Controlled transport and assembly of liquid drops and solid entities over soft substrates

Abstract

newlineSoft substrates are emerging as a versatile alternative to solid surfaces for droplet microfluidics. newlineMajor challenges with conventional solid surface-based microfluidics include difficulty in controlling newlinethe droplet transport direction, speed, transport range, and large contact angle hysteresis (CAH). newlineMoreover, the requirement of complicated and expensive fabrication steps and the need for bulky newlineperipheral components hinder their wide applicability and scope for miniaturization. On the other hand, newlinesoft surfaces are capable of responding to external stimuli such as light, temperature, pressure, etc., newlinewhich could be effectively utilised for realizing droplet manipulations. This PhD work explored the newlinepossibility of two different soft substrate concepts for microscale transport: (a) Deformable polymer newlinesurface and (b) Liquid-air interface. In both platforms, frugal strategies based on wettability-controlled newlinecapillary forces were exploited as transport methods. Capillary force dominates the dynamics of objects newlinein a fluidic environment when the characteristic dimensions are less than the capillary length of the newlinefluid. Therefore, achieving controlled transport of objects in this regime requires exploiting capillary newlineforces to our advantage. Based on how to tackle capillary force for transport of entities over soft newlinesubstrates, the current work was done in three phases. newlineIn the first phase, a fast and reconfigurable droplet manipulation strategy over a Poly Nisopropylacrylamide (PNIPAAm) grafted structured polymer surface using temperature-induced newlinewettability gradient was demonstrated. The experiments showed an unprecedented control over both newlinewater contact angle and contact angle hysteresis, which is crucial for microfluidic droplet transport newlineapplications. This study revealed that the PNIPAAm grafted on intrinsically superhydrophobic surfaces