The advent of microfluidics has provided a tremendous boost to the field of health care for the development of practical in-situ medical diagnoses and Point-of-Care (POC) testing methods. Optical microfluidics offers a lot of scope for carrying out successful biodetections through different target detection techniques such as optical absorption, fluorescence, etc. Two main issues in carrying out successful biodetection on microfluidic platform are the problem of biomolecule immobilization onto the surface of the microfluidic channel and efficient mixing of the bio-fluids necessary to achieve proper bio-interaction. In most cases, the biodetection involves two or more biological specimens, such as enzyme-substrate, antigen-antibody, protein-protein etc., and therefore, it is necessary to discover a solution which addresses to the needs of both immobilization and multi-molecular interactions. In this work, a novel technique of flow controlled molecular sorting is presented, wherein, by appropriate design of the microfluidic channel and by careful control of fluid flow in the system, optimal interaction of the specimens can be achieved through biomolecular sorting, thereby overcoming the problem of bio-immobilization onto the surface of the microfluidic channel. Herein, Finite Element Modeling (FEM) of flow behavior within the microfluidic channel has been carried out for different channel geometries, which is essential for the appropriate choice of microfluidic system for the present application. The technique of implementing the immobilization-free multi molecular bio-interactions in the proposed microfluidic system is explained and the feasibility of carrying out optical microfluidics based biodetection is demonstrated.