5. Some applications

The purpose of this section is to introduce two key example applications of thermal hydrodynamic instabilities in viscoelastic fluids. It is important to mention that these applications were selected because thermal convection is at the core of its working principles. In other words, proper understanding of the hydrodynamic stability in the convection of viscoelastic fluids would improve those applications.

A remarkable fact of the applications described here is that these are placed at the frontier of at least two research topics. The fabrication of corrugated surfaces involved knowledge of hydrodynamics, rheology, heat transfer, optics and polymer chemistry. On the other hand, appliances for DNA replication may need knowledge of genetics, hydrodynamics, heat transfer and rheology.

Steps 4 to 6 in the above are tightly related to thermal convection. As the viscoelastic fluid is heated from below the critical conditions for the onset of convection may be achieved or not. It all depend on the control of the heating source, on the plate thermal and geometrical properties, on the fluid properties [9] and on the desired pattern. This is not an easy task and may resemble a craft work. However very good improvements on this technique has been made so

Figure 3. Brief schematics for the fabrication of corrugated surfaces based on thermal convection. Notice that at the beginning Rayleigh convection dominates the process but at certain fluid layer thickness the Marangoni convection rules the process until the solvent evaporates completely. (a) The suspension is at rest (b) Convective pattern are formed in the fluid layer (c) The solvent starts evaporating and the patterns reduce its size (d) The formed patterns sediment on the

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Several researchers have made advances on this matter which become hot topic with the paper by Nie and Kumacheva [2]. In fact, in 1998 Mitov and Kumacheva [17] reported instability induced patterning as printing technique for the fabrication of corrugated surfaces. More recently, other researchers have made findings on how the theory of Marangoni convection is

Further work is needed for the improvement of the experimental technique. For example, it is known that thickness of the fluid layer decreases in time so that both Rayleigh and Marangoni convection may appear in the fluid. Also, as the fluid layer decreases may be the viscoelastic properties may change and in turn modify the formed patterns. In the literature can be found a few papers dealing with the evaporation rate. On the other hand, thermal control should be robust in order that the fluid layer may sustain the formed patterns. Also, several authors have reported new findings for the proportional and proportional integral control of Rayleigh convection. Although more isolated advances can be done in different fields interesting inte-

that good quality can be achieved.

dried surface.

coupled with the deposition of regular patterns on a surface.

grating work remains for this type of applications.

#### 5.1. Corrugated surfaces

In optics and some electronic appliances [1, 2, 23], it is necessary to print regular patterns over a surface. In fact, there are already technologies that print the mentioned patterns. However, redundant techniques are important for a number of reasons: for reduction of costs and production time, for special cases for which traditional techniques do not work, etc. [1, 2] (Figure 3).

The pattern printed over a surface is in fact a corrugated surface. Precisely, natural convection in thin films of viscoelastic suspensions is used to produced this type of corrugated surfaces. The fabrication process of these corrugated surfaces briefly includes


Applications of Viscoelastic Fluids Involving Hydrodynamic Stability and Heat Transfer http://dx.doi.org/10.5772/intechopen.76122 37

The theoretical results for Ra, k and ω are linked to the experimental observations as follows. The critical value of the Rayleigh number predicts the critical temperature difference at which the convective motions set and the critical value of the wavenumber indicates the number of formed convective cells. Then the theoretical results help for tuning the lab experiments and to

At this point both, theory and experiments are connected for the formation of convective patterns in viscoelastic fluids. As explain in the next section, this background is the foundation

The purpose of this section is to introduce two key example applications of thermal hydrodynamic instabilities in viscoelastic fluids. It is important to mention that these applications were selected because thermal convection is at the core of its working principles. In other words, proper understanding of the hydrodynamic stability in the convection of viscoelastic fluids

A remarkable fact of the applications described here is that these are placed at the frontier of at least two research topics. The fabrication of corrugated surfaces involved knowledge of hydrodynamics, rheology, heat transfer, optics and polymer chemistry. On the other hand, appliances for DNA replication may need knowledge of genetics, hydrodynamics, heat transfer and rheology.

In optics and some electronic appliances [1, 2, 23], it is necessary to print regular patterns over a surface. In fact, there are already technologies that print the mentioned patterns. However, redundant techniques are important for a number of reasons: for reduction of costs and production time, for special cases for which traditional techniques do not work, etc. [1, 2] (Figure 3).

The pattern printed over a surface is in fact a corrugated surface. Precisely, natural convection in thin films of viscoelastic suspensions is used to produced this type of corrugated surfaces.

4. thermal control of the convection process in order to achieve and maintain the critical

The fabrication process of these corrugated surfaces briefly includes

5. formation of the desired pattern in the polymeric suspension,

6. evaporation of the solvent and deposition of the polymeric pattern.

1. preparation of the polymeric suspension,

3. heating from below of the plate

conditions,

2. application of the suspension in a plate surface,

control the formation of patterns in the fluid.

that makes it possible.

36 Polymer Rheology

5. Some applications

5.1. Corrugated surfaces

would improve those applications.

Figure 3. Brief schematics for the fabrication of corrugated surfaces based on thermal convection. Notice that at the beginning Rayleigh convection dominates the process but at certain fluid layer thickness the Marangoni convection rules the process until the solvent evaporates completely. (a) The suspension is at rest (b) Convective pattern are formed in the fluid layer (c) The solvent starts evaporating and the patterns reduce its size (d) The formed patterns sediment on the dried surface.

Steps 4 to 6 in the above are tightly related to thermal convection. As the viscoelastic fluid is heated from below the critical conditions for the onset of convection may be achieved or not. It all depend on the control of the heating source, on the plate thermal and geometrical properties, on the fluid properties [9] and on the desired pattern. This is not an easy task and may resemble a craft work. However very good improvements on this technique has been made so that good quality can be achieved.

Several researchers have made advances on this matter which become hot topic with the paper by Nie and Kumacheva [2]. In fact, in 1998 Mitov and Kumacheva [17] reported instability induced patterning as printing technique for the fabrication of corrugated surfaces. More recently, other researchers have made findings on how the theory of Marangoni convection is coupled with the deposition of regular patterns on a surface.

Further work is needed for the improvement of the experimental technique. For example, it is known that thickness of the fluid layer decreases in time so that both Rayleigh and Marangoni convection may appear in the fluid. Also, as the fluid layer decreases may be the viscoelastic properties may change and in turn modify the formed patterns. In the literature can be found a few papers dealing with the evaporation rate. On the other hand, thermal control should be robust in order that the fluid layer may sustain the formed patterns. Also, several authors have reported new findings for the proportional and proportional integral control of Rayleigh convection. Although more isolated advances can be done in different fields interesting integrating work remains for this type of applications.

#### 5.2. DNA replication

DNA technologies have attracted special attention, first because DNA suspensions are considered viscoelastic fluids because these are a combination of an aqueous solvent and a kind of polymer in the form of DNA chains. Second, hydrodynamics of biological fluids has become a subject of growing interest in recent decades because of its link to a biochemical reaction. For short, DNA replication needs a temperature gradient and an enzyme to be done so that possibly convective motions are involved. DNA replication is common task for geneticists and biotechnologists which also is an important tool for research and development in biotechnology, medicine, molecular biology and other important subjects.

3. a thermal gradient is set vertically in the vial,

important since these are based on the Rayleigh convection.

can be concentrated in suitable spot for further recovery.

place,

5. DNA replication.

6. Discussion

several decades.

7. Conclusions

applications for most humans.

4. a temperature of 95C is maintain in order that the polymerase chain reaction (PCR) takes

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The above given steps lack of some biochemical details but the aim is to show that a physical phenomenon involving thermal gradients and fluid motions is implied. As before, thermal convection is at the core of this appliance and it has been demonstrated the feasibility of cheaper and not too complicated thermocyclers [26]. The lab experiments conducted by Muddu et al. [26] with a very simple cycler prototype based on thermal convection are very

DNA replication is not the only application involving Rayleigh convection. Some other uses in molecular biology have been found like trapping of DNA with help of thermophoresis or the Soret effect [24]. The Soret effect and the Rayleigh and/or Marangoni convection are related, and for short it is mass transfer assisted by heat transfer and vice versa. This is, DNA material

Some applications mainly based on Rayleigh and Marangoni thermal convection were briefly presented. These technologies were developed by researchers working on the frontier of various subjects which, from the authors point of view, is the perfect spot for human advances. Several complicated theories have a similar way finding unexpected common technological

Thermal convection in its general sense has found practical applications in a number of other fields. For example, geology and volcanology use the theory of Rayleigh convection in fluids confined between insulating walls and many industrial equipments for heat transfer use the theory of convection too. On the other hand and to the best knowledge of the authors there are very few new technologies using Rayleigh or Marangoni convection as the ones presented here. It is remarkable that the physical mechanisms of these phenomena are at the core of the above presented technologies and what makes them work is the vast results reported since

Hopefully, researchers working on interdisciplinary projects could take advantage of the large

Here, an interesting connection between rheology and thermal hydrodynamics is presented through some applications. The theoretical approach is linked to an experimental setup by using dimensionless parameters like the Rayleigh number and the wavenumber. Already

amount of reported results on Rayleigh and Marangoni convection.

In DNA replication a very expensive apparatus is involved. The thermocycler is small specialized piece of equipment that has become an indispensable tool for research laboratories for industry and academy. However, thermocyclers work is based on the physical mechanism of thermal convection. This is, the equipment creates a temperature gradient by heating from below small tubes and mixes the fluid by producing soft horizontal motions. Several years ago the process followed by the thermocycler was identified as thermal convection in a vertical cylinder [24] and a number of more simple and cheaper thermocyclers were prototyped [25].

The complicated DNA replication process can now be very briefly outlined. The steps given below should be taken as a general picture and the reader is encouraged to search specialized literature for more details on this matter. Figure 4 shows a very simplified scheme of the thermoconvective motions that take place in this type of biological viscoelastic fluids. Thus, the process is as follows


Figure 4. Brief schematics for thermal convection in a DNA suspension confined in a vertical cylinder. Notice that convection cannot be set if a proper critical Rayleigh number is not achieved. (a) DNA suspension at rest (b) Convective motions in the DNA suspension.


5.2. DNA replication

38 Polymer Rheology

the process is as follows

motions in the DNA suspension.

DNA technologies have attracted special attention, first because DNA suspensions are considered viscoelastic fluids because these are a combination of an aqueous solvent and a kind of polymer in the form of DNA chains. Second, hydrodynamics of biological fluids has become a subject of growing interest in recent decades because of its link to a biochemical reaction. For short, DNA replication needs a temperature gradient and an enzyme to be done so that possibly convective motions are involved. DNA replication is common task for geneticists and biotechnologists which also is an important tool for research and development in biotech-

In DNA replication a very expensive apparatus is involved. The thermocycler is small specialized piece of equipment that has become an indispensable tool for research laboratories for industry and academy. However, thermocyclers work is based on the physical mechanism of thermal convection. This is, the equipment creates a temperature gradient by heating from below small tubes and mixes the fluid by producing soft horizontal motions. Several years ago the process followed by the thermocycler was identified as thermal convection in a vertical cylinder [24] and a number of more simple and cheaper thermocyclers were prototyped [25]. The complicated DNA replication process can now be very briefly outlined. The steps given below should be taken as a general picture and the reader is encouraged to search specialized literature for more details on this matter. Figure 4 shows a very simplified scheme of the thermoconvective motions that take place in this type of biological viscoelastic fluids. Thus,

1. an aqueous dilute suspension of a DNA fragments and the polymerase enzyme is prepared,

Figure 4. Brief schematics for thermal convection in a DNA suspension confined in a vertical cylinder. Notice that convection cannot be set if a proper critical Rayleigh number is not achieved. (a) DNA suspension at rest (b) Convective

2. the suspension is placed in nearly cylindrical containers called vials,

nology, medicine, molecular biology and other important subjects.

The above given steps lack of some biochemical details but the aim is to show that a physical phenomenon involving thermal gradients and fluid motions is implied. As before, thermal convection is at the core of this appliance and it has been demonstrated the feasibility of cheaper and not too complicated thermocyclers [26]. The lab experiments conducted by Muddu et al. [26] with a very simple cycler prototype based on thermal convection are very important since these are based on the Rayleigh convection.

DNA replication is not the only application involving Rayleigh convection. Some other uses in molecular biology have been found like trapping of DNA with help of thermophoresis or the Soret effect [24]. The Soret effect and the Rayleigh and/or Marangoni convection are related, and for short it is mass transfer assisted by heat transfer and vice versa. This is, DNA material can be concentrated in suitable spot for further recovery.
