In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.

transport, disassembly, or reversal of the particle trajectory has not yet been explored. To realize a fluid-fluid interface as a programmable multifunctional platform inspired by nature, [15,22] we need a mechanism, which is capable of generating surface deformation on-demand, also with fine control over the deformation range.The objective of this work is to demonstrate the capability of capillary-driven attraction at the water-air interface for realizing on-demand manipulation of floating objects (transport and assembly) in a reconfigurable manner. We used a confined air bubble, an air bubble trapped between the needle tip and the interface, as the source to create a positive deformation at the water-air interface, and the range of which can be tuned by controlling the radius of curvature of the bubble externally. To the best of our knowledge, this is the first report on the tuning of the surface deformation for transport/patterning processes without the use of external stimuli such as magnetic, [23] acoustic, [24] electrical, [25] or optical fields. [13] Anisotropic objects with hydrophilic side touching the water surface get attracted toward the bubble due to the capillary force. The experiments demonstrate that the objects get accelerated toward the bubble, and reach a maximum velocity of 3-3.5 cm s −1 . The bubble breakage due to the impact of the object leads to its sudden or gradual deceleration, which depends upon its geometry. Long-range transport is realized by creating a series of bubble sources (five bubbles) along the required trajectory, and triggering them sequentially. With this simple method, we were able to guide the motion of the object in a predefined trajectory that covers a distance of about 6 cm. Further increase in the travelling distance could be achieved by increasing the number of bubble sources. Furthermore, this method is capable of assembling floating objects, and if required, breaking the bubble can disassemble the assembled structure, which opens a new paradigm for the designing of smart interfaces capable of on-demand manipulations such as transport, assembly, and disassembly.

Understanding and modulating the cross-stream motion of a surfactant-coated droplet in pressure driven flow has great implications in many practical applications. A combination of interfacial viscosity and Marangoni stress acting over a surfactant-coated droplet in pressure driven flow offers greater flexibility to modulate the cross-stream motion of it. Despite the intense theoretical and numerical research towards manipulating the surfactant-laden Newtonian droplets in Poiseuille flow, the experimental investigations are seldom explored. Herein, we report our study on understanding the influence of interfacial viscosity on the cross-stream motion of a surfactantcoated Newtonian droplet in both isothermal and non-isothermal Poiseuille flow from a theoretical as well as an experimental perspective. A theoretical model has been developed to understand the effect of interfacial viscosity on the lateral migration of a droplet under the assumptions of no shape deformation and negligible fluid inertia or thermal convection. Theoretical analysis is performed under two limiting conditions: (i) when the transport of surfactants is dominated by surface-diffusion and (ii) when the transportation of surfactants is dominated by surface-convection. Our theoretical analysis shows that both the dilatational as well as the shear surface viscosities suppress the lateral migration velocity of the droplet. Experiments have been performed to validate the theoretically predicted droplet trajectories and to understand the influence of channel confinement on the lateral migration of the droplet. It has been observed from the experiments that the droplet travels faster towards the centerline of the flow in a highly confined domain. The results presented in this study could provide new vistas in designing and analyzing various droplet-based microfluidic, biomedical and bio-microfluidic devices.

The dengue virus (DENV) infection commonly triggers threatening seasonal outbreaks all around the globe (estimated yearly infections are in the order of 100 million, combining all the viral serotypes), testifying the need for early detection to facilitate disease management and patient recovery. The laboratory-based testing procedures for detecting DENV infection early enough are challenged by the need of resourced settings that result in inevitable cost penalty and unwarranted delay in obtaining the test results due to distance-related factors with respect to the patient's location. Recognizing that the introduction of alternative extreme point-of-care technologies for early detection may potentially mitigate this challenge largely, we develop here a multiplex paper/polymer-based detection strip that interfaces with an all-in-one simple portable device, synchronizing the pipeline of nucleic acid isolation, isothermal amplification, and colorimetric analytics as well as readout for detecting all the four serotypes of dengue viruses in around 30 min from about 50 μL of human blood serum with high specificity and sensitivity. Aligned with the mandatory guidelines of the World Health Organization, the ultralow-cost test is ideal for dissemination at different community centers via a user-friendly device interface, not only as a critical surveillance measure in recognizing the potential cocirculation of the infection across regions that are hyperendemic for all four DENV serotypes but also for facilitating effective monitoring of patients infected by any one of the particular viral serotypes as well as timely administration of life-saving measures on need.

Advancements in developing antipathogenic interfaces are critical in mitigating the risk of infection spread amid the practical limitations of hygienic control in crowded and resource-limited settings. Such requirements are also extremely compelling in busy patient care centers including intensive care units where the statutory maintenance of environmental standards often appears to be impractical because of the overflooded patient loads. While advances in surface engineering have emerged with great promises to cater these needs, the underlying technological complexities appear to be prohibitive against practicable applications amid constrained technological resources. Here, we harnessed the role of unique topographical features of the skin of Ptyas mucosa (oriental rat snake), a commonly found snake species in south and southeast Asia, in terms of exhibiting supreme antifouling properties via natural inheritance, leading to pathogenic resistance. Our characterization studies unveiled that unlike the previously reported vertical pillars, hairs, and needles, arrays of horizontal denticulation, offering favorable topographical characteristics of structured roughness and hierarchical features, emerged to be responsible for exhibiting the desired functionalities. We subsequently adapted these structures with certain simplifications by biomimicking artificially engineered topologies on a polydimethylsiloxane (PDMS) surface. The resulting surfaces were proven to offer dual antimicrobial mechanisms such as resistances to adhesion or colonization of different bacteria (Escherichia coli, Enterococcus faecalis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus mutans) and facilitation for cell wall deformation and programmed cell death as evidenced by an abundance of oxidative stresses. These results opened up strategies of producing biomimetic surface textures and their effective implementation against pathogenic invasion in a plethora of applications ranging from medical implants to marine propulsion.