Self-running droplets
 
by: David K. N. Sinz , Jorge A. Vieyra-Salas, and Anton A. Darhuber  
Submitted: 15 May 2012
 
Mesoscopic Transport Phenomena Group, Department of Applied Physics
Eindhoven University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
 
Duration: 00:03:23
 

Introduction:   Besides Marangoni driven sub-phase redistribution¹, the deposition of a droplet of insoluble surfactant on thin liquid films can give rise to fascinating flow phenomena such as contact-line instabilities2. In this video the fascinating self-sustained motion of such droplets on thin liquid films is shown along with an outline of application potential in chemically confined microfluidics. Fascinating modes of motion arise from adjusted experimental conditions.

Chemicals and substrate patterning:   First we explain our experimental procedure. Silicon substrates were obtained from Silicon Quest (batch number SQ13869), glass substrates were Gold Seal coverslips 48 mm×60 mm (product number 3334). Sample clean- ing and chemical pattern creation by photolithography and subsequent vapor deposition of 1H,1H,2H,2H- perfluorooctyl-trichlorosilane (PFOTS, purity > 97%, Sigma Aldrich product number 448931), were previously described¹. Sub-phase films consisted of glycerol or a 0.55 w% solution of sodium dodecyl sulfate (SDS, Aldrich product number 71727, purity 99%) in an- hydrous glycerol (purity 99%, Sigma Aldrich product number 49767). Self-propelling surfactant droplets were cis-9-octadecen-1-ol (oleyl alcohol, Sigma O8880, 99% purity) or cis-9-Octadecenoic acid (oleic acid Sigma O1008-1G 99% purity).

Propulsion:   We show real time experimental images of a basic propulsion experiment as explained in the introductory part. The width of the sub-phase rivulet in this experiment was 1.5 mm, the illuminating light was passband limited around a centerwavelength of 650 nm. Besides the propulsion of the oleyl alcohol drop, pro- nounced flow in the sub-phase is evident associated with a strong film thinning behind the droplet.

Cargo transport:   In a side-view configuration the possibility to use a hydrophilic wedge geometry to load a solid cargo particle into a droplet is shown. Following the loading of the cargo, a PMMA bead (diameter  300 μm), the droplet, containing the cargo, can be mobilized and propels along a sub-phase film.

Routing:   The following part of the video is illustrating how localized thermal modulation of the sub-phase interfacial tension can be utilized to implement a routing procedure for these droplets at fluidic junctions. First, the thermal routing is implemented via a PDMS flow cell attached on the bottom of the substrate beneath one of the fluidic branches. As the droplet reaches the junction, the flow cell is flushed with water 10 K warmer than the ambient temperature.

In the second routing experiment, a cargo transporting droplet is approaching a fluidic junction and an infrared laser is used to control its trajectory. During the experiment a guide laser is constantly indicating the approximate position of the laser beam, the infrared laser however is more focused and only switched on for  1 s at an output power of 1 W as the droplet reaches the junction. To ensure complete absorption of the laser beam, the entire substrate was placed onto an infrared absorbing glass and the experiment was monitored from below.

Complex modes of motion:   In the last parts of our video we present experiments in which the boundary conditions have been adjusted to give rise to complex modes of drop motion.

Here we used Oleic acid droplets on pure glycerol. The first experiment shows a drop that propels against the flow direction of the underlying bulk phase and as a consequence exhibits a meandering motion. In the final sequence a droplet on a glycerol sub-phase containing DI-water breaks up as the flow rate of the sub-phase is increased, subsequently the resulting daughter droplets exhibit a synchronized meandering motion.

Acknowledgments:   We would like to thank Steffen Berg and Axel Makurat from Shell International Exploration and Production (Rijswijk, The Netherlands) for the in- spiring collaboration and gratefully acknowledge the par- tial support by the Dutch Technology Foundation STW, applied science division of NWO and the Technology Pro- gram of the Ministry of Economic Affairs.

¹David K. N. Sinz and Myroslava Hanyak and Jos C. H. Zeegers and A.A. Darhuber, Phys. Chem. Chem. Phys., 2011, 13, 9768-9777.

²David K. N. Sinz and Myroslava Hanyak and A.A. Darhuber, JCIS, 2011, 364, 519-529.