RICERCA ITALIANA     

Technological Applications of Microfluidics

Università degli Studi di Parma

Abstract

The goal of the proposed research project is the analysis of transport phenomena in single and two-phase flows in microducts having a hydraulic diameter less than 4 mm, with the aim to obtain correlations able to give the designers the tools required for the correct choice of dimensions and configurations of microdevices. The investigation shall be carried out both theoretically and experimentally, with the intent of exploring how the transport phenomena are modified by scaling effects, which are typical of the reduced dimensions, and by the effects which induce a new modelization of the balance equations for continuum flow, and/or of the boundary conditions (micro-effects).
The investigation shall aim at thoroughly investigating the role played by the single scaling effects and micro effects, on the pressure drop and the convective heat transfer coefficient. The project shall deal with the analysis of single-phase flows (liquid and gaseous) and two-phase flows (flow boiling and condensation). The working fluids shall be chosen bearing in mind possible technological applications of microfluidics in the thermal control of electronic components (refrigerants such as HCF, FC72, R404a, CO2, air, N2) and in the development of "lab-on-a-chip" (H2O). Table 1 shows the program assigned to the different research Units. One can observe that single phase flows will be analysed both theoretically and experimentally. The study of two-phase flows shall be carried out with experimental activities and data analysis. Indeed results and models developed for single-phase flows will be useful also for the determination of pressure drop and heat transfer coefficient in condensation and boiling. In fact, often the two-phase models refer to single phase flows, introducing corrective factors in those correlations.

Table 1

Table 1 shows that the research fields of the Research Units involved in this project are complementary. This aspect can be better understood if one considers how the scaling and micro effects have been distributed among the Units (see Table 2).
The main scaling effects that shall be investigated during the Research Program are the following:
- Viscous dissipation effects,
- Conjugate heat trasfer (wall-fluid) effects,
- Compressibility effects,
- Liquid-vapour interactions.

The main micro-effects that shall be investigated are the following:
- Rarefaction effects,
- Electro-osmotic effects.

The analysis of these effects has been subdivided among the Research Units with the aim to couple the experimental and the theoretical aspects of each effect (see Table 2). As it is possible to note by observing Table 2, there exist common research fields among the Research Units to improve the interactions and the exchange of scientific knowledge and of numerical and experimental data.
Table 2

The research activities are complementary: the overlaps will improve the exchange of data, especially between experimental Units and theoretical ones.
The present project is the natural development of the previous PRIN03 project; also this Project involves the support of the Microfluidics Laboratory of ENEA (Casaccia). <<<

Principal Investigator

Marco SPIGA Università degli Studi di PARMA

Research Objectives

Microfluidics, involving fluid-dynamics and heat transfer in microchannels, is a paramount topic for scientists and technicians. This is proved by the many papers published in this area, by the birth of several new International Journals (since Microscale Thermophysical Engineering in 1998, to Microfluids & Nanofluids in 2004), by several International Conferences (since Int. Conference on Heat Transfer and Transport Phenomena in Microscale, Banff 2000, to the next third edition of Int. Conference on Microchannels and Minichannels which will be held in Toronto, in June 2005).
In this last decade, the papers appeared in literature highlights how the classical correlations used in macro-scale to evaluate the pressure drops and the convective heat transfer coefficients can predict values well different from those obtained experimentally. The reason for these discrepancies can be explained, because the miniaturization is responsible for scale-effects which imply the correction of classical models or the proposal of new models to represent the fluid-mechanics and heat transfer in isothermal and non-isothermal flows. The continuum flow theory, based on the Navier-Stokes and energy equations with no-slip boundary conditions for velocity and temperature, must be revised.
At the moment it is not yet clear how and when the micro scale-effcts and the micro-effects play a significant role on the transport phenomena.
For this reason, the basic objective of this Project is the theoretical and experimental investigation of the micro scale effects, aiming at proposing new correlations useful for the microfluidics design.
The main scale effects analysed in this Project are the following:
•viscous dissipation;
•conjugate heat transfer (fluid-wall);
•fluid comprimibility;
•interfacial stresses in condensation and boiling;

The main micro-effects are the following:
•rarefaction,
•electro-osmotic flows and Electric Double Layer.

The investigation of these effects has been subdivided and shared among the different Research Units, trying to link the theoretical analysis with the experimental activity.
In detail, the aims of the Project can be summarized as follows.

1.Analysis of the importance of the scale-effects and micro-effects on pressure drop and heat transfer coefficient for single-phase (liquid and gas) and two-phase (condensation and boiling) flow.

2.Assessment of a database containing all the obtained experimental results. It will be integrated by the already available data referred to macro-scale. This database will constitute a common reference point for all the Research Units, it will be useful to enhance cooperation between different Units working on shared topics.

3.Set up of prototype for the cooling of electronic components, based on microchannels, and designed according to the results of the present Project.

4.Strengthen of the Research Laboratories, whose activity is aimed at microfluidics (test sections devoted to experiments on microcomponents and/or numerical simulations) recently born at the University of Bologna (Microfluidics Lab), at the University of Bergamo (Robotics and Microdevices Lab), at the University of Padova.

5.Personnel to be hired, among young scientists and Ph. Doctors. In this frame, 6 work contracts (UniPd, UniUd, UniBg, UniPd, UniPr, UniBg) have been proposed. <<<

Timescale

24 months

National and international background

Fluid dynamical and thermal analysis of microchannels is a subject of remarkable scientific and technological interest, as testified by several works being published on the most renowned journals in this field or presented at conferences dedicated to the subject in the last years (from the held in Banff in the year 2000 to the oncoming third edition of ASME's Int. Conference on Microchannels and Minichannels which shall be held in Toronto in June 2005) and by the foundation of periodicals dedicated to the subject (from Microscale Thermophysical Engineering to Micro- and Nanofluidics)
Most of the research in this field is of the applicative kind and is oriented to the fabrication of microcomponents with well-defined functions. The enormous progress that surface microfabrication and micromachining techniques enjoyed in recent years allowed a proliferation of technical applications of microcomponents using or circulating fluids (Micro Flow Devices, MFDs). MFDs allow a vast range of tasks: from microcomponents devised for elementary operations such as transport or dosing of a fluid up to complex MFDs that are miniaturised chemical labs (lab-on-a-chip) where mixing and synthesis of fluid and fluid mixtures – sometimes quite complex in nature – take place. One of the main difficulties in engineering these components lies in the control of flow rates, of the volumes of the fluids to handle and of the temperature. In order to successfully design such components it is thus necessary to study how transport phenomena change when the characteristic geometric dimension of the flow shrinks to the microscale. Several theoretical and experimental investigations aimed at the study of gaseous and liquid flows within conduits having characteristic dimensions smaller than one millimetre proved how the technologies successfully employed at the common scale become ineffective or even useless when applied at the microscale. A detailed analysis and comparison of the experimental results published in the open literature was conducted by Morini [1] in the framework of PRIN03, named "Dynamical and thermal analysis of single-phase and two-phase flows in microconduits". This project is the national scientific starting point of this new project, the research project proposed here being its natural continuation.
In the framework of the past project it was possible to set up some University Labs dedicated to the experimental study of microfluidics (DIENCA's Microfluidics Lab at Bologna University and the Lab for Robotics and Microdevices at Bergamo University), to construct dedicated experimental test rigs (such as that for the study of condensation in microtubes at Padua University), to create numerical and analytical models (UniPd, UniPr) for the analysis of transport phenomena in microconduits. The results obtained are testified by the works published by the Research Units [1-22]. The main results achieved by the Units partaking in this project pertain convective heat transfer in single-phase (liquid and gaseous) flows within metallic and silicon microchannels [1-6], the study of the no-slip condition at the walls for gas flows in microconduits [7-11], the effect of electro-osmotic interaction between fluid and wall in silicon conduits [12], the design of new micro-heat sinks [13-16], the effects of wall roughness on pressure drop and heat transfer coefficients [17-18], the study of condensation of refrigerants within minichannels and microchannels [19-22]. These works highlighted how – in some cases – the classical correlations used to calculate pressure drop and heat transfer coefficients in conduits of ordinary dimensions may give results that differ from those obtained experimentally. The reason of this disagreement between theoretical predictions and experimental results can be explained by recognising that miniaturisation of the components introduces some "scaling effects" and true "micro-effects" which make a reformulation of the classical study of isothermal and non-isothermal flows (Navier-Stokes and energy equations, no-slip and temperature continuity at the wall boundary conditions) mandatory.
"Scaling effects" are those effects which may be neglected at the reference geometrical scale, but which become important when the scale changes [23-24]. As an example, the reduction of the hydraulic diameter of a channel implies a larger reduction in its volume than in the corresponding surface, so that the area-to-volume ratio tends to be very high in microfluidic systems. This implies a prevalence of surface forces over body forces, which renders the advective terms prevailing over those related to the volume forces in the conservation equations. Owing to this reason, the behaviour of microflows can be completely different from that of flows in ducts of ordinary dimensions. Another example is the evolution of the flow regimes for boiling or condensing fluid within microtubes: the prevalence of interface (capillary) forces over body forces changes dramatically the sequence of flow regimes and cancels the phase of liquid flow stratification.
In the open literature a number of scaling effects having a dominating role in transport phenomena for single-phase flows at the microscale have been determined, namely:
(i) Conjugate wall-fluid heat transfer effects [25-27]
(ii) Effects related to the microchannels' surface roughness [28-29]
(iii) Effect of viscous dissipation [30-32]
(iv) Effects due to axial conduction [33-35]
(v) Effects related to compressibility [36-38]
(vi) Effects of interface tension (in boiling and condensation) [39-42]

"Micro-effects" are those which determine a reformulation of the equations of thermal fluid dynamics and/or associated boundary conditions as the characteristic scale is reduced. To this category belong:
(i) Effects due to rarefaction [43-45];
(ii) Electro-osmotic effects (EDL) [46-48].
The present project aims at studying each one of the above-mentioned effects, so as to understand how microscale transport phenomena are influenced by them.
As can be noticed by reading papers [1-22], some of these effects have been studied by the various Research Units during the project PRIN03. The proposal to continue the investigation on some effects also in this project stems indeed from the need to continue along those research paths which showed the need for further experimentation and theoretical investigation.
The present project is thus seen as the natural continuation of the PRIN03 project, which gave the partaking units very satisfactory scientific acknowledgments and achievements and gave birth to several activities shared by the research groups.
The work done in the last years by the different Research Units indeed suggested in which direction to deepen the research and which new research paths to undertake. This led to a broadening of the list of the scale and micro effects which are to be investigated in this new project.
The investigation proposed shall be both theoretical and experimental and shall be focused on the establishment of correlations useful for the thermal and fluid dynamical design of microfluidic components. <<<