Koray Sekeroglu

An engineered anisotropic nanofilm with unidirectional wetting properties

Anisotropic textured surfaces allow water striders to walk on water, butterflies to shed water from their wings and plants to trap insects and pollen. Capturing these natural features in biomimetic surfaces is an active area of research. Here, we report an engineered nanofilm, composed of an array of poly(p-xylylene) nanorods, which demonstrates anisotropic wetting behaviour by means of a pin-release droplet ratchet mechanism. Droplet retention forces in the pin and release directions differ by up to 80 μN, which is over ten times greater than the values reported for other engineered anisotropic surfaces. The nanofilm provides a microscale smooth surface on which to transport microlitre droplets, and is also relatively easy to synthesize by a bottom-up vapour-phase technique. An accompanying comprehensive model successfully describes the films anisotropic wetting behaviour as a function of measurable film morphology parameters.

Transport of a soft cargo on a nanoscale ratchet

Surface ratchets can guide droplet transport for microfluidic systems. Here, we demonstrated the actuation of microgels encapsulated in droplets using a unidirectional nanotextured surface, which moves droplets with low vibration amplitudes by a ratcheting mechanism. The nanofilm carries droplets along the ratchets with minimal drop shape deformation to move the encapsulated soft cargo, i.e., microscale hydrogels. The tilted nanorods of the nanofilm produce unidirectional wetting, thereby enabling droplet motion in a single direction. Maximum droplet translation speed on the nanofilm was determined to be 3.5 mm∕s, which offers a pathway towards high throughput microgel assembly applications to build complex constructs.

Neuronal alignment on asymmetric textured surfaces

Axonal growth and the formation of synaptic connections are key steps in the development of the nervous system. Here, we present experimental and theoretical results on axonal growth and interconnectivity in order to elucidate some of the basic rules that neuronal cells use for functional connections with one another. We demonstrate that a unidirectional nanotextured surface can bias axonal growth. We perform a systematic investigation of neuronal processes on asymmetric surfaces and quantify the role that biomechanical surface cues play in neuronal growth. These results represent an important step towards engineering directed axonal growth for neuro-regeneration studies.

Noncovalent mechanism for the conformal metallization of nanostructured parylene films.

We describe a rapid, reliable method of preparing nanoporous Ni or Co films using nanostructured poly(chloro-p-xylylene) (nanoPPX) films as templates. The nanoPPX films are vapor deposited onto Si substrates using oblique angle polymerization (OAP), resulting in the formation of an obliquely aligned PPX nanorod array on the substrate. The nanoPPX films are then subjected to noncovalent functionalization using an aromatic ligand (i.e., pyridine) by means of treatment with either an aqueous solution of the ligand or ligand vapor. The results of quartz crystal microbalance and X-ray diffraction studies support a model in which pyridine adsorption is facilitated by the formation of pi-pi interactions with aromatic moieties in the amorphous surface regions of nanoPPX. The physisorbed pyridine in the nanoPPX film can subsequently bind a catalytic Pd(II)-based colloidal seed layer. Continuous, conformal Ni or Co films, characterized by FIB/SEM and AFM, are grown on the Pd(II)-laden nanoPPX films using electroless metallization. Analogous metallization of a conventionally deposited planar PPX film results in noncontinuous or patchy metal deposits. Such behavior is attributed to the sluggish adsorption of pyridine in the planar PPX film, resulting in an approximately 22-fold decrease in the quantity of pyridine adsorbed compared to that in a nanoPPX film. Consequently, the level of Pd(II) bound by pyridine on a planar PPX film is insufficient to catalyze continuous metallization. Results of a statistical two-level factorial design indicate that the morphology of the metal layer formed on a nanoPPX film is profoundly influenced by the ligand adsorption condition (i.e., aqueous ligand vs ligand vapor treatment) and is correlated to the catalytic activity of Co films for the production of hydrogen from sodium borohydride decomposition.

Fibroblast adhesion on unidirectional polymeric nanofilms

Nanotextured polymeric surfaces with inclined rods reveal highly anisotropic properties concerning wetting and adhesion. In this work, we report on the interaction of fibroblast cells with these highly anisotropic materials. The authors quantified removal of adherent cells from such surfaces by a laminar flow. The critical shear force needed for cell removal from the surface depends on the inclination direction. Based on electron microscopy cross sections we deduce that interactions of cellular filopodia extending into the nanotextured surface are causing the direction depending removal.

Drainage from a Fluid-Handling Component Due to Centrifugal Acceleration

The onset of drainage of liquids from plastic tubes was evaluated. One end of the tubes was plugged, filled with liquid, oriented horizontally, and attached to a rotor. With their open end facing outward, the filled tubes were spun at progressively higher speeds until they began to drain. Resistance to drainage was independent of the tube length, but depended on the surface tension and density of the liquids as well as the diameter and wettability of the tubing. We found that the onset of drainage could be explained in terms of a critical centrifugal acceleration.

Liquid Leaks: Dripping Versus Evaporation

Liquid leaks often reveal themselves as pendant drops or drips emanating from a low point on a fluid handling component. For volatile liquids, understanding the contributions of interfacial properties, such as diffusivity of the liquid and wettability of the solid, is crucial to determining leak rates. To estimate the resolution of hydrostatic leak testing, the competing factors of leak and evaporation rates were analyzed. We used drop volumes and contact angles along with intrinsic fluid properties to calculate the detection limit of hydrostatic leak tests. For water and ethanol, we reckon that it is approximately 10-4 to 10-5 cm3/s in dry air.