Slide electrification-spontaneous charge separation at receding contact line has been reported to result in an electrostatic potential of around 1 kV and significantly affect drop motion. In order to find out how slide electrification influences contact angle, we examined the dynamic contact angles of aqueous droplets at various salt concentrations sliding down surfaces. We compare drops...
The analysis of sliding drop videos have emerged as a valuable tool for investigating physical phenomena (1-3). Hereby, automatic image processing enables the extraction of various measures from these videos (4, 5). On homogeneous surfaces, hydrodynamic dissipation increases with velocity. Recently, Li et al. studied drops sliding down an inclined surfaces (3) and reported an empirical...
Adaptation may play a role in materials science, biology, and engineering1, 2. Dynamic adaptation is more relevant than static wettability due to allowing for greater versatility, functionality, and control over surface properties3. The dynamic adaptation process of liquids on solid surfaces is a complex subject due to its involvement with different lengths and time scales4, 5. The challenge...
Droplets are set in motion on substrates with a spatio-temporal wettability pattern as generated, for example, on light-switchable surfaces. To study such cases, we implement the boundary-element method to solve the governing Stokes equations for the fluid flow field inside and on the surface of a droplet and supplement it by Cox–Voinov friction for the dynamics of the contact line. One...
Slide electrification of water droplets over CYTOP electrets surfaces
Shalini1,4, Dhiman Mallick1, 2, Rüdiger Berger4 and Ankur Goswami1,3
1School of Interdisciplinary Research, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India 2Department of
Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India ...
Despite its importance in high temperature processes (e.g. welding and coating), the wetting behavior of liquid metals on solid metals (or ceramics) at high temperatures remains poorly understood due to complex physiochemical processes e.g. surface oxidation, Marangoni flows, and interfacial reactions controlling the interfacial properties. Performing experiments above 1000°C requires a strict...
The manipulation with topography of surfaces, which can be achieved through the application of external stimuli, including temperature, magnetic fields, pH, light, and others stimuli, allows switching of wetting and adhesion. Shape-memory polymers is one kind of materials can alter their appearance under applied stress and can be used for fabrication of such smart surfaces. Particularly,...
Directional spreading and wicking of liquid drops on topographic micropatterns are studied on regular arrays of triangular posts breaking the reflection symmetry of the lattice. To test for directional wicking on this class of patterns, we employ heptane drops forming a material contact angle of 50° on Teflon coated samples fabricated using standard photolithography processes. Depending on...
Soft and wet contacts are ubiquitous across scales from geology to physiology and are crucial for engineering. Interestingly, when an object moves near a soft substrate, the generated hydrodynamic pressure would deform the substrate, then resulting in the hydrodynamic coupling. Based on the EHD coupling, we proposed a contactless way to probe the mechanical properties of soft substrate using...
Water drops moving on many surfaces experience electrical charge separation. This process leads to opposite charging of the surfaces and drops, which changes properties of this system. Electrostatic forces affect the dynamic contact angles and sliding velocity of drops.[1] In addition, charges on the surface decreases its surface energy. Recently, charging of water drops inspires many...
Electro-dewetting (EDeW) was introduced as a new method in order to change the wetting properties of surfaces. Using ionic surfactants such as dodecyl tetrethyl ammonium bromide (DTAB) the wettability of hydrophilic silicon oxides on a a conductive Si substrate can be controlled by an applied electrod potential, which leads to an increase in the contact angle. This behavior critically depends...
In this study, we aim to investigate the influence of substrate properties on biofilm spreading using a simplified model tailored to elastic substrates. The contact angle plays a key role in biofilm spreading and is related to the spreading velocity. To facilitate our mesoscale analysis, we use a model of wetting energy as a function of film height, denoted
Wetting has been recently identified as physiologically important in fundamental cellular processes: phase-separated condensates (e.g., proteins and RNA) form liquid droplets in cells and interact with membranes, e.g., during autophagy in eukaryotic cells or protein storage in plant vacuoles. Upon contact, the droplets can exert wetting forces on the membrane that deforms. This creates a...
Four polymer brush coatings containing photoresponsive arylazopyrazole (AAP)-based methacrylate monomers are reported. The glass substrates were functionalized using microcontact printing (µcP) and surface-initiated atom transfer radical polymerisation (SI-ATRP). These polymer thin films were characterised by atomic force microscopy (AFM) and resulted in a height of 15 to 25 nm depending on...
Liquid crystal elastomers (LCEs) are an exceptional framework for highly active and functional soft matter. Briefly, LCEs couple the anisotropic LC director to a soft polymer network resulting in a zoo of phase-dependent responsive properties including reversible optical switching, mechanical stiffening, and shape-shifting in response to heat, light, or magnetic/electric fields. Here, we...
We are interested in the analysis of the (de)hydration dynamics of polymer-coated surfaces monitored with time-resolved fluorescence microscopy. In particular, the reversible changes of the wetting state in thermally-switchable polymer layers is imaged in a confocal microscope via changes of the fluorescence intensity, spectrum and/or decay time of the reporter dye due to changes of local...
Leveraging our established droplet model developed using the boundary element method (BEM), we investigate dynamic wetting on thin flexible sheets. One promising application arising from this research is the potential for designing tunable fluidic lenses.
Central to our study is the nuanced interplay between the mechanical attributes of the sheet and droplet behavior, with a particular...
Biological cells are dynamic systems that undergo complex transformations (e.g., division, growth, and translocation) in the absence of external control. Neutrophils of the immune system are a prime example of such complex, cell-autonomous locomotion. These cells exhibit directed motion as they sense and move towards bacteria while navigating a forest of red blood cells. An artificial system...
Water drops sliding on insulating, hydrophobic substrates can become electrically charged [1–3]. Despite many decades of research, this spontaneous electrification of moving drops is still far from being understood. By precisely measuring charge and voltage, we found that moving water drops accumulate a voltage of several kilovolts after sliding for just a few centimeters. To enable an...
Conducting polymers (CPs) are discussed in a huge variety of electronic devices including organic field effect transistors, batteries, actuators and (bio)electronic sensors. Compared to other conducting materials, CPs are light-weight, low cost, non-toxic, flexible and allow easy processing, low voltage operation (around 1 Volt) and low power consumption.
Here, poly(3-hexylthiophene) (P3HT)...
The breakup dynamics of highly viscous capillary bridges on grounded hydrophobic surfaces is investigated. The breakup dynamics bears a very close resemblance to that of highly viscous free capillary bridges. However, due to the strong dependency of the dynamics on the surface properties, the wettability of the substrate must be taken into account. In this regard, it is demonstrated that under...
Nanometer thick layers of polydimethylsiloxane (PDMS) are widely applied as hydrophobic coatings because they are environmentally friendly and chemically inert. In many applications, low friction of water drops is required. While the onset of motion (static friction) has already been studied, dynamic friction is less explored. It is not understood which processes lead to energy dissipation and...
The static contact angle of a liquid droplet on a substrate is often used to describe its wetting properties, which are influenced by various factors such as the chemistry of the substrate, properties of the liquid, and environmental conditions. Our study shows that the wetting of a microscopic droplet can systematically depend on its size. This dependency can be described with consideration...
Protein condensates inside human cells are liquid-like droplets composed of protein and RNA. These condensates interact with the heterogeneous, active and dense environment of the cytoplasm, crossed by various cytoskeletal filaments such as microtubules and actin. Wetting interactions with the cytoskeleton lead to stereotypical positioning of such protein droplets inside the cell. Using...
Wetting and adhesion of lipid vesicles and biological cells are distinct from that of Newtonian fluid, because of their viscoelasticity. The use of switchable substrates based on polymer brushes and hydrogels offers a unique advantage for the adjustment of interfacial interactions on demand.
In the first of my talk, I will introduce our collaborative activities with Müller group (Göttingen)...
Soft materials can undergo irreversible shape changes when driven out of equilibrium [1,2]. When shape changes are triggered by processes at the surface, geometry-dependent feedback can arise. Motivated by the mechanochemical feedback observed in multicellular systems [1,3-5], we study incompressible droplets that adjust their interfacial tensions in response to shape-dependent signals. We...
A liquid bridge is a liquid droplet suspended between solid surfaces. We study pendent liquid bridges between two horizontally aligned cylindrical rods using experiments, direct numerical simulations and reduced-order model equations obtained by minimizing an appropriate Rayleighian according to Onsager’s variational principle. Additionally, we analyse the influence on the dynamics of perfect...
Liquid droplets on soft, solid, elastic substrates tend to deform the substrate on which they sit due to the interaction of interfacial and elastic forces. This deformation is the more pronounced the softer the substrate. Our project aims at understanding the mechanisms that determine the resulting morphology of equilibrium droplets.
To study these phenomena, we explore the dewetting of...