Control of drop motion by means of optical patterns imprinted on Fe:LiNbO3 crystals.

Controlling and predicting the mobility of sessile droplets in contact with a solid surface has fundamental implications and technological applications. In the last decade, the use of liquid impregnated surfaces has become the best way to improve the wettability of a substrate and promote the motion of sessile droplets. In this thesis we will couple the realization of a liquid impregnated surface with the photovoltaic effect exhibited by Lithium Niobate crystals to control in an active way the motion of sessile droplets. The resulting method is simple and versatile, enabling the successful manipulation of many consecutive drops having typical volumes of micro-liters, without the need for electrodes or power supplies. In particular, the pattern of illumination arriving on the Lithium Niobate is generated by a spatial light modulator which permits to write any shape to control the droplet. After a preliminary exploration of the dynamics of the system, the main goal of this thesis work consists in finding the best parameters to realize and control the motion of a train of droplets in a reproducible way.ope

Moto di gocce di ferrofluido su piano inclinato soggette a un campo magnetico esterno

Questa tesi presenta i risultati del lavoro svolto presso il Laboratorio di Fisica delle Superfici e Interfacce (LaFSI) di Padova riguardante il controllo del moto di gocce di ferrofluido su piano inclinato attraverso campi magnetici variabili. Il lavoro parte da presupposti di microfluidica aperta riguardanti il controllo attivo e passivo del moto di gocce in generale e del moto di gocce di ferrofluido, in particolare attraverso l’utilizzo di una LIS dove far scorrere la goccia. La novità introdotta dallo studio è l’utilizzo di un elettromagnete per generare campi magnetici variabili. Il risultato è il controllo completo del moto della goccia di ferrofluido che può scendere, risalire e restare immobile sul piano inclinato in base all’intensità del campo magnetico.ope

Moto di gocce di ferrofluido su piano inclinato soggette a un campo magnetico esterno

Questa tesi presenta i risultati del lavoro svolto presso il Laboratorio di Fisica delle Superfici e Interfacce (LaFSI) di Padova riguardante il controllo del moto di gocce di ferrofluido su piano inclinato attraverso campi magnetici variabili. Il lavoro parte da presupposti di microfluidica aperta riguardanti il controllo attivo e passivo del moto di gocce in generale e del moto di gocce di ferrofluido, in particolare attraverso l’utilizzo di una LIS dove far scorrere la goccia. La novità introdotta dallo studio è l’utilizzo di un elettromagnete per generare campi magnetici variabili. Il risultato è il controllo completo del moto della goccia di ferrofluido che può scendere, risalire e restare immobile sul piano inclinato in base all’intensità del campo magnetico

Determination of the Dielectrophoretic Force Induced by the Photovoltaic Effect on Lithium Niobate

The actuation of droplets on a surface is extremely relevant for microfluidic applications. In recent years, various methodologies have been used. A promising solution relies on iron-doped lithium niobate crystals that, when illuminated, generate an evanescent electric field in the surrounding space due to the photovoltaic effect. This field can be successfully exploited to control the motion of water droplets. Here, we present an experimental method to determine the attractive force exerted by the evanescent field. It consists of the analysis of the elongation of a pendant droplet and its detachment from the suspending syringe needle, caused by the illumination of an iron-doped lithium niobate crystal. We show that this interaction resembles that obtained by applying a voltage between the needle and a metallic substrate, and a quantitative investigation of these two types of actuation yields similar results. Pendant droplet tensiometry is then demonstrated to offer a simple solution for quickly mapping out the force at different distances from the crystal, generated by the photovoltaic effect and its temporal evolution, providing important quantitative data for the design and characterization of optofluidic devices based on lithium niobate crystals

Sliding and rolling of yield stress fluid droplets on highly slippery lubricated surfaces

Hypothesis: Droplets of yield stress fluids (YSFs), i.e. fluids that can flow only if they are subjected to a stress above a critical value and otherwise deform like solids, hardly move on solid surfaces due to their high viscosity. The use of highly slippery lubricated surfaces can shed light on the mobility of YSF droplets, which include everyday soft materials, such as toothpaste or mayonnaise, and biological fluids, such as mucus. Experiments: The spreading and mobility of droplets of aqueous solutions of swollen Carbopol microgels were studied on lubricant infused surfaces. These solutions represent a model system of YSFs. Dynamical phase diagrams were established by varying the concentration of the solutions and the inclination angle of the surfaces. Findings: Carbopol droplets deposited on lubricated surfaces could move even at low inclination angles. The droplets were found to slide because of the slip of the flowing oil that covered the solid substrate. However, as the descending speed increased, the droplets rolled down. Rolling was favored at high inclinations and low concentrations. A simple criterion based on the ratio between the yield stress of the Carbopol suspensions and the gravitational stress acting on the Carbopol droplets was found to nicely identify the transition between the two regimes