Micro Bubble Lithography

Physics of Micro-bubbles

Bubbles are fascinating as they have versatile applications ranging from soft matter, fluid dynamics to medical sciences. Researchers are using bubbles as micro robot to manipulate organic cells, nanoparticles even using them for drug delivery and ultrasonic imaging of human organs. Micro-bubbles under OT is especially interesting- firstly since they cannot be trapped by conventional Gaussian beams as their RI is less than the surrounding water medium, and secondly since they can actually be used to probe fluid flows around them in the micro-scale. We have a novel technique of generating micro-bubbles with ease, by having an absorbing species of material (guess what this material is? Soft-oxometalates or SOMs - the same stuff with which the peapods are made up of - provided by who else but Dr. Roy!!!) deposited/ absorbed in-situ on either of the surfaces of our glass sample chamber that is made of a standard microscope glass coverslip and a glass slide along with double sided sticky tape in-between. A local hot spot is created when we focus our trapping laser on the adsorbed SOMs, following which the bubble can be controllably manipulated as it follows the laser hot-spot, which we carry out by controlling the translation stage. We have observed fascinating effects with micro bubbles. Presently there are three classes of experiments that we perform with micro-bubbles under OT. 

Controlled microlithography using Optical Tweezers 

We have developed an emerging technique of micro patterning of materials using laser induced micro bubbles (presently called LIMBT- Laser Induced Micro Bubble Technique) under OT, where for the first time, we achieved continuous micro patterning of materials (guess the material again!!!) on transparent surfaces (see our Langmuir paper).This has been extended to pattern various different classes of materials (once again in collaboration with Dr. Soumyajit Roy). We have successfully developed catalytic micro-chip by micro-patterning the catalytic reagent on the patterns (see our JMCA paper). We have also extended this technique towards solution processed micro-circuits by having conductive micro patterning with doped polymers inside SOMs (see our JMCC paper). As a part of this we are on the process of developing micro capacitors using the same technique. Presently we are also extending this technique towards patterning of organic materials, thus developing bio-chips and sensors using micro patterning under OT.

You may like to watch this animation to understand the schematic of the pattern formation process better.

Study of growth dynamics of micro-bubbles 

The growth rate of micro-bubbles after formation follows very interesting dynamics. No existing theory have been able to predict this dynamics with confidence simply because no exact solution of time dependent Navier-Stokes equation in 3D has been achieved that can predict all the features going around at the time of bubble nucleation. Our technique of micro bubble generation allows us to passively confine the bubble on the hot-spot that opens the opportunity of looking closely into the growth dynamics of bubbles. 

Flow-induced self-assembly and complex manipulation of particles 

When a bubble is generated using LIMBT, it reaches to a steady state within a few seconds where the size and shape are maintained over the time scale of minutes. This allows us to further investigate the surrounding fluid motion in the presence of the bubble. The thermo-capillary stress produced by the micro-bubble generates flows in the ambient medium. Due to the temperature gradient present across the surface of the bubble, a surface stress is developed on the bubble surface which induces Marangoni flow around it. We are studying this fluid flow, generated and manipulated by the micro-bubbles under OT. With a single and multi-bubble(s) system we are able to generate controlled flow patterns which are fascinating. The flows can lead to particle sorting, generate angular momentum in the flow causing both radial and axial rotation in particles, and so on! 

Wave-guiding through self-assembled micro structures using bubbles 

Using LIMBT, we have been able to develop directed self-assembled micro-rods of diphenylanaline, the core recognition motif of the Alzheimer’s β-amyloid polypeptides. These self-assembled microstructures show wave guiding properties (see figure). We are currently working to couple this wave guiding modes with the continuum modes through fano resonance, which has possible applications in sensing. This work is done in collaboration with Dr. Nirmalaya Ghosh of DPS, IISER Kolkata and Dr. Debashish Haldar of DCS, IISER Kolkata. Roshan is working in this project.