We report irreversible Cassie–Wenzel wetting transition on a nanostructured superhydrophobic surface employing surface acoustic wave (SAW) vibration. The transition is achieved upon penetration of the liquid into the nanogrooves driven by the inertial energy of the drop imparted by the SAW. However, the filling up of nanopores imposes an energy barrier (Eb) to the transition, which requires the displacement of the initial solid–air interface inside the pores with a solid–liquid interface. We unravel that the relative magnitudes of the input acoustic energy (Eac), and this energy barrier, hence, dictate the occurrence of the wetting transition, with the irreversibility in the transition, therefore, being explained from energy minimization of the system following the transition. In addition, observing the dynamics of the wetting front allowed the different regimes of the wetting transition process to be identified.
Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet–droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications.
The complexities involved to achieve tailor-made evaporative deposition pattern has remained a challenge. Here, we show that the morphological pattern of drying suspension droplets can be altered by varying substrate elastic modulus 𝐸. We find the particle spot diameter and spacing between the particles scale with substrate stiffness as 𝑑 𝑠 ~𝐸−0.15 and 𝑠~𝐸 −1.23 , respectively. We show that the larger spot diameter and spacing between particles on a softer substrate is attributed to a higher energy barrier 𝑈 associated with stronger pinning of the contact line. The particle deposition pattern is characterized in terms of deposition index, 𝐼 𝑑 , whose value is < 0.50 and > 0.75 for centralized (multilayer) and uniform (monolayer) deposition patterns observed for stiffer and softer substrates, respectively. The outcome of the present study may find applications in biochemical characterization and analysis of micro/nano particles.Droplet evaporation, apart from the association with the natural processes such as rain, fog and dew, has found applications in inkjet printing 1 , spray cooling 2 , DNA microarrays 3 , biochemical assays 4 and spraying of pesticides 5 etc. The topic has been extensively studied over the past two decades giving rise to important scientific advancements and technological developments [6][7][8][9] . Evaporation of particulated droplets involves rich physicochemical phenomena such as particle/particle interaction, particle/substrate interaction, patterning and wetting. The seminal work by Deegan et al. 10 illustrated that an outwardly driven flow resulting from the differential evaporation flux drags micro/nano particles toward the three-phase contact line, which gives rise to the accumulation of particles in the form of a ring; famously known as "the coffee-ring effect".The coffee ring effect has been exploited in various applications such as detection of malaria and other biomarkers 11,12 , nanochromatography 13 and disease diagnostics 14 . On the contrary, the performance of the matrix assisted laser deposition ionization spectrometry (MALDI) 15 , surface enhanced Raman spectroscopy (SERS) 16 , fluorescent microarrays 17 , and color filters in LCDs 18 are greatly hampered by this effect. Thus an in-depth understanding of the kinetics of evaporation and the subsequent morphological pattern would have utmost importance for such applications. By varying the physicochemical parameters such as ambient pressure 19 , substrate temperature 20,21 , relative humidity 22 , substrate wettability 23 , properties of the solute (shape, size and wettability) 24 and solvents (pH) 25 , presence of the surfactants 26 and additives 27 and external flow
Cell lysis is a critical step in genomics for the extraction of cellular components of downstream assays. Electrical lysis (EL) offers key advantages in terms of speed and non-interference. Here, we report a simple, chemical-free, and automated technique based on a microfluidic device with passivated interdigitated electrodes with DC fields for continuous EL of cancer cells. We show that the critical problems in EL, bubble formation and electrode erosion that occur at high electric fields, can be circumvented by passivating the electrodes with a thin layer (∼18 μm) of polydimethylsiloxane. We present a numerical model for the prediction of the transmembrane potential (TMP) at different coating thicknesses and voltages to verify the critical TMP criterion for EL. Our simulations showed that the passivation layer results in a uniform electric field in the electrode region and offers a TMP in the range of 5-7 V at an applied voltage of 800 V, which is well above the critical TMP (∼1 V) required for EL. Experiments revealed that lysis efficiency increases with an increase in the electric field (E) and residence time (t r ): a minimum E ∼ 10 5 V/m and t r ∼ 1.0 s are required for efficient lysis. EL of cancer cells is demonstrated and characterized using immunochemical staining and compared with chemical lysis. The lysis efficiency is found to be ∼98% at E = 4 × 10 5 V/m and t r = 0.72 s. The efficient recovery of genomic DNA via EL is demonstrated using agarose gel electrophoresis, proving the suitability of our method for integration with downstream on-chip assays.
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