Measuring the bending of asymmetric planar EAP structures

The geometric characterization of low-voltage dielectric electro-active polymer (EAP) structures, comprised of nanometer thickness but areas of square centimeters, for applications such as artificial sphincters requires methods with nanometer precision. Direct optical detection is usually restricted to sub-micrometer resolution because of the wavelength of the light applied. Therefore, we propose to take advantage of the cantilever bending system with optical readout revealing a sub-micrometer resolution at the deflection of the free end. It is demonstrated that this approach allows us to detect bending of rather conventional planar asymmetric, dielectric EAP-structures applying voltages well below 10 V. For this purpose, we built 100 μm-thin silicone films between 50 nm-thin silver layers on a 25 μm-thin polyetheretherketone (PEEK) substrate. The increase of the applied voltage in steps of 50 V until 1 kV resulted in a cantilever bending that exhibits only in restricted ranges the expected square dependence. The mean laser beam displacement on the detector corresponded to 6 nm per volt. The apparatus will therefore become a powerful mean to analyze and thereby improve low-voltage dielectric EAP-structures to realize nanometer-thin layers for stack actuators to be incorporated into artificial sphincter systems for treating severe urinary and fecal incontinence.

Casting materials and their application in research and teaching

From a biological point of view, casting refers to filling of anatomical and/or pathological spaces with extraneous material that reproduces a three-dimensional replica of the space. Casting may be accompanied by additional procedures such as corrosion, in which the soft tissue is digested out, leaving a clean cast, or the material may be mixed with radiopaque substances to allow x-ray photography or micro computed topography (µCT) scanning. Alternatively, clearing of the surrounding soft tissue increases transparency and allows visualization of the casted cavities. Combination of casting with tissue fixation allows anatomical dissection and didactic surgical procedures on the tissue. Casting materials fall into three categories namely, aqueous substances (India ink, Prussian blue ink), pliable materials (gelatins, latex, and silicone rubber), or hard materials (methyl methacrylates, polyurethanes, polyesters, and epoxy resins). Casting has proved invaluable in both teaching and research and many phenomenal biological processes have been discovered through casting. The choice of a particular material depends inter alia on the targeted use and the intended subsequent investigative procedures, such as dissection, microscopy, or µCT. The casting material needs to be pliable where anatomical and surgical manipulations are intended, and capillary-passable for ultrastructural investigations.

The 'triple contrast' method in experimental wound ballistics and backspatter analysis.

In practical forensic casework, backspatter recovered from shooters' hands can be an indicator of self-inflicted gunshot wounds to the head. In such cases, backspatter retrieved from inside the barrel indicates that the weapon found at the death scene was involved in causing the injury to the head. However, systematic research on the aspects conditioning presence, amount and specific patterns of backspatter is lacking so far. Herein, a new concept of backspatter investigation is presented, comprising staining technique, weapon and target medium: the 'triple contrast method' was developed, tested and is introduced for experimental backspatter analysis. First, mixtures of various proportions of acrylic paint for optical detection, barium sulphate for radiocontrast imaging in computed tomography and fresh human blood for PCR-based DNA profiling were generated (triple mixture) and tested for DNA quantification and short tandem repeat (STR) typing success. All tested mixtures yielded sufficient DNA that produced full STR profiles suitable for forensic identification. Then, for backspatter analysis, sealed foil bags containing the triple mixture were attached to plastic bottles filled with 10 % ballistic gelatine and covered by a 2-3-mm layer of silicone. To simulate backspatter, close contact shots were fired at these models. Endoscopy of the barrel inside revealed coloured backspatter containing typable DNA and radiographic imaging showed a contrasted bullet path in the gelatine. Cross sections of the gelatine core exhibited cracks and fissures stained by the acrylic paint facilitating wound ballistic analysis.

Posterior Vertical Bite Reconstructions of Erosively Worn Dentitions and the "Stamp Technique" - A Case Series with a Mean Observation Time of 40 Months

PURPOSE In the present case series, the authors report on seven cases of erosively worn dentitions (98 posterior teeth) which were treated with direct resin composite. MATERIALS AND METHODS In all cases, both arches were restored by using the so-called stamp technique. All patients were treated with standardized materials and protocols. Prior to treatment, a waxup was made on die-cast models to build up the loss of occlusion as well as ensure the optimal future anatomy and function of the eroded teeth to be restored. During treatment, teeth were restored by using templates of silicone (ie, two "stamps," one on the vestibular, one on the oral aspect of each tooth), which were filled with resin composite in order to transfer the planned, future restoration (ie, in the shape of the waxup) from the extra- to the intraoral situation. Baseline examinations were performed in all patients after treatment, and photographs as well as radiographs were taken. To evaluate the outcome, the modified United States Public Health Service criteria (USPHS) were used. RESULTS The patients were re-assessed after a mean observation time of 40 months (40.8 ± 7.2 months). The overall outcome of the restorations was good, and almost exclusively "Alpha" scores were given. Only the marginal integrity and the anatomical form received a "Charlie" score (10.2%) in two cases. CONCLUSION Direct resin composite restorations made with the stamp technique are a valuable treatment option for restoring erosively worn dentitions.

Space requirement of a prefabricated bar on two interforaminal implants: a prospective clinical study

OBJECTIVES This clinical study measured the dimensional changes of existing lower complete dentures due to the integration of a prefabricated implant bar. Additionally, the impact of this dimensional change on patient satisfaction and oral function was analyzed. METHODS Twenty edentulous patients (10 men/10 women; aged 65.9 ± 11.8 years) received two interforaminal implants. Subsequent to surgery, a chair side adapted, prefabricated bar (SFI Bar(®), C+M, Biel, Switzerland) was inserted, and the matrix was polymerized into the existing lower denture. The change of the denture's lingual dimension was recorded by means of a bicolored, silicone denture duplicate that was sectioned in the oro-vestibular direction in the regions of the symphysis (S) and the implants (I-left, I-right). On the sections, the dimensional increase was measured using a light microscope. Six months after bar insertion, patients answered a standardized questionnaire. RESULTS All dentures exhibited increased lingual volume, more extensively at S than at I (P = 0.001). At S, the median diagonal size of the denture was doubled (+4.33 mm), and at I, the median increase was 50% (I-left/-right = +2.66/+2.62 mm). The original denture size influenced the volume increase (P = 0.024): smaller dentures led to a larger increase. The amount of denture increase did not have negative impact on either self-perceived oral function or patient satisfaction. Approximately, 95% of the patients were satisfied with the treatment results. CONCLUSIONS The lingual size of a lower denture was enlarged by the integration of a prefabricated bar without any negative side effects. Thus, this attachment system is suitable to convert an existing full denture into an implant-supported overdenture.

Tomographic PIV behind a prosthetic heart valve

The instantaneous three-dimensional velocity field past a bioprosthetic heart valve was measured using tomographic particle image velocimetry (PIV). Two digital cameras were used together with a mirror setup to record PIV images from four different angles. Measurements were conducted in a transparent silicone phantom with a simplified geometry of the aortic root. The refraction indices of the silicone phantom and the working fluid were matched to minimize optical distortion from the flow field to the cameras. The silicone phantom of the aorta was integrated in a flow loop driven by a piston pump. Measurements were conducted for steady and pulsatile flow conditions. Results of the instantaneous, ensemble and phase averaged flow field are presented. The three-dimensional velocity field reveals a flow topology, which can be related to features of the aortic valve prosthesis.

Transfer and effects of 1,2,3,5,7-pentachloronaphthalene in an experimental food chain.

Polychlorinated naphthalenes are environmentally relevant compounds that are measured in biota at concentrations in the μg/kg lipid range. Despite their widespread occurrence, literature data on the accumulation and effects of these compounds in aquatic ecosystems are sparsely available. The goal of this study was to gain insights into the biomagnification and effects of 1,2,3,5,7-pentachloronaphthalene (PeCN52) in an experimental food chain consisting of benthic worms and juvenile rainbow trout. Worms were contaminated with PeCN52 by passive dosing from polydimethylsiloxane silicone. The contaminated worms were then used to feed the juvenile rainbow trout at 0.12, 0.25 or 0.50 μg/g fish wet weight/day, and the resulting internal whole-body concentrations of the individual fish were linked to biological responses. A possible involvement of the cellular detoxification system was explored by measuring PeCN52-induced expression of the phase I biotransformation enzyme gene cyp1a1 and the ABC transporter gene abcb1a. At the end of the 28-day study, biomagnification factors were similar for all dietary intake levels with values between 0.5 and 0.7 kg lipid(fish)/kg lipid(worm). The average uptake efficiency of 60% indicated that a high amount of PeCN52 was transferred from the worms to the fish. Internal concentrations of up to 175 mg/kg fish lipid in the highest treatment level did not result in effects on survival, behavior, or growth of the juvenile trout, but were associated with the induction of phase I metabolism which was evident from the significant up-regulation of cyp1a1 expression in the liver. In contrast, no changes were seen in abcb1a transcript levels.

Proceedings of the Twentieth Annual Biochemical Engineering Symposium

The 20th Annual Biochemical Engineering Symposium was held at Kansas State University on April 21,1990. The objectives of the symposium were to provide: (i) a forum for informal discussion of biochemical engineering research being conducted at the participating institutions and (ii) an opportunity for students to present and publish their work. Twenty-eight papers presented at the symposium are included in this proceedings. Some of the papers describe the progress of ongoing projects, and others contain the results of completed projects. Only brief summaries are given of the papers that will be published in full elsewhere. The program of the symposium and a list of the participants are included in the proceedings. ContentsCell Separations and Recycle Using an Inclined Settler, Ching-Yuan Lee, Robert H. Davis and Robert A. Sclafani Micromixing and Metabolism in Bioreactors: Characterization of a 14 L Fermenter, K.S. Wenger and E.H. Dunlop Production, Purification, and Hydrolysis Kinetics of Wild-Type and Mutant Glucoamylases from Aspergillus Awamori, Ufuk Bakir, Paul D. Oates, Hsiu-Mei Chen and Peter J. Reilly Dynamic Modeling of the Immune System, Barry Vant-Hull and Dhinakar S. Kompala Dynamic Modeling of Active Transport Across a Biological Cell: A Stochastic Approach, B.C. Shen, S.T. Chou, Y.Y. Chiu and L.T. Fan Electrokinetic Isolation of Bacterial Vesicles and Ribosomes, Debra T.L. Hawker, Robert H. Davis, Paul W. Todd, and Robert Lawson Application of Dynamic Programming for Fermentative Ethanol Production by Zymomonas mobilis, Sheyla L. Rivera and M. Nazmul Karim Biodegradation of PCP by Pseudomonas cepacia, R. Rayavarapu, S.K. Banerji, and R.K. Bajpai Modeling the Bioremediation of Contaminated Soil Aggregates: a Phenomenological Approach, S. Dhawan, L.E. Erickson and L.T. Fan Biospecific Adsorption of Glucoamylase-I from Aspergillus niger on Raw Starch, Bipin K. Dalmia and Zivko L. Nikolov Overexpression in Recombinant Mammalian Cells: Effect on Growth Rate and Genetic Instability, Jeffrey A. Kern and Dhinakar S. Kompala Structured Mathematical Modeling of Xylose Fermentation, A.K. Hilaly, M.N. Karim, I. C. Linden and S. Lastick A New Culture Medium for Carbon-limited Growth of Bacillus thuringiensis, W. -M. Liu and R.K. Bajpai Determination of Sugars and Sugar Alcohols by High Performance Ion Chromatography, T. J. Paskach, H.-P. Lieker, P.J. Reilly, and K. Thielecke Characterization of Poly-Asp Tailed B-Galactosidase, M.Q. Niederauer, C.E. Glatz, l.A. Suominen, C.F. Ford, and M.A. Rougvie Computation of Conformations and Energies of cr-Glucosyl Disaccharides, Jing Zepg, Michael K. Dowd, and Peter J. Reilly Pentachlorophenol Interactions with Soil, Shein-Ming Wei, Shankha K. Banerji, and Rakesh K. Bajpai Oxygen Transfer to Viscous Liquid Media in Three-Phase Fluidized Beds of Floating Bubble Freakers, Y. Kang, L.T. Fan, B.T. Min and S.D. Kim Studies on the Invitro Development of Chick Embryo, A. Venkatraman and T. Panda The Evolution of a Silicone Based Phase-Separated Gravity-Independent Bioreactor, Peter E. Villeneuve and Eric H. Dunlop Biodegradation of Diethyl Phthalate, Guorong Zhang, Kenneth F. Reardon and Vincent G. Murphy Microcosm Treatability of Soil Contaminated with Petroleum Hydrocarbons, P. Tuitemwong, S. Dhawan, B.M. Sly, L.E. Erickson and J.R. Schlup

Properties of seawater (partial pressure of carbon dioxide (pCO2)) from a ProOceanus CO2-Pro sensor mounted on the continuous surface water sampling system during the Tara Oceans expedition 2009-2013

The Tara Oceans Expedition (2009-2013) sampled the world oceans on board a 36 m long schooner, collecting environmental data and organisms from viruses to planktonic metazoans for later analyses using modern sequencing and state-of-the-art imaging technologies. Tara Oceans Data are particularly suited to study the genetic, morphological and functional diversity of plankton. The present data set provides continuous measurements of partial pressure of carbon dioxide (pCO2), using a ProOceanus CO2-Pro instrument mounted on the flowthrough system. This automatic sensor is fitted with an equilibrator made of gas permeable silicone membrane and an internal detection loop with a non-dispersive infrared detector of PPSystems SBA-4 CO2 analyzer. A zero-CO2 baseline is provided for the subsequent measurements circulating the internal gas through a CO2 absorption chamber containing soda lime or Ascarite. The frequency of this automatic zero point calibration was set to be 24 hours. All data recorded during zeroing processes were discarded with the 15-minute data after each calibration. The output of CO2-Pro is the mole fraction of CO2 in the measured water and the pCO2 is obtained using the measured total pressure of the internal wet gas. The fugacity of CO2 (fCO2) in the surface seawater, whose difference with the atmospheric CO2 fugacity is proportional to the air-sea CO2 fluxes, is obtained by correcting the pCO2 for non-ideal CO2 gas concentration according to Weiss (1974). The fCO2 computed using CO2-Pro measurements was corrected to the sea surface condition by considering the temperature effect on fCO2 (Takahashi et al., 1993). The surface seawater observations that were initially estimated with a 15 seconds frequency were averaged every 5-min cycle. The performance of CO2-Pro was adjusted by comparing the sensor outputs against the thermodynamic carbonate calculation of pCO2 using the carbonic system constants of Millero et al. (2006) from the determinations of total inorganic carbon (CT ) and total alkalinity (AT ) in discrete samples collected at sea surface. AT was determined using an automated open cell potentiometric titration (Haraldsson et al. 1997). CT was determined with an automated coulometric titration (Johnson et al. 1985; 1987), using the MIDSOMMA system (Mintrop, 2005). fCO2 data are flagged according to the WOCE guidelines following Pierrot et al. (2009) identifying recommended values and questionable measurements giving additional information about the reasons of the questionability.

Caracterización Dinámica de Partículas Magnéticas y su Uso en Aplicaciones Biomédicas

El uso de las partículas magnéticas ha venido a más durante los últimos 10 años. Lo anterior ha estado condicionado por factores como la amplia investigación en materiales a nivel atómico, los retos propuestos por las diferentes áreas de investigación donde los niveles nanométricos cobran importancia, la inherente innovación tecnológica que pueden aportar a la industria dichas escalas y finalmente, el impacto socioeconómico que pueden tener estos avances tecnológicos. Sin embargo, la traslación a la industria de los avances tecnológicos se ha visto limitada por la falta de estrategias, ya que el 88% del conocimiento se queda en Universidades y Centros de Investigación y el resto, 12%, pasa a la industria o es obtenido en ella. En este trabajo de investigación se ha tratado de completar el círculo de idea, conceptualización, generación de conocimiento, prototipo, prueba in vitro y prueba in vivo; todo ello para dejar el menor espacio posible entre Investigación e Industria. Se ha fabricado un recinto que permite la observación del comportamiento de las partículas magnéticas cuando están inmersas en un medio con viscosidad controlada y con una fuente magnética estática. De la observación experimental se han encontrado fenómenos magnéticos como la aglomeración y la autoorganización, que han sido aprovechados, no solo para ofrecer una solución alternativa a la corrección del desprendimiento de retina, sino también para promover las mejoras de métodos de separación y/o filtrado celular, ayuda en correcciones ortopédicas o en el diagnóstico y tratamiento de enfermedades degenerativas. Particularizando en la corrección del desprendimiento de retina, se ha desarrollado y patentado un Dispositivo Ocular cuya función es la de tapón del agujero retiniano, ayudando en la cicatrización de la retina. Dicho dispositivo ha sido evaluado en conejos albinos neozelandeses obteniéndose un éxito en la reaplicación de la retina del 80% y resultando su toxicidad en condición difusa tras evaluación anatomopatológica. Así mismo, el ferrofluido diseñado y fabricado se ha localizado en la zona de interacción magnética siempre y su disposición ha sido en el mayor de los casos en forma de placa pero también se han formado anillos; lo anterior no ha afectado en ningún sentido el éxito en la reaplicación de la retina. Por otro lado, no se han encontrado indicios de siderosis en la zona de interacción magnética, sus alrededores o el resto del globo ocular; ni las células han presentado apoptosis inducida por los materiales introducidos en el ojo. Sin embargo, se ha encontrado una fuerte necrosis inducida por la presión ejercida sobre la retina por el dispositivo ocular, evidenciando la necesidad de un mejor control de dicha presión. Tras la caracterización de los copos y la presión que éstos ejercen, se ha podido identificar que el dispositivo ocular se puede sintonizar para inducir una presión entre 0.5 y 2.5 mmHg; por tanto se puede vislumbrar que es posible minimizar la muerte celular causada en las células retinianas. Finalmente, tras la experimentación con conejos se ha observado que el dispositivo ocular resuelve los problemas inducidos por las siliconas o los gases e inclusive aporta ventajas competitivas de alto valor añadido como: no requerir de posiciones incómodas en el post-operatorio, no modifica las propiedades ópticas del ojo ya que no es necesario dejar silicona o gases en el interior del ojo, se pueden sellar múltiples agujeros retinianos, corrección de desprendimiento de retina inferior sin complicaciones asociadas, etc.; en definitiva mejora la calidad de vida domiciliaria del paciente además de solventar los problemas inducidos por las soluciones actuales. The using of magnetic particles has become more important during last 10 years. This event has been conditioned by several factors like a wide researching on materials at atomic level, the challenges proposed by different research areas where nanometric levels became important, the inherent technological innovation to the industry and finally, the socioeconomic impact of these kinds of technologies. However, the nanotechnological advances have not been well addressed to the industry since 88% of the acknowledge keeps on Universities and Research Centers, and the remaining 12% goes through the industry or is obtained in it. In this research work, we have tried to complete the entire process about idea, concept, acknowledge generation, prototyping and in vitro and in vivo testing; all to leave as little space as possible between Research and Industry. We have built an experimental hall which allows us the observation of magnetic particles' behavior when are immersing in a controlled viscous medium and under a static magnetic field. The experimental observation has been useful to identify and use two magnetic phenomena: agglomeration and self-organization; no only for retinal detachment, but also to improve cellular separation and/or filtered methods, orthopedic solutions or the diagnostic and treatment of degenerative diseases. Particularizing on retinal detachment, we have developed and patented an Ocular Device which its function is acting like a tampon of the retinal hole for helping with the scarring retina. The device has been evaluated in animals and the results show that we can achieve a success of 80% before rabbit’s death and non cell apoptosis, only necrosis, over the retina attached by the ocular device after anatomopathological evaluation. Also we have identified a diffuse toxicity after anatomopathological evaluation, so more experimenting must be proposed. Otherwise, the ferrofluid has been localized where the magnetic interaction area is located and its deposition has been in most cases plaque shape like but rings have also been observed; this has not affected in any way the successful reattachment of the retina. On the other hand, we have not found any evidence of siderosis in the magnetic interaction zone, near of or some other place of the ocular globe; and we did not find any apoptosis induced by the materials inserted within the eye. However full necrosis, exactly on the magnetic interaction zone, has been found and induced by the pressure exerted over the retina by the ocular device, evidencing the needing of its better control. After flakes’ characterization and its pressure exerted, we identified that the ocular device can be tuned in order to induce a pressure between 0.5 and 2.5 mmHg; therefore we can perceive that death caused to the retinal cells can be minimized. Finally, the experimentation has shown that the ocular device solves all problems induced by silicone or gases an even contributes with some other competitive advantages of high added value like: no uncomfortable postoperative positions, it does not modify any optical property of the eye because there is no needing of leaving silicone or gases inside the eye, many holes can be sealed, the inferior retinal detachment can be solve without any complications, etc.; definitively the ocular device improves patients’ quality of life.

Performance Modeling of Fresnel-Based CPV Systems: Effects of Deformations under Real Operation Conditions

Getting a lower energy cost has always been a challenge for concentrated photovoltaic. The FK concentrator enhances the performance (efficiency, acceptance angle and manufacturing tolerances) of the conventional CPV system based on a Fresnel primary stage and a secondary lens, while keeping its simplicity and potentially low‐cost manufacturing. At the same time F‐XTP (Fresnel lens+reflective prism), at the first glance has better cost potential but significantly higher sensitivity to manufacturing errors. This work presents comparison of these two approaches applied to two main technologies of Fresnel lens production (PMMA and Silicone on Glass) and effect of standard deformations that occur under real operation conditions

Multi-criteria selection of structural adhesives to bond ABS parts obtained by rapid prototyping

One of the most used methods in rapidprototyping is Fused Deposition Modeling (FDM), which provides components with a reasonable strength in plastic materials such as ABS and has a low environmental impact. However, the FDM process exhibits low levels of surface finishing, difficulty in getting complex and/or small geometries and low consistency in “slim” elements of the parts. Furthermore, “cantilever” elements need large material structures to be supported. The solution of these deficiencies requires a comprehensive review of the three-dimensional part design to enhance advantages and performances of FDM and reduce their constraints. As a key feature of this redesign a novel method of construction by assembling parts with structuraladhesive joints is proposed. These adhesive joints should be designed specifically to fit the plastic substrate and the FDM manufacturing technology. To achieve this, the most suitable structuraladhesiveselection is firstly required. Therefore, the present work analyzes five different families of adhesives (cyanoacrylate, polyurethane, epoxy, acrylic and silicone), and, by means of the application of technical multi-criteria decision analysis based on the analytic hierarchy process (AHP), to select the structuraladhesive that better conjugates mechanical benefits and adaptation to the FDM manufacturing process

Control by ATR-FTIR of surface treatment of cork stoppers and its effect on their mechanical performance

Fourier transform infrared (FTIR) spectroscopy was applied to determine the type of surface treatment and dose used on cork stoppers and to predict the friction between stopper and bottleneck. Agglomerated cork stoppers were finished with two different doses and using two surface treatments: P (paraffin and silicone), 15 and 25 mg/stopper, and S (only silicone), 10 and 15 mg/stopper. FTIR spectra were recorded at five points for each stopper by attenuated total reflectance (ATR). Absorbances at 1,010, 2,916, and 2,963 cm -1 were obtained in each spectrum. Discriminant analysis techniques allowed the treatment, and dose applied to each stopper to be identified from the absorbance values. 91.2% success rates were obtained from individual values and 96.0% from the mean values of each stopper. Spectrometric data also allowed treatment homogeneity to be determined on the stopper surface, and a multiple regression model was used to predict the friction index (If = Fe/Fc) (R 2 = 0.93)

Towards materials with enhanced electro-mechanical response: CaCu3Ti4O12-polydimethylsiloxane composites

We describe a straightforward production pathway of polymer matrix composites with increased dielectric constant for dielectric elastomer actuators (DEAs). Up to date, the approach of using composites made of high dielectric constant ceramics and insulating polymers has not evidenced any improvement in the performance of DEA devices, mainly as a consequence of the ferroelectric nature of the employed ceramics. We propose here an unexplored alternative to these traditional fillers, introducing calcium copper titanate (CCTO) CaCu3Ti4O12, which has a giant dielectric constant making it very suitable for capacitive applications. All CCTO-polydimethylsiloxane (PDMS) composites developed display an improved electro-mechanical performance. The largest actuation improvement was achieved for the composite with 5.1 vol% of CCTO, having an increment in the actuation strain of about 100% together with a reduction of 25% in the electric field compared to the raw PDMS matrix.

Hydrothermal waves and corotating rolls in laterally heated convection in simple liquids.

The stability of a liquid layer with an undeformable interface open to the atmo- sphere, subjected to a horizontal temperature gradient, is theoretically analysed. Buoyancy and surface tension forces give rise to a basic flow for any temperature dif- ference applied on the system. Depending on the liquid depth, this basic flow is desta- bilised either by an oscillatory instability, giving rise to the so-called hydrothermal waves, or by a stationary instability leading to corotating rolls. Oscillatory perturba- tions are driven by the basic flow and therefore one must distinguish between convec- tive and absolute thresholds. The instability mechanisms as well as the di¿erent re- gimes observed in experiments are discussed. The calculations are performed for a fluid used in recent experiments, namely silicone oil of 0.65 cSt ðPr 1?4 10Þ. In partic- ular, it is shown that two branches of absolute instability exist, which may be related to the two types of hydrothermal waves observed experimentally