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Interfaces, separating bulk fluids of the same or multiple phases, play an important role in controlling the transport processes, which is critical in natural and industrial systems. A delicate balance between various forces acting on the interface dictates its shape, geometry, and stability. In many systems, disturbances in bulk fluids often lead to dynamic changes in the interface, affecting the transport processes across it. This thesis is focused on two types, non-reactive and reactive interfaces, and investigates their dynamics under unsteady forcing.The first part of the thesis focuses on investigations of droplets impacting vibrating substrates. Here, the interface between liquid droplets and their surroundings undergoes significant changes influenced by surface tension, inertia, and viscosity forces. The study explores the spreading of impacting droplets on both low-frequency and ultrasonically vibrating substrates, providing insights into the distinct mechanisms at play. The examination extends to delineating the influence of substrate oscillation on post-impact jet formation and droplet ejection behavior, offering a detailed understanding of the dynamics involved.Shifting to the second part of the thesis, our investigations delve into the response of premixed counterflow flames to unsteady forcing, particularly variations in the upstream reactant flow rate. Using experiments and numerical simulations, the study explores the effect of mono-frequency forcing on flame extinction in non-equidiffusive flames. This reveals insights into the relationship between the extinction strain rate under oscillating incoming flow as a function of the mean flow rate for different Lewis numbers of incoming reactants.Impacting droplets and premixed flames under unsteady forcing is important for various applications. For impacting droplets, the findings find applicability in coating processes, lab-on-chip analyzers, spray cooling systems, and advanced printing technologies. In the realm of premixed flames, understanding their extinction response to unsteady conditions is critical for optimizing combustion processes in gas turbines, rocket engines, and industrial burners. Furthermore, it provides fundamental insights into the behavior of turbulent flames. This research not only advances specific technologies but also enhances our fundamental understanding of complex fluid dynamics, offering potential avenues for innovation in diverse fields.

Icing on aircraft wings is one of the leading causes of aircraft crashes. It is mainly caused due to accumulation of ice or snow on the wing surface due to impact with supercooled droplets when passing through clouds at high altitudes, causing loss of lift obtained by the wings. It was found that droplet impact characteristics are dependent on droplet size, surface roughness, surface material hydrophobicity, and droplet impact velocity. As a continuation of the study of droplet impact contact characteristics by varying surface roughness and impact velocity, this study focuses on droplets impacting the vibrating surface at frequencies between 2-7 kHz. Atomization (water drop splitting into smaller droplets and ejecting from the surface) has been observed at different rates for all frequencies. The first set of data is collected by keeping roughness constant and increasing the amplitude of the vibration to observe the critical amplitude at which atomization is initiated. The surface roughness is varied for the second set of experiments. The data is quantified using image processing of the high-speed videos to obtain the rate of ejection for each case.

Superhydrophobic surfaces have many potential applications from self-cleaning textiles to ice-free aircraft. Their low affinity to water stems from a heterogeneous wetting state predicated on the stability of an intervening air layer between liquid and substrate. However, this layer is fragile and irreversible transitions to a homogeneous wetting state are possible and devastating to their performance. Recent studies have examined how pressure-based transitions, known as impalement, occur for droplets impacting superhydrophobic surfaces and have proposed solutions to mitigate against this under ambient conditions. Yet to this point, little consideration has been given to how the interplay between environmental effects and the dynamics of droplet-substrate interactions affect the behaviour of the intervening layer and the different phenomena that may result; especially in relation to phase change. In thiswork, I explore how departures from the standard ambient environments influence the local thermodynamic conditions and stability of the intervening layer, elucidating the wetting state transitions that arise from these volatile conditions. In the context of droplet impact, I investigate how the pressure and gas composition of the intervening air layer critically influence the ability of droplets to rebound from a superhydrophobic surface. Through high-speed imaging and theoretical modelling, I expose a previously unknown condensation-based wetting state transition mechanism in humid conditions and explain a trend of increasing impalement with decreasing environmental pressure. When studying the freezing of sessile droplets on superhydrophobic surfaces, in a low-pressure environment I deduce the presence of a force engendered by the evaporation difference across a droplet undergoing recalescence capable of either driving it into the texture or explosively expelling it, depending on the characteristics of the substrate in question. Exploring the complementary case of ambient pressure and low-temperature, I uncover a novel condensation-based filling mechanism induced by the temperature rise during recalescence that is endemic to freezing events on superhydrophobic surfaces that require a degree of subcooling with acute repercussions for their use in ice-repellency applications. Finally, whilst developing a superhydrophobic coating robust to impinging warm water droplets using a photothermal metasurface, I establish the theoretical foundations of condensation filling in a diffusion-limited regime based on acompetition between the filling and droplet-substrate contact times. Furthermore, I rationalise the performance of a hierarchical texture by considering the resulting shift to a nucleationlimited condensation regime. The understanding of phase change-induced wetting transitions accrued here should herald advancements in the design of superhydrophobic surfaces, broadening their working envelope and extending water repellency far beyond the state-of-the-art.

Droplet freezing is important both in nature and in technology. In this thesis I investigate the fundamentals of freezing water droplets and derive design criteria for the development of intrinsically ice-repellent materials. Such icephobic surfaces could improve the performance and safety of a multitude of technical processes in energy and transport. This includes for example heat exchangers, where ice built-up reduces thermal transport, and airplane flight, where freezing of water on airfoils can result in catastrophic events. The thesis consists of three individual studies. In the first study we investigated how the environmental conditions during droplet freezing affect the freezing outcome. We found that evaporatively or convectively supercooled water droplets resting on solid substrate can self-remove during freezing. This phenomenon, which we termed self-dislodging, requires that the heat removal from the droplet’s free surface dominates the heat removal through the solid substrate. Consequently, the freezing front moves from the outside of the droplet towards the center and from the top to the bottom, resulting in a solid ice shell with an unsolidified core and an unfrozen droplet-substrate interface. We observed experimentally that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, due to the volumetric expansion associated with freezing, semi-frozen water is displaced towards the droplet-substrate interface lifting the freezing droplet away from the substrate. The combined effects of dewetting and lifting result in droplet self-removal. We found that the more the substrate is hydrophobic the more robust self-dislodging occurs. In the second study we examined how multiple water droplets interact during freezing in a low-pressure environment. Understanding droplet interactions during freezing is important as droplets do not appear in isolation, but always in groups. We found that the freezing of a supercooled droplet results in self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We suggest that as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble resulting in ice coverage of the solid surface. In the third study we introduced a controllable and upscalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture for improved repellency of viscous liquids. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7mPa s, which covers the viscosity of supercooled water down to -17 °C. Based on the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces that can repel supercooled water before it freezes and sticks to the surface. The three studies presented in this thesis address the topic of surface icing from three different angles, collaboratively covering a broad range of the problem. Only when taking into account the environmental conditions, freezing group dynamics and liquid solid interactions, robust icephobic surfaces can be designed in the future. With my thesis I contribute to this development process.

The present thesis focuses on the study of droplet behavior within the framework of two classes of emerging technologies, related to the development of elastic superhydrophobic or icephobic materials with extreme performance and to the realization of contactless acoustophoretic DNA transfection, respectively. To this end, first we examine the collaborative effects of substrate flexibility, topography, and composition on enhancing surface superhydrophobicity and icephobicity, as defined by droplet impalement resistance, contact time and restitution coefficient, and use such knowledge in redesigning superhydrophobic and icephobic materials to repel supercooled droplets. Subsequently, we investigate the use of acoustophoretic actuation for contactless processing of liquid droplets containing living cells and design a containerless biological reactor appropriate to carry out DNA transfection experiments in a contactless manner. Flexible materials are ubiquitous in everyday life and technology, from textiles to insect wings and plant leaves; however, the effect of flexibility on the wetting and droplet impact dynamics under broad environmental conditions is not well understood. The concept investigated here is how the flexibility of the substrate—defined by its stiffness, damping and areal density—can be made to work collaboratively with surface chemistry and texturing in order to enhance superhydrophobicity. Droplet impact experiments reveal that the rational tuning of flexibility is able to achieve improved water repellency as characterized by the impalement and breakup resistance, contact time reduction and restitution coefficient increase. Designing flexible substrates with low-areal density and stiffness imparts immediate responsiveness to the substrate for impacting droplets, mitigating the collision and lowering the impalement probability without the need for an active mechanism. The aforementioned effects on drop impact dynamics are rationalized through appropriate modeling and exemplified by man-made and natural materials. The previous knowledge of manipulating droplets with flexible substrates is then applied in the design of icephobic materials. Surface icing is omnipresent in equipment installed outdoors and can cause deterioration of performance, e.g. in wind turbine blades and power lines, and serious safety hazard, e.g. in aviation. The flexible superhydrophobic materials were designed to absorb the energy of incoming droplets and to boost droplet mobility by releasing the absorbed energy at a later time. The design was optimized using highly viscous droplets for simplicity (mimicking supercooled water viscosity), like glycerol and honey, which do not rebound from rigid superhydrophobic surfaces. Then it was validated with water droplets at supercooled temperatures. The partial solidification of the supercooled droplets in the presence of nucleation promoting particles on the surface did not impede the solidifying droplets from rebounding, demonstrating a passive mechanism for shedding ice-water slurries. The findings of this investigation were underpinned using appropriate modeling. With reference to contactless droplet manipulation, acoustophoresis has been shown to be able to effect planar motion of solid particles and liquid droplets, with a purposely designed spatial and temporal regulation of the acoustic field. Therefore there exists the potential for single or multiple microliter volume droplets to be moved and mixed, avoiding contamination or loss of reagents to the container walls, enabling applications in chemistry, engineering and biology appealing. Here, targeting towards biological applications, an acoustophoretic concept is presented, able to manipulate suspensions of living cells and DNA plasmids in a controlled environment and eject them onto tissue culture plates. The cell viability of the procedure is assessed by detecting perturbations in the neuronal differentiation pathways of a cell model sensitive to even small levels of toxicity. Contactless DNA transfection of single or double plasmids is performed in the acoustophoretic setup and favorably compared to container-based methods in order to demonstrate its capabilities.

Laser treatment of Port-Wine Stains (PWS) was studied computationally by assuming a two dimensional skin model and using the reduced scattering method for the photon energy transport. The energy deposited during the laser irradiation of skin was based on a Gaussian laser profile. Alternating Direction Implicit finite difference method was used to investigate the effect of cryogenic spray cooling during laser irradiation on the skin temperature response. The computational model was developed in MATLAB and validated using experimental results available in the literature. Simulations were first run for the case of laser irradiation with a continuous spurt of cryogen which showed a drastic drop in the temperature rise as compared to the case without any cooling method. Then the effect of intermittent cryogen cooling accompanying laser irradiation was analyzed. To understand the outcome of preheating on the treatment procedure, preheating was computational modeled with continuous as well as intermittent cryogen cooling. It was found that by preheating the skin it is possible to raise the temperature of the blood vessels (responsible for PWS), by minimizing the danger of vessel necrosis. Preheating the skin increases the temperature of the entire skin model (layers of epidermis, dermis and blood) but since cryogen provides only selective cooling it protects the epidermis from any form of thermal injury, without affecting the temperature of blood vessels present in the dermis layer. It was also found that using cryogen spurt in intermittent intervals instead of a long continuous spurt further reduces the risk of vessel necrosis and other forms of thermal injury. Results showed that the maximum temperature rise is a weak function of the surface heat transfer coefficient however the temperature in the epidermis layer is strongly dependent on the surface heat transfer.

The impact of liquid drops on solid surfaces is a ubiquitous phenomenon in our everyday experience; nevertheless, a general understanding of the dynamics governing droplet impact remains elusive. The impact event is understood within a commonly accepted hydrodynamic picture: impact initiates with a rapid shock and a subsequent ejection of a sheet leading to beautiful splashing patterns. However, this picture ignores the essential role of the air that is trapped between the impacting drop and the surface. We describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. We directly visualize the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. We show that the approach of the spreading liquid toward the surface is unstable, and lift-off of the spreading front away from the surface occurs. Lift-off ensues well before the liquid contacts the surface, in contrast with prevailing paradigm where lift-off of the liquid is contingent on solid-liquid contact and the formation of a viscous boundary layer. We show that when a drop impacts an atomically smooth mica surface, a strikingly stable nanometer thin layer of air remains trapped between the liquid and the solid. This layer occludes the formation of contact, and ultimately causes the complete rebound of the drop.

Cell printing is an emerging technology that uses droplets to deliver cells to desired positions with resolution potentially comparable to the size of single cells. In particular, ink–jet based cell printing technique has been successfully used to build simple bio–constructs and has shown a promise in building complex bio–structures or even organs. Two important issues in ink–jet based cell printing are the moderate survival rate of delicate cells and the limited cell placement resolution. Resolving these issues is critical for the ink–jet based cell printing techniques to realize their full potential. In this work, we use numerical simulations to reconstruct the impact of a droplet loaded with a single cell onto a pool of viscous fluids to gain insights into the droplet and cell dynamics during cell printing. We developed a mathematical model for this process: the droplet, pool and air are modeled as Newtonian fluids, and their flow is modeled as a laminar flow governed by the Navier–Stokes equation. The cell is modeled as an axisymmetric solid object governed by the neo–Hookean law and also has a shear viscosity that is the same as that of its host droplet. To numerically solve the coupled fluid and cell motion, we used a hybrid method in which fluid flow is solved on a fixed Cartesian grid and the deformation of solid body is solved on a Lagrangian mesh. We also developed a new full Eulerian method, termed the solid level set (SLS) method, to simulate cell printing. The key idea is to track the deformation of the solid body using four level set functions on a fixed Cartesian grid instead of using a Lagrangian mesh. The SLS method is easy to implement and addresses several challenges in simulations of fluid–tructure interactions using hybrid Eulerian/Lagrangian meshes. Using codes developed based on the above methods, we systematically investigated the fluid and cell dynamics during the cell printing process. We studied how the droplet penetration depth, droplet lateral spreading, cell stress and cell surface area change are affected by printing conditions such as impact velocity, pool depth, and cell stiffness. Our simulations indicate that cell experiences significant stress (∼20kPa) and local surface area dilation (∼100%) during the impact process. The latter suggests that cell membrane is temporally ruptured during the printing process, and is consistent with the gene transfection observed during cell printing. We speculate that the survival of cell through the rather violent cell printing process may be related to the briefness of the impact process, which only lasts about 0.1 milliseconds. Based on our simulation results, several strategies have been proposed to reduce the stress and membrane dilation of cells during cell printing.

The study of impact, spreading, and recoil of droplets of aqueous solutions of polymeric and surfactant additives on horizontal hydrophobic (Teflon) and hydrophilic (glass) surfaces is presented in this thesis. The non-Newtonian aqueous polymer solutions are prepared by mixing the water-soluble hydroxyethyl cellulose (HEC) with varying degree of polymerization (QPC 300, 250 HR, and 250 HHR) at different concentrations. The solution rheological and interfacial properties are characterized to understand the role of wettability, surface tension, and viscosity on the droplet surface interactions. For each polymer solution, the surface tension is measured by the maximum bubble pressure method and the static contact angle is measured using a contact angle/wettability analyzer. Intrinsic viscosities of the three polymers are determined from the viscosity measurements of their dilute solutions carried out using a capillary tube viscometer. A high speed digital camera is used to capture the droplet impact behavior at 4000 frames per second. The captured images of the droplet are analyzed using an image-processing software and the temporal variations of the spreading factor and the flattening factor of the droplet are determined. Results show that the higher viscosity coupled with lower surface tension of the polymer solutions leads to larger spread compared to a water droplet and inhibits strong recoil on a hydrophobic surface.Computational simulations of the fluid flow and heat transfer during spreading, recoil, rebound/break up of hot droplets of water and aqueous solutions of two surfactants (SDS and Triton-X 100) on a Teflon surface have been carried out at We ∼ 28. The continuity, momentum conservation, and the energy conservation equations are solved simultaneously using a finite volume method to determine the drop shape evolution and the drop-substrate heat transfer. The Volume-of-fluid or VOF method is used to track the liquid-gas interface deformations during spreading and recoil. By comparing the water and surfactant solution droplet impact dynamics at the same Weber number, it is evident that the surfactant solution drops produce larger initial spread and weaker recoil due to the reduction in surface tension at the liquid-air interface and the change in the wettability of the liquid-solid interface. The increased contact area leads to higher rate of heat transfer for a surfactant solution droplet compared to that for a water drop. The Triton X-100 with its lower mobility is less effective in increasing the drop-substrate heat transfer compared to SDS which has a higher mobility.