Manipulation of micro droplets has attracted a great deal of attention in the field of chemistry, bio and life sciences. Sample size reduction takes advantage of high reaction rate, high throughput and space saving. For these reasons, new techniques are required for liquid handling in a micro scale. We have applied a simple technique using a small rod for nano-liter droplet formation. When a rod is dipped into the liquid and pulled up, a droplet is formed at an end of the rod. It is known that the droplet volume is strongly affected by the rod diameter and its wettability. In the present study, the thermocapillary flow was induced by imposing a temperature difference on the free surface in order to control the droplet volume. The droplet formation and resulted droplet volume were observed through a high-speed camera. Numerical simulation has also been performed to obtain a static meniscus shape and critical height. The experiment showed that the droplet volume decreased linearly with increasing the intensity of the thermocapillary flow. The droplet volume was reproducible with an uncertainty of 1 % under the present experimental condition. The meniscus shape and critical height by the experiment and analysis were in good agreement with each other. It was found that the thermocapillary flow induced slipping of contact line, and thus the dewetting took place on the rod end face. As the result, the meniscus volume was reduced and a smaller droplet was obtained. Therefore, reduction of the droplet volume was caused by the thermocapillary effect. It was concluded that the thermocapillary flow was able to realize volume control of a nano-liter droplet.

View full abstract

Using the system consist of water and small number of sulfuric acid molecules, we performed molecular dynamics simulation of vapor-liquid nucleation and examined the impact of addition of sulfuric acid molecules on the homogeneous nucleation of the water vapor. The nucleation rate, the critical cluster size, the cluster formation free energy and the cluster structure were compared with that in the homogeneous case. It was found that sulfuric acid reduced the critical cluster size, but did not affect the barrier height of the nucleation. Correspondingly, the nucleation rate was observed to be only moderately accelerated. Also, the curve of the cluster formation free energy is rather different from homogeneous one, and indicates that the cluster larger than the critical size cannot grow rapidly. It was found that these observations come from the process of cluster growth. In the early phase, each sulfuric acid molecule rapidly forms a stable small cluster. This cluster is difficult to absorb monomer, and the main way of growth is the fusion of clusters. Because the fusion does not happen very often, cluster growth becomes slow.

View full abstract