Microanalysis of clouding process at the single droplet level

We have applied laser tweezers equipped with a position sensitive photodetector (PSD) to investigate the clouding process in individual droplets with micrometer to nanometer in radius. The employment of PSD ensures a highly sensitive and non-intrusive detection for micro or nanodroplets. Systems as diverse as triethylamine (TEA, a small molecule), poly(N-isopropylacrylamide) (PNIPAM, a polymer), and protein elastin, have been studied using this method. Compared to widely used approaches such as UV–vis determination of clouding temperature, our methods have shown an improvement of at least two orders of magnitude in detection limit. The trapping of droplet by laser tweezers creates a wall-less environment to prevent surface effects. In addition, it avoids gravity implications that interfere with bulk investigations on the clouding transition. The capability of our method is further demonstrated by an unambiguous observation that PNIPAM droplets in the clouding process grow through intermolecular aggregation after following coil to globule transition in the polymer.