Pharmacists, biotechnologists, cell biologists and genetic engineers have to work precisely with minimal amounts of liquid. One droplet too much or too little can lead to extreme deviations in the end product - possibly with devastating side effects for the user. A new method for dosing the droplets has now been developed at the university.
Chinese scientist Dr Yanzhen Zhang, who is conducting research as an Alexander von Humboldt scholarship holder in the Physical Chemistry working group at the University of Oldenburg, has developed a completely new process for producing the smallest possible droplets. The current issue of the renowned journal "Nature Communications" presents the project. Co-author of the study is the head of the Oldenburg working group, chemist Prof Dr Gunther Wittstock.
The dosing and manipulation of small liquids in the micro and nano range is of central importance for many applications. In order to produce the smallest possible droplets, nozzles are usually used that shoot droplets through a valve at high pressure. "The problem with conventional nozzle processes, however, is that the volume of the droplet is very closely linked to the diameter of the opening. So if I want to produce very small droplets, I also need smaller and smaller openings," explains Wittstock. It is even more difficult to press highly viscous, i.e. viscous liquids such as resins or solutions with solid particles through these tiny openings that are invisible to the naked eye. The result: the passages stick together or become blocked.
In contrast, the method used by the Oldenburg scientists is very insensitive. The researchers are able to produce droplets in the nanoscale range and smaller ones with the help of larger openings. "Our method even allows liquids to pass through the opening over a very high viscosity range. The droplet is much smaller than the opening. So we don't even have to produce such small openings in order to produce droplets. Larger openings are also much easier to maintain than smaller ones," continues Wittstock.
The secret behind this: Instead of a nozzle that uses pressure to force the liquid through an opening, the researchers use a pump that sucks the liquid back into the capillary. Only a single drop remains - similar to the drop at the tap. This so-called mother drop is sucked empty at a precisely defined speed. "When I suck the drop empty, its shape cannot follow the change in volume infinitely quickly. That's why the droplet thins at its neck until it breaks off. What remains is a tiny 'daughter' droplet that re-optimises on the surface," explains Wittstock.
The method has a second decisive advantage: If the droplet is released from a stainless steel cannula to which a voltage is applied, the droplet can be controlled in an electric field - so precisely, for example, that it hits other droplets and - depending on the control - either just touches them or combines with them. This opens up a wide range of applications. The individually generated droplets could potentially act as a "postman" and transport certain drugs, which are used to inhibit the growth of malignant cells in tumour therapy, for example, to specific receptors. Or they could collect specific substances and the contents of the drops could then be analysed. Controlled reactions in microarray reactors are also conceivable. "With our method, individual ultra-small droplets can be produced and deposited in a controlled manner, even with large openings. In addition, the volume of the droplets can be changed over several orders of magnitude using the same experimental setup. This has never been done before," summarises Wittstock.
"Hydrodynamic dispensing and electrical manipulation of attolitre droplets", Yanzhen Zhang, Benliang Zhu, Yonghong Liu and Gunther Wittstock, NATURE COMMUNICATIONS, 7:12424, DOI: 10.1038/ncomms12424, www.nature.com/naturecommunications