1,721,010 research outputs found
Conical micro-structures for super-repellent surfaces and their effect on droplet impact
Full text linkSurface wetting properties control is crucial to various applications, such as anti-wetting, self cleaning and heat transfer processes. Design of surface wetting properties can be achieved by mechanical and chemical methods. The mechanical methods include surface roughness design, while chemical methods involve changing the intrinsic wetting properties of the surface. The intrinsic wetting properties, which can be characterized by the contact angle on a flat surface, can reach a maximum of 120°. The combination of roughness and chemical treatment can distinctly enlarge the contact angle range, including wicking (zero contact angle), partial wetting (finite contact angle) and superhydrophobicity (contact angle larger than 150°). Thus, a lot of works showing various types of structure design with different wettability properties can be found in the literature.
Among the various possible micro-structures, conical structures (well-known structures found in the Lotus leaf) are frequently used due to their unique properties. The Lotus leaf has a lot of micro-scale tapered bumps and also nano-scale roughness. Various previous works artificially produce this kind of conical structures and achieve similar wetting properties. However, a lot of these works use non-regular conical like structures, while the number of works using patterned conical structured surfaces is limited. Therefore, it remains unclear how the different conical geometries can affect the wetting properties of the surface. To bridge this gap, we fabricate patterned conical micro-structured surfaces with different cones geometry and topography and study how the static and dynamic wetting properties are affected by the structures. The conical micro-structures are produced on silicon substrates using photo-lithography and plasma etching techniques. By varying the fabrication process recipe, different types of conical structures are fabricated. This thesis presents the study of both static and dynamic wetting properties for various conical structured surfaces. In addition, cylindrical pillar structured surfaces are also used for comparison.
It is found that the conical structured surface can be designed to be super-repellent for intrinsic contact angles larger than 90°. The conical half-apex angle of the cones is important for suppressing the Cassie-Wenzel transition. This work not only provides more insights into the effect of conical structures on wetting but also shows that conical structures can be a good path for achieving superhydrophobicity.
In addition to the previous study of Cassie wetting state on conical structures, we subsequently investigate how the partial wetting Wenzel droplet shape is affected by the conical pillars sidewall geometry. We compare truncated cone pillars with cylindrical pillar surfaces. Previous works show that pillar height/pitch can affect the liquid droplet final shape. However, we observe that the drop shape on truncated cones and on cylindrical pillars is different even when they have the same pitch and height. Besides, the drop shape on these two types of surfaces is also evolving in a different way as the impact Weber number increases. This work reveals that the micro-structures side wall topography can influence the final drop shape.
We further investigate conical structures as a means to increase the anti-wetting properties of surfaces during impact of low surface tension droplets. We fabricate conical pillars surfaces with re-entrant like side wall roughness all along the side wall, which looks like a tree-branch topography. Low surface tension drop impact experiments are conducted on these surfaces and we show that the tree-branch like structure does improve the anti-wetting performance by exhibiting a higher critical Weber number (the Weber number starting to show partial rebound), compared with conical structures without a sidewall roughness. The tree-branch like structures can reduce the solid-liquid contact and have higher resistance to penetration, and thus can reach higher anti-wetting performance than other reported rigid surfaces.
Finally, we explore how conical and cylindrical pillar structures behave for water droplets impacting at different Weber numbers. We show how the liquid residue size is affected when the droplet impacts above the critical Weber number for conical and cylindrical pillars. It is shown that the conical pillar surfaces have higher contact angle and lower hysteresis while the cylindrical pillars show lower contact angle and higher hysteresis for dense array surfaces. At low Weber number, conical structures surfaces show less energy dissipation compared with cylindrical structures surfaces. For the same height and pitch, the cylindrical pillars show a higher critical Weber number compared with conical pillars due to the large solid-liquid contact at the pillar top. However, the liquid residue when the Weber number is above the critical Weber number for the cylindrical case is larger than for the conical pillar case. We propose that the liquid residue size is affected by the We number, anti-penetration ability and liquid mobility inside the structures. Liquid mobility within the conical structures is lower than for the cylindrical ones, which leads to less wetted area due to less open space inside the structures.
This work not only reveals how the conical geometry can affect the wetting properties but also shows that conical structured surfaces are a good candidate for anti-wetting performance enhancement, which can be useful for various applications
Two-phase Flow Instabilities during Flow Boiling and Control of Wettability by Micro-structured Surfaces
Full text linkBoiling heat transfer is of relevance in a variety of applications in power and process industries. In spite of the vast research performed in the last decades, fundamental questions about mechanisms related to the boiling regime, heat transfer mechanisms, pressure drop and two-phase flow instabilities still remain unanswered. This thesis deals with two important points within boiling heat transfer: (i) the first part of the thesis is focused on understanding the dynamics of two-phase flow instabilities in small-diameter channels, in particular, pressure drop oscillations and their effect on the heat transfer coefficient; (ii) the second part of the thesis deals with manipulation of surface wettability by modification of the surface structure its effect on evaporative heat transfer, more precisely the Leidenfrost phenomenon.
In flow boiling systems, two-phase flow instabilities such as density wave oscillations and pressure drop oscillations have been identified as one of the impediments for achieving high heat flux due to the potential heat transfer deterioration. However, most of the fundamental characteristics of the two-phase flow instabilities and the mechanisms leading to the heat transfer deterioration remain uncharted. In this thesis, contradictions between previous studies regarding the effect of the mass flux on amplitude and period of oscillations during pressure drop oscillations are clarified experimentally. A series of designed experiments are able to show the conditions for the interaction between density wave oscillations and pressure drop oscillations. Especially, it is shown that pure pressure drop oscillations can occur in a horizontal heated tube, something that has not been reported in previous studies. When both types of oscillations occur simultaneously, it is shown that the amplitude of the superimposed density wave oscillations on the pressure drop oscillations is controlled by the surge tank upstream of the test section. This previously unknown effect of the surge tank on the density wave oscillations contributes to improving the understanding of the complex dynamics of twophase flow instabilities. Furthermore, the deterioration of the heat transfer coefficient under controlled flow oscillations is investigated. It is observed that the averaged heat transfer coefficient can be deteriorated by flow oscillations. In particular, the deterioration is negligible until the amplitude of the oscillations become higher than a given threshold. Above this threshold, the deterioration is attributed to the dry-out of the wall during the low mass flux part of the oscillations. Based on these findings, it can be suggested that a severe deterioration of the flow boiling heat transfer coefficient can occur because of two-phase flow instabilities, but only when the amplitude of the oscillations is above a given threshold.
Looking more in the microscale level, when it comes to the enhancement of the heat transfer from the surface to the cooling fluid, recent progress in controlling the properties of the surfaces at the micro-nanoscale by micro/nano fabrication techniques has motived a vast amount of research. However, the effect of the micro-nanoscale surface properties into the overall performance remains an open question. In the second part of the thesis, a fabrication process for nature-inspired microstructures is introduced. By mimicking conical microstructures found in nature, which can present superhydrophilic and superhydrophobic properties, a broad range of surface wettability is obtained. The fabricated surfaces show that the wettability can be controlled by adjusting the geometric parameters of the microstructure without external excitation. Remarkably, a drastic wetting transition from the Cassie-Baxter to the Wenzel state is observed by varying the spacing between the microstructures. Furthermore, the effect of the surface topology and chemical coating on the surface wettability is investigated. Silanization and replication of the geometrical microstructures into polydimethylsiloxane (PDMS) are considered to modify the chemical composition of the surfaces and to keep the similar topography of the surfaces. It is shown that the wettability in the Cassie-Baxter state is determined by the geometrical aspect in the microscale while wettability in the Wenzel state is decided by the chemical aspect. Regarding heat transfer, enhancement of the Leidenfrost temperature on the microstructured surfaces is investigated.
The tested microstructures are able to increase the Leidenfrost point. Differences between all the definitions of the Leidenfrost point found in the literature are investigated. It is observed that the difference between the Leidenfrost points which is less than 10K in the smooth surface can become larger up to 70K when microstructures exist. Results indicate that with a microstructured surface, not only can the Leidenfrost be shifted to higher temperatures but phenomenological differences compared to what is observed on smooth surfaces occur, namely the observation of the transition film boiling regime
A new simplified model for condensation heat transfer of zeotropic mixtures inside horizontal tubes
No full textCell Lysis via Surface Acoustic Waves (SAW)
No full textAt the laboratory scale, chemical lysis is the most commonly used method for cell disruption due to its high efficiency, low cost, and compatibility with analytical techniques like PCR and electrophoresis. However, chemical lysis has drawbacks related to the use of reagents, such as the need for downstream chemical removal or long processing times. Therefore, new cell lysis methods that minimize chemical use and equipment complexity are being investigated. Among these, acoustic-based lysis, particularly using Surface Acoustic Wave (SAW) technology, shows promise. SAW generates high-shear regions in the fluid that can damage the cell membrane, but this technology presents significant limitations, such as difficulty in generating strong enough shear forces for effectively disrupting cell membranes, low volume capacity, and poor compatibility with lab workflows.
The goal of this thesis is to advance the application of SAW-based cell lysis by optimizing the control and manipulation of acoustic streaming in open microfluidic systems. The objectives are to enhance the acoustic-induced shear stress through the design and optimization of interdigital transducer (IDT) configurations, to maximize the exposure of cells to acoustic-induced shear stress lysis efficiency by optimizing droplet size, to extend the application of these optimized SAW techniques to larger volumes and to investigate the role of shear stress magnitude and exposure duration in the cell lysis performance. In this work, the use of high-frequency SAW-devices for cell lysis is studied. Short attenuation lengths associated with high-frequency SAW (>100 MHz), lead to shorter recirculation lengths, enhancing acoustic streaming velocity and shear forces. We demonstrate that by using a 120 MHz SAW-device, acoustic-induced shear stress is sufficient for achieving efficient cell disruption. Chemical-free cell disruption due to acoustic streaming induced shear stress is achieved in 20 μL droplets with an efficiency of 50-60% for a total process time of 4 minutes. Further, we demonstrate the possibility of enhancing cell lysis efficiency by reducing the droplet size, which increases cell exposure to high shear stress. A challenge of SAW actuation in small droplets is the droplet displacement and nebulization when the size of the droplet becomes comparable to the SAW width. These effects are counteracted by reducing the aperture of the IDT, which concentrates the acoustic energy in a narrower region of the droplet, enabling cell lysis in small droplets. By reducing the droplet size to 5 μL and using a micro-sized IDT, cell lysis efficiency increases up to 80%. The total lysis time is reduced to 60 seconds. These results can be explained by an enhancement of cell exposure to shear stress. These observations suggest an accumulative damage of the cell membrane exposed to repetitive, short-term shear. A deeper analysis of these observations reveals that there is a shear stress threshold of 0.26 Pa over which if it is exceeded, cells lysis occurs. Above this threshold value, the extracted DNA gets proportional to the shear stress. A relation between cell lysis and the accumulative damage on the cells is suggested. Last, the limitations in volume capacity and compatibility with lab workflows of SAW-based cell lysis methods are addressed with the development of a mobile IDT. This device allows more precise and effective transmission of acoustic energy throughout the sample, enhancing acoustic streaming in larger volumes. Cell lysis with an efficiency of 90-100% is achieved in 50 μL droplets in 75 seconds. In addition, this device enables direct transmission of the acoustic energy into samples contained in assay tubes or microwells, avoiding wave damping in the container walls. This method brings an alternative to in-droplet SAW-based lysis, improving SAW-based compatibility with lab workflows. Cell lysis is also enabled in assay tubes containing 100 μL of sample, with an efficiency of 90-100% in 45 seconds. Compared to chemical lysis, this method presents equal compatibility with lab workflows and offers comparable efficiencies, while reducing the process time by 15 times and avoiding the use of reagents.
In summary, this work advances the application of Surface Acoustic Wave (SAW) technology for cell lysis by optimizing acoustic streaming and addressing key limitations associated with its use. The scientific impact of these findings lies in their potential for wide-ranging applications in cell biology, diagnostics, and molecular biology, particularly in settings where reagent-free, rapid, and scalable cell lysis is critical. Additionally, the results provide a strong foundation for future research into the development of more efficient and versatile SAW-based lysis systems that can be integrated into lab workflows
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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