Spatial Control of Dropwise condensation

Schematic illustrating the dropwise condensation on hydrophilic spots of a patterned surface. Controlled condensation allows the easier removal of condensed droplets and thus increasing the condensation heat transfer.

Understanding the condensation mechanism is crucial to enhance the heat transfer performance of numerous industrial applications such as power generation, water harvesting, water desalination, cooling of the nuclear reactor, and the thermal management of electronic devices. Condensation is classified into two types, namely filmwise condensation (FWC) and dropwise condensation (DWC). In FWC, condensate forms a liquid film on the surface. This liquid film act as an additional thermal resistance to heat transfer between the surface and the vapor. In DWC, vapor forms distinct liquid drops on the surface. DWC is often preferred since it provides higher heat transfer coefficients than the filmwise condensation.

A novel method of improving heat transfer and maintianing DWC was recently observed by creating periodic hydrophilic spots on hydrophobic surfaces. The idea originated from observing the for harvest on the back of Namibia beetle which has hydrophilic bumps on its back which is a hydrophobic surface. However, several questions remain unanswered to commercially develop such surfaces. The spatial distance between hydrophilic spots varies with difference in the sub cooling temperature, surface energy combinations among others. Different regimes such as jumping of droplets, flooding, etc. were observed during the coalescence of individual droplets.

Our research focusses on controlling the spatial distance between the hydrophilic spots based on the application. We study the condensation of micro-droplets on both mixed wettable flat and textured surfaces using lattice Boltzmann modeling simulations.

Droplet Dynamics during Inkjet Printing

Schematic illustrating the impingment of droplets on surface during inkjet printing. Spreading and retracting behavior of droplets plays an important role during the printing. In the manufacturing technologies, drop on demand printing can be used to create period surface roughness. Dynamics of polymer droplets on surface play key role in determining the shape and accuracy of roughness.

Inkjet printing has gained lot of interest in recent years due to its applications in the field of printing technologies and Drop on demand manufacturing. Advangates on inkjet printing in these technologies is due to limited use of ink or polymer, and precise control of ink. The study is still in its infancy due to several technological and conceptual questions that need to be answered.

First and foremost improvement should be to reduce the size of droplets while jetting at fast rate. Through various techniques such as using electric field, research community tried to inject nanosized droplets for inkjet printing. However, high surface energy prevents formation of droplets of nanosize during inkjet printing and thus more research is necessary to be able to obtain better results.

Another factor to be considered before inkjet technology for manufacturing field becomes a reality is the control of droplet dynamics when the droplet hits the surface. Usually the droplets on impinging on to the surface either spread or break up depending on the velocity of impingement. The objective of inkjet technologies is to prevent the break up and understand the spreading and retracting behaviour of droplets at nanolevel in order to control the textured roughness being achieved on the surface.

Inertial microfluids for droplet path control

Microfluidic channels can be designed to sort droplets and cells based on their size, and rigidity. Sorting phenomena depends on the inertial, drag and lift forces acting on the droplet.

Traditional view of microfluidics was the low Reynolds number flows with fluid motion dominated by viscosity. Recent high speed image capturing techniques have allowed the research community to study the microfluidic flows when Reynolds number is greater than 1. This new fluid flow phenomena has been termed as inertial microfluidics.

Inertial microfluidics garnered wide interest due to its applicability in sorting of diseased cells and detection of diseases at high through put. Droplets are commonly used as analogues to cells for studying the sorting behavior in microchannels with numerous outputs. While the drag and lift forces acting on droplets were used to control the path selection of droplets in the traditional microfluidics, inertial force can also play major role in controlling the droplet sorting.

Objective of the research is to theoretically, experimentally, and numerically understand the sorting behavior of droplets in inertial microfluidics and apply it to design microchannels for diseased cell sorting.