**6.1. Fundamentals of THM**

Thermophilic particles diffused to the region with higher temperature, while thermophobic particles move to the opposite direction, as shown in **Figure 4a**.

Liquid flow density, *J*, driven by thermophoretic field, is governed by

$$J = D\left[\nabla \mathbf{C} + \mathbf{S}\_\mathbf{r} \mathbf{C} (\mathbf{1} - \mathbf{C}) \nabla T\right] \tag{10}$$

where *D* is the diffusion coefficient, *C* is the concentration, *T* is the temperature, and *ST* is the Soret coefficient, defined as the ratio of thermal diffusion coefficient, *DT*, over diffusion coefficient, *D*, which is given by

$$\mathbf{S}\_r = \frac{D\_r}{D} \tag{11}$$

Steady-state concentration changes for a given spatial temperature difference, *ΔT*, which is given by

$$\frac{C\_{\text{het}}}{C\_{\text{cold}}} = \exp\{-S\_{\text{r}}\Delta T\} \tag{12}$$

where *Chot* is the molecule concentration in the hot area, while *Ccold* is in the cold area.

Studies prove that a temperature difference between 2 and 8 K in the beam center with a 1/*e*<sup>2</sup> diameter of 25 μm managed to induce thermophoretic motion, while the "1/*e*<sup>2</sup> diameter" (*e* = 2.71828) indicates the beam diameter where intensity drops to 13.5% of the maximum value. The temperature rise of the suspending medium must be kept low to avoid bioparticle damage, such as in the case of DNA, which is from 23 to 31°C [47].

the thermophoretic behavior of the monomeric, oligomeric, and fibrillary forms of the protein. An infrared laser is focused inside a capillary borosilicate where homogenous protein suspension is placed, to generate localized heating, resulting in the motion of the molecules along the

Biological Particle Control and Separation using Active Forces in Microfluidic Environments

http://dx.doi.org/10.5772/intechopen.75714

95

Optical tweezing or trapping (OPT) indicates the manipulation of particles using optical forces, referring to the exploitation of light radiation pressure to displace and demobilize

Emission of light by a light source induces scattering and gradient forces, which affect particle in the light propagation axis. Scattering force, *Fscat*, affects in the direction of propagation, pushing the particle away from the light source. Gradient force, *Fgrad*, on the other hand, affects the direction of the optical field gradient, attracting the particle to the region with peak spatial

Optical tweezing or trapping depends on the dimension range of the particle under manipulation, which is governed by two physical principles, i.e., (1) *Mie* scattering and (2) *Rayleigh* scattering [12]. *Mie* regime condition governs for the condition where the particle dimension range is greater than the wavelength of light (*d >> λ*) which can be explained by ray optics. Cell-type bioparticle (micron-sized) manipulation lies on this regime. Rays of light carry momentum and

**Figure 5.** (a) Optical tweezing or trapping. (Left) Scattering force, *Fscat*, pushes bioparticle away from the light source following the direction of light propagation. Gradient force, *Fgrad*, attracts bioparticle to the spatial light intensity peak according to the direction of the optical field gradient. (Right) Bioparticle is trapped at the highest intensity region, i.e., light focus, as emission of light through a high numerical aperture number lens generates *Fgrad* > *Fscat* condition (Reprinted with permission from Md Ali et al. [13]. Copyright 2016 Royal Society of Chemistry). (b) Manipulation of protein by optical tweezing or trapping: (1) Double-nanohole optical tweezer setup. (2) Details of manipulation of microfluidic device construct. Proteins are trapped at the focus of optical trap at the vicinity of the gold nanohole surface while at the same time unfolding between the double nanoholes. (3) SEM image of the double nanohole (ODF, optical density filter; HWP, half-wave plate; BE, beam expander; MR, mirror; MO, microscope objective; OI MO, oil immersion microscope objective; APD, avalanche photodiode) (Reprinted with permission from Pang and Gordon [52]. Copyright

temperature gradient until the steady state is established.

**7. Optical tweezing/trapping**

light intensity [11] as shown in **Figure 5a**.

target particles [11, 12].

**7.1. Fundamentals of OPT**

2011 American Chemical Society).

#### **6.2. Thermophoretic manipulation of bioparticles**

Thermophoretic manipulation of yeast cells has been demonstrated by Lin et al. [46] using low-power and flexible all-optical manipulation method, which presented in **Figure 4b**. They generate light-controlled temperature gradient field thus to trap the suspended cells due to permittivity gradient in the electric double layer of the cell membrane-charged surface. In fact, they manage to realize arbitrary spatial arrangement, as well as precise rotation of single-cell assemblies, with resolution down to 100 nm. J. Chen et al. [48] demonstrate thermophoretic manipulation of *E. coli* by inducing thermal gradient by microscale electric thermal heater. The electric thermal heater is fabricated by gold thin film by means of direct writing with a femtosecond laser. A thin SiO<sup>2</sup> is coated over the gold thin film to electrically isolate the sample as well as to ensure that the trapping is entirely due to thermal effect. Osterman et al. [49] use microscale thermophoresis to study all potential intraviral protein-protein interactions of hepatitis E virus in order to understand the viral replication cycle. The thermal gradient is generated using 100 V-powered red LED for 25 s radiation. Remarkable thermophoretic manipulation of nucleic acids has been demonstrated by He et al. [50], in which translocation of DNA through nanopore utilizing cross-pore thermal gradient has been achieved. Heating of *cis* chamber is performed by exterior heater to maintain the environment at the melting temperature of the double-strand DNA to transform into single-strand DNA. Extensive temperature drop across the pore is caused by thermal-insulating membrane that separate the *cis* and *trans* chambers, resulting in the thermophoretic translocation of the DNA from *cis* to *trans* chamber. Wolff et al. [51] perform microscale thermophoresis upon different forms of the protein α-synuclein, which is associated with Parkinson's disease, to quantitatively characterize

**Figure 4.** (a) Thermophoresis. Thermophilic bioparticles A (purple) diffuse to regions with higher temperature, while thermophobic bioparticles B (yellow) diffuse to lower temperature regions (Reprinted with permission from Md Ali et al. [13]. Copyright 2016 Royal Society of Chemistry). (b) Manipulation of model organisms by thermophoresis. Optothermal thermophoretic tweezer manipulation is performed by the following: (1) Laser is directed to the digital micromirror device (DMD). (2) Resultant image is focused on gold nanoparticle (AuNP) substrate for surface plasmon excitation. (3) Plasmon-enhanced optothermal potentials defined by the DMD-controlled optical images are exploited to trap and arbitrarily manipulate colloidal particles or biological cells (Adapted with permission from Lin et al. [47]. Copyright 2017 American Chemical Society).

the thermophoretic behavior of the monomeric, oligomeric, and fibrillary forms of the protein. An infrared laser is focused inside a capillary borosilicate where homogenous protein suspension is placed, to generate localized heating, resulting in the motion of the molecules along the temperature gradient until the steady state is established.
