.5 �C, followed by volcanic ash (-11.2 �C), and then soot (-25.6 �C). In addition, we have used classical nucleation theory to identify the contact parameters and nucleation rates for the compositions explored.

An experimental study of phase transitions of long-chain n-alkanes induced by the effect of interfaces is described. ¶ The phase behaviour of long-chain n-alkanes (carbon number 14, 16, 17, 18) adsorbed at isolated mica surfaces and confined between two mica surfaces has been studied in the vicinity of and down to several degrees below the bulk melting points, Tm. Using the Surface Force Apparatus we have measured the thickness of alkane films adsorbed from vapour (0.97 [equal to or greater-than] p/p[subscript o] [equal to or greater-than] 0.997), studied capillary condensation transition, subsequent growth of capillary condensates between two surfaces, and phase transitions in both the adsorbed films and the condensates. By measuring the growth rate of the capillary condensates we have identified a transition in the lateral mobility of molecules in the adsorbed films on isolated mica surfaces. This transition to greater mobility occurs slightly above Tm for n-hexadecane, n-heptadecane and n-octadecane but several degrees below Tm for n-tetradecane, and is accompanied by a change in wetting behaviour and a measurable decrease in adsorbed film thickness for n-heptadecane and n-octadecane. Capillary condensates that form below Tm remain liquid, but may freeze if the degree of confinement is reduced by separation of the mica surfaces. An increase in the area of the liquid-vapour interface relative to that of the liquid-mica interface facilitates freezing in the case of the long-chain alkanes, which show surface freezing at the liquid-vapour interface. ¶ Although thermodynamic properties of the surface freezing transition have been rather well documented, the kinetics involved in formation of such ordered monolayers has so far received very little attention. We studied the surface tension of n-octadecane as a function of temperature in the vicinity of Tm, using the static Wilhelmy plate and the dynamic maximum bubble pressure methods. The two methods give different results on cooling paths, where nucleation of the surface ordered phase is involved, but agree on heating paths, where both methods measure properties of the equilibrium surface phase. On cooling paths, the surface of bubbles may supercool below the equilibrium surface freezing temperature. The onset of surface freezing is marked by a sharp drop in the surface tension. The transition is accompanied by an increased stability of the films resulting in longer bubble lifetimes at the liquid surface, which suggests that the mechanical properties of the surfaces change from liquid-like to solid-like. Our results suggest occurrence of supercooling of the monolayer itself.

This hydrogeophysical field experiment evaluated the ability of high frequency (450 & 900 MHz) ground penetrating radar (GPR) to characterize the release of gasoline over an annual cycle of in situ conditions. In August 2008, 200 liters of E10 gasoline were released into the unconfined sand aquifer at CFB Borden. The 900 MHz profiling clearly shows the development of shallow (i.e., above 10 ns) high reflectivity in the vicinity of the trench immediately after the release. Additional lateral extension of high reflectivity zone was observed over the following 20 days until the seasonal water table low stand occurred, after which no further lateral movement was observed. Throughout the remainder of the monitoring, the 900 MHz profiling observed a long-term dimming of reflectivity at the periphery of the impacted zone. While direct imaging of the shallow impacted zone by the 450 MHz antennas was significantly obscured by the superposition with the direct air-ground wave arrival; its improved depth of penetration allowed the measurement of a velocity “pull-up” of an underlying stratigraphic interface resulting from the displacement of low velocity water by high velocity gasoline. The maximum pull-up was observed during the water table low stand. The ongoing changes in the pull-up magnitude during the remainder of the observation period suggest the continued redistribution of fluids in the impacted zone. Because of the shallow depth of the gasoline impacted zone, the effects of freezing during the winter period were observed in the GPR imaging. The presence of the gasoline impacted zone appears to have affected the depth of freezing, causing a depression of the frozen soil base. The dimming of the direct air-ground wave complex indicates that the contaminant phase brought to the surface by the water table fluctuations have impacted the nature of the near-surface freezing.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008.
Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3241. Advisers: Anthony M. Jacobi; John G. Georgiadis. Includes bibliographical references (leaves ) Available on microfilm from Pro Quest Information and Learning.

A new class of molecules possessing amphipathic character was prepared. These compounds possessed a hydrophobic region capable of intercalation into a lipid bilayer, a hydrophilic linker group capable of extending beyond the surface of a membrane, and a carbohydrate attached at the end of this linker group. These features of this class of compounds permitted their direct incorporation into vesicle formulations and hence the direct examination of interactions occuring in the dry state between carbohydrates and lipid groups in such vesicle membrane systems. Samples of treated vesicle preparations were subjected to freezing and thawing as well as to direct dehydration via lyophilization. Under these conditions, the stability and integrity of the membrane was examined via several spectroscopic techniques. Through these studies of systems in which a carbohydrate is directly bound to a membrane surface, it was possible to determine a defined ratio, independent of solution and concentration effects, at which carbohydrates can afford protection to dehydrated membranes. In addition, the interactions responsible for conferring the protection were determined. It was found that direct intercalation of the carbohydrates into a membrane interface preserves the membrane structure and organization that is normally observed in the presence of water. This behavior prevents the phase transitions, lipid phase separations, and fusion phenomena that normally compromise dehydrated membrane systems. This phenomena is directly related to the amount of carbohydrate that is present and the structure of the carbohydrate that is used. These results indicate that the partitioning behavior of the carbohydrates at the interface is of prime importance in determining the effectiveness in this regard.