Ana Sofia Moita

Enhancement of pool boiling heat transfer by surface micro-structuring

The present paper addresses the use of surfaces structured with arrays of square micro-cavities to enhance pool boiling heat transfer. The heat transfer performance, obtained with the structured surfaces is evaluated based on the measured boiling curves and on the heat transfer coefficients. Two new parameters are suggested to relate the bubble dynamics (and consequently the surface topography) with the heat transfer coefficients: the modified dimensionless cavity spacing and the dimensionless distance, which cover the governing parameters of the phenomena. Correlations of these parameters with the heat transfer coefficients allowed to identify the best performing patterns, from those tested so far. Based on this progress it is expected that optimization of these relations will lead to precise relations which allow a systematic optimization of the surface pattern leading to an effective heat transfer enhancement, for situations involving high heat fluxes.

Characterization of the Topography and Wettability of English Weed Leaves and Biomimetic Replicas

The topography and wettability of the underside of English weed (Oxalis pes-caprae) leaves and of their biomimetic replicas are investigated. Polyvinyl siloxane molds were cast from the leaves and then filled with an epoxy pre-polymer to produce replicas. The particular topographical structures of leaves and replicas were evaluated by optical microscopy and Scanning Electron Microscopy (SEM) analysis. The static wettability of leaves and replicas was assessed by contact angle measurements, while the dynamic wettability was characterized by estimating contact angle hysteresis and studying the dynamic behavior of impacting water droplets. A smooth glass slip and its replica were used as control surfaces. The replica moulding method used was able to transfer the characteristic pattern of irregular 100 µm − 200 µm × 60 µm convex papillae interspersed with stomata of the original leaf to the epoxy replicas. The static contact angle of 143° ± 3° and the contact angle hysteresis of 2° indicate that the underside of the English weed leaf is close to superhydrophobic. The lower contact angles (130° ± 4°) and higher hysteresis (31°) observed for the replica when compared with the original leaves were associated to an inaccurate replication of the chemistry and structures of the three-dimensional wax projections covering the plant surface. Also, trichomes in the original leaves could not be accurately reproduced due to their flexibility and fragility. Differences in wetting behavior were also evident from droplet impact experiments, with rebound regimes prevailing in the original leaves and regimes characterized by higher adhesion and larger dissipation predominating in the replicas. Nevertheless, the morphological features of the leaf transferred to the replica were sufficient to promote a clear hydrophobic behavior of the replica when compared with the smooth epoxy reference surface.

The Deformation of Single Droplets Impacting onto a Flat Surface

This paper presents an experimental study of the deformation of spherical liquid droplets impinging onto dry and flat surfaces making use of a CCD-camera with a high spatial resolution. The experiments consider different liquids (water and Diesel oil) and the effects of droplet velocity and diameter at the impact in a range of Weber numbers up to 1100 and Reynolds numbers up to 77400. Emphasis is put on the nature of the surface target. To consider this effect, two surface materials were used (Perspex and aluminium) with surface roughness varying from less than 5µm (considered as a smooth surface) up to Ra=66,6µm. For the range of droplet diameters considered in the experiments, the corresponding dimensionless values of Ra/Rdroplet vary from 1,5x10 -5 and 2,5x10 -2 . In a first step, the experiments focus on the spread of the liquid film and analysis of the results suggest that, provided that the Reynolds number of the droplet at the impact is large (Re>2000), the energy dissipated at the wall is not affected by the nature of the surface. The effect of surface roughness appears to be important for low Reynolds numbers, typically Re<1000. Depending upon the physical parameters at the impact, the droplet may splash at the first contact with the surface (prompt splash) or the liquid film at the wall may break-up on secondary droplets during spread. In a second step, the experiments emphasize the effect of surface roughness on the onset of splash for different liquids. The experimental results are analysed in terms of the critical Weber number for which splash occurs, and compared with correlations reported in the literature accounting for the effects of surface roughness and droplet liquid and suggest the likely influence of, not only the nature of the surface (e.g. surface profile and material), but also of the liquid.

Dynamics of droplets of biological fluids on smooth superhydrophobic surfaces under electrostatic actuation

This study describes the dynamic behaviour of droplets of biological liquids on hydrophobic surfaces under electrostatic actuation, to devise sample handling in lab-on-chip diagnostic tools. Bovine Serum Albumin (BSA) is taken as a representative biomolecule, since it is often used in adsorption studies. Green Fluorescence Protein (GFP) is also considered, given its natural fluorescence. Several effects such as sample concentration and pH are discussed. The results show negligible effects of proteins concentration in electrowetting, although increased concentrations endorse passive adsorption mechanisms, which alter the local wettability of the substrates precluding droplet motion. Bioinspired surfaces promote the largest spreading diameter, which is beneficial for droplet motion. However, surface roughness promotes energy dissipation limiting the receding droplet motion. Hence, the most effective approach is altering the surface chemistry. The coating is applied to a surface with a mean roughness smaller than 20 nm and does not alter significantly the topography, thus leading to the so-called smooth superhydrophobic surface. This coating also reduces passive proteins adsorption, as confirmed by Confocal Microscopy (CM), which is beneficial for droplet motion. Evaluating absorption spectra of protein solutions evidences an increase in protein concentration ascribed to droplet evaporation as confirmed by theoretical analysis and time resolved infrared visualization.

Droplet Impact on a Solid Surface

This chapter considers droplet-wall interaction and droplet impact and splashing on a solid surface. The discussion on droplet-wall interaction considers thermo-fluid-dynamic processes associated with droplet impact onto solid surfaces. The emphasis is put on the disintegration mechanisms as an introduction to the intricate interaction phenomena occurring at spray impingement. The analysis starts with the simplest situation of single droplet impacts onto non-heated and dry surfaces; further complexities are then introduced which consider the interaction with a liquid film and the combined effects of heat transfer. The discussion on droplet impact and splashing on a solid surface includes splashing and fragmentation of molten metal and other liquid droplets landing on a solid surface. Issues such as different types of splashing, corona splashes, freezing induced splashing are considered from an experimental point of view.

Application of bioinspired superhydrophobic surfaces in two-phase heat transfer experiments

This paper addresses the potential to use Lotus leaf bioinspired surfaces in applications involving heat transfer with phase change, namely pool boiling and spray impingement. Besides describing the role of bioinspired topographical features, using an innovative technique combining high-speed visualization and time-resolved infrared thermography, surface durability is also addressed. Water is used for pool boiling and for spray impingement systems (simplified as single droplet impact), while HFE7000 is used in a pool boiling cooler for electronic components. Results show that surface durability is quickly compromised for water pool boiling applications, as the chemical treatment does not withstand high temperatures (T > 100 °C) during long time intervals (3 h - 4 h). For HFE7000 pool boiling (depicting lower saturation temperature - 34 °C), heat transfer enhancement is governed by the topography. The regular hierarchical pattern of the bioinspired surfaces promotes the heat transfer coefficient to increase up to 22.2%, when compared to smooth surfaces, while allowing good control of the interaction mechanisms until a distance between micro-structures of 300 µm - 400 µm. Droplet impingement was studied for surface temperatures ranging between 60 °C - 100 °C. The results do not support the use of superhydrophobic surfaces for cooling applications, but reveal great potential for other applications involving droplet impact on heated surfaces (e.g. metallurgy industry).

2 phase microprocessor cooling system with controlled pool boiling of dielectrics over micro-and-nano structured Integrated Heat Spreaders

The present work addresses a microprocessor cooling technique based on pool boiling of a dielectric fluid, HFE-7000 with a compact closed loop thermosyphon, which requires no pumping or auxiliary components to operate. Aiming at modern desktop CPU cooling, the devised system is modular to infer on the optimization of several parameters influencing the system performance. The evaporator bottom surface is enhanced with micro-structured cavities to increase the liquid/solid contact area and optimize nucleation and bubble dynamics within the heterogeneous nucleation process. Optimization of surface structuring must account for several interaction mechanisms and assure that the flow near the surface maximizes the heat transfer mechanisms present in pool boiling heat transfer. This optimization is based on the minimization of steady-state overall thermal resistance of the system and on transient power conditions to control the onset of nucleate boiling and the inherent temperature overshoot upon regime transition at start-up. The condenser tilt angle is optimized as well as the effect of evaporator dimensions, orientation (horizontal and vertical positioning) and liquid fill charges. Based on the outcomes of this exploratory research, a cooling system is implemented in a working computer, cooling a modern CPU, mounted vertically.

Bubble dynamics and heat transfer for pool boiling on hydrophilic, superhydrophobic and biphilic surfaces

This paper proposes a detailed analysis of bubble dynamics to describe pool boiling heat transfer in extreme wetting scenarios (superhydrophobic vs hydrophilic). A mechanistic approach, based on extensive post-processing allows quantifying the relative advantage of the superhydrophobic surfaces to endorse the onset of boiling at very low superheats (1-2K) vs their worse heat transfer performance associated to the swift formation of an insulating vapour film. Based on this analysis, a simple biphilic surface is created. The results suggest that for high heat fluxes, bubble dynamics is dominated by the emission of very small bubbles, which seems to affect the interaction mechanisms, precluding the emission of the large bubbles from the surface, thus compromising the good performance of the biphilic surfaces.

Microfluidic Prototype of a Lab-on-Chip Device for Lung Cancer Diagnostics.

Cell sorting for disease diagnostics is often achieved by fluorescence based identification of specific markers. However, in lung cancer diagnostics, cytological analysis of pleural fluids is not always reliable and immunofluorescence essays demand for specific sample preparation. Hence, this paper addresses the development of a microfluidic device for lung cancer diagnostics which infers on the potential of a diagnosis based on analysing the cell deformability (stiffness) that alters the rheological properties and consequently the flow characteristics. Cell deformability will be induced by external actuation. Electrowetting is used to transport the samples in an open configuration system using microdroplets. Effects of the test chip configuration, sample physico-chemical properties and potential adsorption mechanisms are discussed. Wettability plays here a vital role in the sample transport and in the diagnostic method to be tested. Hence, an innovative approach is presented, the 3D Laser Scanning Fluorescence Confocal Microscopy (3D-LSCFM) to provide a detailed reconstruction of the surface topology at the liquid-solid interface region thus allowing contact angles measurement with high spatial resolution.

Design, Test and Fabrication of a Droplet based Microfluidic Device for Clinical Diagnostics

Despite the intensive research performed towards the development of biomicrofuidic devices, information on the design, test and microfabrication of the devices is scarcely reported. Following our previous work, this paper describes the design, microfabrication and test of an electrowetting chip to transport and manipulate biosamples, towards the development of a microfluidic device for cancer diagnostics. As a first approach, experiments are performed to infer on the basic chip dimensions and configuration (size and positioning of the electrodes), allowing its best performance, evaluated based on droplet dynamics (spreading/receding diameter and contact line velocity). Then, to scale down this section, for its proper integration in the device, these basic dimensions are introduced as first guess values in a numerical model, used to optimize the distance between the electrodes, the thickness of the dielectric and the electric potential and frequency to be applied.