**5. Role of In situ surface wettability for the development of microfluidic devices**

Surface wettability or wetting is the ability of the liquid to maintain contact and interact with the solid surface over which it is flowing. It results from the interaction of intermolecular forces between the molecules of liquid and molecules over the surface of the solid. Surface wettability measurement is a very critical technique to measure the flow of micro fluid in microfluidic applications. In microfluidic devices the detection of small volume change with change in fluid properties is very small because of the micrometer range. In order to detect this small change in micrometer range *in situ* wetting measurement is required. Microfluidic devices offer innumerable application in the field of science and technology. The scopes of these types of devices have been increasing for recent decades. For example, in clinical trials for drug development the amount of antibodies used is very high and cover large cost. On the other hand microfluidic devices reduce amount and cost of antibodies as well as time.

In microfluidic devices the motion of chemical reaction governed by chemo taxis gradient and this gradient is responsible for the motion of droplet. The motion of droplet generally measured with the help of wettability and wettability of droplet depends upon the surface. Wettability has a dynamic impact on the displacement of fluid inside micro fluidic device. The change in displacement of any fluid inside any microfluidic device measured in term of spreading of fluid. The spreading behavior of any flowing liquid measured with its wetting behavior and it is generally measured in term of contact angle. The magnitude of contact angle formed by micro fluid with micro-channel wall has great importance to study the characteristics of micro fluidic device.

Example: Suppose a static fluid is placed at the center of any plate and we apply taxis gradient at the two end of plate. The taxis gradient (magnetic, chemotaxis) is responsible for the displacement of fluid inside. The fluid try to spread both in linear (parallel to gradient axis) and lateral (perpendicular to direction of applied gradient) direction. The two directional spreading of liquid makes difficulty in the quantitative measurement in displaced liquid. In order to overcome this issue microfluidic devices play the important role in various scientific testing applications.

The contact angle measurement is carried out using young's equation is given in Eq. (1). The equation is derived by balancing different interfacial energy in all direction.

$$\mathbf{c}\cos\Theta = \left(\boldsymbol{\sigma}\_{\text{sv}} - \boldsymbol{\sigma}\_{\text{sl}}\right) / \boldsymbol{\sigma}\_{\text{lv}}\tag{1}$$

**157**

**Figure 3.**

*Microfluidic device for mixing different liquid.*

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication*

Wettability of fluid over the solid surface is measured in terms of contact angle (θ). The higher value of contact angle leads to lower wettability (low spreading area of displaced fluid) as shown in **Figure 1c**. The contact angle close to 0°, as droplet turns into flat puddle shows complete wetting (highest spreading area) as shown in **Figure 1e**, if angle exceeds zero but is less than 90° as shown in **Figure 1d**

In microfluidic devices the fluid displacement takes place only in linear direction because of micro channel cavity. The quantitative measurement of displaced fluid inside micro channel can be made by measuring the dimension of micro channel and displace length of fluid. The measurement of displace volume with little change in taxis gradient improves the overall sensitivity of device. Sensitivity of device is defined as the measurement of small change in the system by varying input

The existing process of antibiotic susceptibility measurement uses Petri dish coated with bacteria and divides the Petri dish into required number of segment using marker. An antibiotic dish (different concentrations) is then placed over the bacterial coated Petri dish. The petri dish is then placed over incubator for 24–48 hours. The reaction of bacteria with antibiotic takes place in petri dish and reaction takes place in radial outward direction as shown in **Figure 1a**. The measurement of reaction in radial direction is difficult to quantify in required scale. To overcome this issue a microfluidic device can be used as antibiotic susceptibility testing device. In this type of device, glass slide micro channel is coated with bacterial coating like petri dish as shown in **Figure 1b**. Different antibodies are then placed over bacterial coated micro channel for measuring the spreading of reaction

*DOI: http://dx.doi.org/10.5772/intechopen.93480*

σsl = solid/liquid interfacial energy σlv = liquid/vapor interfacial energy.

shows wetting [66].

parameter.

**5.1 Case study 1**

where θ = contact angle

σsv = solid/vapor interfacial energy

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication DOI: http://dx.doi.org/10.5772/intechopen.93480*

σsl = solid/liquid interfacial energy σlv = liquid/vapor interfacial energy.

Wettability of fluid over the solid surface is measured in terms of contact angle (θ). The higher value of contact angle leads to lower wettability (low spreading area of displaced fluid) as shown in **Figure 1c**. The contact angle close to 0°, as droplet turns into flat puddle shows complete wetting (highest spreading area) as shown in **Figure 1e**, if angle exceeds zero but is less than 90° as shown in **Figure 1d** shows wetting [66].

In microfluidic devices the fluid displacement takes place only in linear direction because of micro channel cavity. The quantitative measurement of displaced fluid inside micro channel can be made by measuring the dimension of micro channel and displace length of fluid. The measurement of displace volume with little change in taxis gradient improves the overall sensitivity of device. Sensitivity of device is defined as the measurement of small change in the system by varying input parameter.

## **5.1 Case study 1**

*21st Century Surface Science - a Handbook*

with other existing techniques.

**devices**

micro fluidic device.

for size based separation of particles including microorganisms and therefore have implications as miniature filter for analysis of water samples. Living radical photopolymerization technique using wide range of polymers was used for fabrication of these devices. Salt-leaching technique was used for placement of porous plug in the microfluidic channels. Pore size of the porous plug in this device was determined using flow field-flow fractionation. It is a new and cost efficient simple tool for water assessment [65]. Research is moving at a fast pace for development and commercialization of such paper based microfluidic devices that can be conjugated

**5. Role of In situ surface wettability for the development of microfluidic** 

Surface wettability or wetting is the ability of the liquid to maintain contact and interact with the solid surface over which it is flowing. It results from the interaction of intermolecular forces between the molecules of liquid and molecules over the surface of the solid. Surface wettability measurement is a very critical technique to measure the flow of micro fluid in microfluidic applications. In microfluidic devices the detection of small volume change with change in fluid properties is very small because of the micrometer range. In order to detect this small change in micrometer range *in situ* wetting measurement is required. Microfluidic devices offer innumerable application in the field of science and technology. The scopes of these types of devices have been increasing for recent decades. For example, in clinical trials for drug development the amount of antibodies used is very high and cover large cost. On the other hand

In microfluidic devices the motion of chemical reaction governed by chemo taxis gradient and this gradient is responsible for the motion of droplet. The motion of droplet generally measured with the help of wettability and wettability of droplet depends upon the surface. Wettability has a dynamic impact on the displacement of fluid inside micro fluidic device. The change in displacement of any fluid inside any microfluidic device measured in term of spreading of fluid. The spreading behavior of any flowing liquid measured with its wetting behavior and it is generally measured in term of contact angle. The magnitude of contact angle formed by micro fluid with micro-channel wall has great importance to study the characteristics of

Example: Suppose a static fluid is placed at the center of any plate and we apply taxis gradient at the two end of plate. The taxis gradient (magnetic, chemotaxis) is responsible for the displacement of fluid inside. The fluid try to spread both in linear (parallel to gradient axis) and lateral (perpendicular to direction of applied gradient) direction. The two directional spreading of liquid makes difficulty in the quantitative measurement in displaced liquid. In order to overcome this issue microfluidic devices play the important role in various scientific testing applications. The contact angle measurement is carried out using young's equation is given in Eq. (1). The equation is derived by balancing different interfacial energy in all

cos θ= σ −σ σ ( sv sl lv )/ (1)

microfluidic devices reduce amount and cost of antibodies as well as time.

**156**

direction.

where θ = contact angle

σsv = solid/vapor interfacial energy

The existing process of antibiotic susceptibility measurement uses Petri dish coated with bacteria and divides the Petri dish into required number of segment using marker. An antibiotic dish (different concentrations) is then placed over the bacterial coated Petri dish. The petri dish is then placed over incubator for 24–48 hours. The reaction of bacteria with antibiotic takes place in petri dish and reaction takes place in radial outward direction as shown in **Figure 1a**. The measurement of reaction in radial direction is difficult to quantify in required scale. To overcome this issue a microfluidic device can be used as antibiotic susceptibility testing device. In this type of device, glass slide micro channel is coated with bacterial coating like petri dish as shown in **Figure 1b**. Different antibodies are then placed over bacterial coated micro channel for measuring the spreading of reaction

**Figure 3.** *Microfluidic device for mixing different liquid.*

due to chemo taxis in one direction and chemo taxis spreading phenomenon can be quantify using microfluidic chip time lapse microscopy as shown in **Figure 1d**. The spreading of reaction is then measured by the dimension of micro channel as shown in **Figure 1c**.

#### **5.2 Case study 2**

In this study, microfluidic device for mixing three liquid is used. In this device, three different liquid A, B and C is used to mix in different concentration and their mixing reaction is measured with the range of output mixing micro channel as shown in **Figure 3**. In this type of device the change in output parameter can be detect significantly my using small volume of liquid droplet. These devices are very useful to measure mixing behavior of two or more liquid for various chemical mixing applications.

## **6. Factor affecting wettability**

The wettability is generally are properties of displaced liquid measured in term of contact angle. The surface morphology, material impurity and porosity are the properties which affect the wettability.

Effect of surface roughness: All smother surfaces look rough in microscopic level. The rough surface of solid specimen affects the wettability of liquid over the solid surface. The contact angle formed with flat surface is called apparent contact angle θa and it is consider by considering ideal surface condition. The actual contact angle θA is generally higher than that of apparent contact angle θa as shown in **Figure 4a,b**. To calculate real surface free energies of liquid actual contact angle is used. Generally hydrophilic surface is considered to be the best surface where lower

**159**

**Figure 5.**

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication*

value of contact angle is obtain. The wettability of liquid surface generally increases as we decrease the surface roughness of solid surfaces. The relation between roughness and wettability was explained by Wenzel and stated that if the surface is chemically hydrophobic it will become more hydrophobic when surface roughness

R is the surface ratio between actual and projected area of solid surface over which fluid is flowing. For smother surface R = 1 and apparent contact angle becomes equal to actual contact angle. Other than surface roughness, impurity and

In microfluidic device, the displacement of fluid takes place continuously, and it is very difficult to measure wettability (contact angle formed by moving fluid with the wall of micro channel). The Sessile drop method and image analysis techniques are the method used only for measuring the static contact angle of liquid in micro channel device. For biomedical and clinical application the chemotaxis reaction takes place continuously and required continuous monitoring of contact angle *in situ* image capturing system is used. In this technique the position of chemical reaction captured with the help of microscope and high speed camera installed over the viewing point of microscope [67]. The camera records the position of reaction in different time interval and measured the contact angle and contact angle is further used to measure interfacial energy of fluid in microfluidic devices. The schematic of *in situ* image capturing system is

**7.** *In situ* **wettability measurement in microfluidic devices**

*In situ image capturing system for measuring wettability of microfluid devices.*

cos R cos (θ= θ A a ) ( ) (2)

*DOI: http://dx.doi.org/10.5772/intechopen.93480*

porosity in solid surface effect wettability.

is added. According to Wenzel,

as shown in **Figure 5**.

**Figure 4.** *Effect of surface roughness on the wettability of fluid.*

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication DOI: http://dx.doi.org/10.5772/intechopen.93480*

value of contact angle is obtain. The wettability of liquid surface generally increases as we decrease the surface roughness of solid surfaces. The relation between roughness and wettability was explained by Wenzel and stated that if the surface is chemically hydrophobic it will become more hydrophobic when surface roughness is added. According to Wenzel,

$$\cos\left(\theta\_{\Lambda}\right) = \mathbb{R}\cos\left(\theta\_{\mathfrak{a}}\right) \tag{2}$$

R is the surface ratio between actual and projected area of solid surface over which fluid is flowing. For smother surface R = 1 and apparent contact angle becomes equal to actual contact angle. Other than surface roughness, impurity and porosity in solid surface effect wettability.

### **7.** *In situ* **wettability measurement in microfluidic devices**

In microfluidic device, the displacement of fluid takes place continuously, and it is very difficult to measure wettability (contact angle formed by moving fluid with the wall of micro channel). The Sessile drop method and image analysis techniques are the method used only for measuring the static contact angle of liquid in micro channel device. For biomedical and clinical application the chemotaxis reaction takes place continuously and required continuous monitoring of contact angle *in situ* image capturing system is used. In this technique the position of chemical reaction captured with the help of microscope and high speed camera installed over the viewing point of microscope [67]. The camera records the position of reaction in different time interval and measured the contact angle and contact angle is further used to measure interfacial energy of fluid in microfluidic devices. The schematic of *in situ* image capturing system is as shown in **Figure 5**.

**Figure 5.**

*In situ image capturing system for measuring wettability of microfluid devices.*

*21st Century Surface Science - a Handbook*

shown in **Figure 1c**.

mixing applications.

**6. Factor affecting wettability**

properties which affect the wettability.

**5.2 Case study 2**

due to chemo taxis in one direction and chemo taxis spreading phenomenon can be quantify using microfluidic chip time lapse microscopy as shown in **Figure 1d**. The spreading of reaction is then measured by the dimension of micro channel as

In this study, microfluidic device for mixing three liquid is used. In this device,

The wettability is generally are properties of displaced liquid measured in term of contact angle. The surface morphology, material impurity and porosity are the

Effect of surface roughness: All smother surfaces look rough in microscopic level. The rough surface of solid specimen affects the wettability of liquid over the solid surface. The contact angle formed with flat surface is called apparent contact angle θa and it is consider by considering ideal surface condition. The actual contact angle θA is generally higher than that of apparent contact angle θa as shown in **Figure 4a,b**. To calculate real surface free energies of liquid actual contact angle is used. Generally hydrophilic surface is considered to be the best surface where lower

three different liquid A, B and C is used to mix in different concentration and their mixing reaction is measured with the range of output mixing micro channel as shown in **Figure 3**. In this type of device the change in output parameter can be detect significantly my using small volume of liquid droplet. These devices are very useful to measure mixing behavior of two or more liquid for various chemical

**158**

**Figure 4.**

*Effect of surface roughness on the wettability of fluid.*

In this system a microscope is just place at the top of microfluidic device and it captures the motion of chemical reaction change in micro channel. A light source is applied from the side to capture the video with more celerity with the help of high speed camera and store video into computer. The video is than sliced into image in required time interval as shown in lower left corner of figure. The Enlarge version of captured screen is shown in lower right corner of **Figure 5** which shows the measurement of contact angle variation at different time interval.
