##Introduction The term Optofluidics was for the first time coined in 2003. Currently it is defined as a very rapidly emerging research field the main focus of which is combining microfluidics and optical technology. If we look back, in the last 5 years this terminology has become widely adopted for a large number of research directions. It is easy to check this because currently the input of the term “optofluidics” in Google search now yields more than 30,000 results. It early became apparent that the concept of optofluidics, to take the idea of combining the advantages of microfluidics and optics could bring much more to both of these fields than they are separately. Some of the projects for which links between the birth of the term optofluidics and the initiation of the projects exists are for example the optofluidic microscope and optofluidic lasers.This leads to the question: What exactly is optofluidics? In this short article we will address and try to answer this question. Here we will also try to briefly examine the advantages of optics and microfluidics, discuss some of the ways these two disciplines can combine, and how the combination can lead to and generate optofluidic technologies with unique capabilities.
##What is Optofluidics? In 2007 a review paper  was released which started the shift to a more systematic definition of optofluidics in which the advantages of optofluidic technologies were discussed as well as possible benefits to both the optics and the microfluidics fields. In the present context, optofluidics can be defined as the combination of optics and microfluidics in the same platform to use specific advantages of these two disciplines. Optofluidics brought us new and potentially more useful and better ways to build and use already well established optical technologies, structures and devices. It is also fair to say that some of the growth directions in recent years have also been totally unanticipated.
##Advantages As mentioned above gave us a new way to think about either optics and fluidics. At the moment, key advantages of optofluidics are: 1. The ability to change the optical properties of the fluid medium within a device by replacing one fluid with another. 2. Immiscible fluid-fluid interfaces are smooth. It has been known for a very long time that the optical smoothness of fluid interfaces can be very useful property for various applications. For example it could provide a cost-effective way to create optical surfaces. This happens due to surface tension, and as a result - an immiscible fluid-fluid interface is uniform and smooth. 3. Diffusion can create controllable blend of optical properties. This property is of extreme importance for various applications. The solid-based structures fail to provide this property that can be created by the diffusion across the interface of two liquids. Flow parameters, fluid choices, and the device structures can be tuned in such a way as to provide full controllability and flexibility and enable the creation of novel optical interconnects. For example, an optical splitter and wavelength filter based on the selective mixing of two fluid jets in a third fluidic medium has been demonstrated recently. This differs from a conventional beamsplitter because the split ratio of the optofluidics based beamsplitter can be dynamically tuned for any given wavelength. 4. Fluid can act as an excellent transport medium. This is so because it is relatively easy to input, move, and manipulate fluid in an optofluidic device. As in previous example pressure differential is a common and convenient property to achieve this. One of such examples of optofluidic technology that makes good use of fluid transport is the optofluidic mask-less lithography approach. 5. Optofluidics could also help to build optofluidic lasers, the working principle of which depends on the switching of laser dye medium as a way to achieve wavelength tuning over the range of interest. 6. Last but not least is that fluid can be an excellent buoyancy mediator. The density of fluid media ranges widely. By mixing two miscible fluids, fluid with arbitrary intermediate density values can be created.
Although not mentioned above, recently, there has also been a significant progress made in the use of waveguides to exert new types of controls. Beyond direct force use, like momentum transfer, there are other but more subtle ways in which light can be used to manipulate and move fluids and objects in fluids. This can be achieved with the use of optically induced heating and fluid vaporisation. These phenomena act in a way that helps to manipulate fluid in totally new ways and therefore show significant advantages of optofluidics technology. We can therefore, reasonably expect and anticipate rapid growth of this field during coming decades at an increasing pace.
As described in this short article, the field of optofluidics emerged from a series of efforts in trying to fuse advanced planar optics with micro and nanofluidics. One of the current aims of this new and rapidly emerging field is the development of optical devices that have new functionalities enabled by microfluidic elements. Main advantages of these devices are associated with their ability to exploit various old and new fluidic transport phenomena. This is in order to change optical properties like refractive index, gain, and nonlinearity, over very small length scales.
The opposite is of course also possible and already used, that is optical effects can be used to enhance microfluidic transport. Such techniques range from traditional optical tweezing, rotational manipulation of components of fluid to more recent electro-optic approaches where electro optical phenomena come into play. Optofluidics will find a lot of different applications in the field of biomedicine, and especially in biomedical analysis devices. This is so because of precision with which particles or fluids can be transported and separated with these optical techniques. We believe that the field of optofluidics will continue to surprise us with its new and unique devices and techniques.
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