Thesis title: 3D microstructures for active and soft matter studies
Microfabrication techniques have opened up new ways to study the dynamics of microsystems expanding the range of applications in microengineering and cell biology. Among three-dimensional microfabrication techniques, two-photon polymerization enjoys a unique set of characteristics that make it appealing for designing complex structures of arbitrary form. During the last decades, two-photon polymerization has evolved from the first structure fabricated with this technique, a coil with a diameter of $7\;\mu m$ and a total length of approximately $34\; \mu m$ (by Maruo \textit{et al.}), to generate sophisticated systems like remotely driven micromachines. In the present thesis, we address two main applications of microfabrication.
On the first line of research, the design and fabrication of efficient and self-powered micro-robots have been a very active research topic. Motile micro-organisms like \textit{E. coli} may provide an optimal solution to generate propulsion in artificial microsystems. It has been demonstrated that microstructures can be transported when released on a layer of swarming bacteria, suspended in a bacterial bath, or covered by surface adhering bacteria. Although it is possible to obtain a net movement in the mentioned cases, the displacement is stochastic and self-propulsion characteristics are hard to reproduce. In this thesis, we investigate possible design strategies for bio-hybrid micro shuttles having a defined number of propelling units that self-assemble onto precisely defined locations. One of the biggest issues involved in the optimization design process of the microshuttles is an irreversible adhesion of structures in the substrate, which often is caused by Van der Waals attraction. To overcome this problem we use different stabilization methods with unsuccessful results. Looking for a less invasive and biocompatible strategy we investigate the possibility of changing the sign of Van der Walls forces turning them from attractive to repulsive. To this aim, we develop a method that demonstrates to reduce the adhesion observed before. So, the final design aims at minimizing friction and adhesion with the substrate while optimizing propulsion speed and self-assembly efficiency. Finally, using a mutated strain of \textit{E. coli} the microshuttle can be remotely controlled by dynamic structured light patterns for reaching an optimal control of the motion of the structure.
In a different direction of microfabrication applications, 3D microstructures can also offer new opportunities to address more fundamental problems in the soft matter dynamic. On this second line of research, we have designed and used complex 3D microstructures to investigate the Brownian dynamics and hydrodynamics of propeller shaped particles, as well as to probe effective interactions in colloidal systems, like critical Casimir forces.
In the dynamics of microhelices we use optical tweezers to study the mechanic and hydrodynamic properties of micro-fabricated helices suspended in a fluid. For the case of rigid helices, we track Brownian fluctuations around mean values with a high precision and over a long observation time. Through the statistical analysis of fluctuations in translational and rotational coordinates, we recover the full mobility matrix of the micro-helix including the off diagonal terms related with roto-translational coupling. Exploiting the high degree of spatial control provided by optical trapping, we can systematically study the effect of a nearby wall on the roto-translational coupling, and conclude that a rotating helical propeller moves faster near a no-slip boundary. We also study the relaxation dynamics of deformable micro-helices stretched by optical traps. We find that hydrodynamic drag only weakly depends on elongation resulting in an exponential
relaxation to equilibrium.
In connection with the versatility of microfabrication by two-photon polymerization, we find the study of interaction in colloidal systems. At macroscopic scales, thermal fluctuations of a physical property on a system are typically negligible, but at the micrometer and nanometer scales instead, fluctuations become generally relevant and they give rise to novel and intriguing phenomena such as critical Casimir effect. Critical Casimir forces are induced between colloidal objects suspended in a critical binary mixture undergoing strong thermal fluctuations. So far, most of the experiments and proposed models consider the interaction between simple geometrical objects such as two spheres, or a single sphere and a plate. In the last part of this thesis, we propose a novel 3D printed microprobes consisting of the main body and two handles that can be optically trapped to directly measure effective forces and torques between colloidal objects with non spherical shapes.
The organization of this thesis is as follows. Chapter 1 gives a general introduction to the physical phenomenon behind the 3D microfabrication technique employed in our experiments, two-photon polymerization. We describe the differences between two phenomena: single-photon absorption and two-photon absorption, and explain the effectiveness of using two-photon polymerization for reaching a resolution of 100 nanometers in microfabrication. Then we present an experimental characterization of the voxel size of our custom-built two-photon polymerization set-up. We explain the sample preparation steps for microfabrication as well as the development of an innovative low-refractive index layer for eliminating irreversible adhesion of SU-8 microstructures.
Chapter 2 provides a general introduction to \textit{E. coli} motility, the propulsion mechanism of these bacteria, and the circular trajectory developed by the microorganism when swimming near a rigid boundary. Besides, we briefly explain the possibility of using synthetic biology to obtain light driven strains of \textit{E. coli} by the expression of Proteorhodopsin on the bacteria membrane.
In Chapter 3 we combine two-photon polymerization technique and genetically modified bacteria to create a biohybrid microshuttle. We start with a basic microshuttle design whose propulsion is obtained from four \textit{E. coli} bacteria. After integrating ramps in another microshuttle model for minimizing the circular trajectory showed in the microstructure trajectory, we make major changes in the distribution of microchambers inside the last model named catamaran microshuttle. Exploiting the ability of a mutated strain of \textit{E. coli} expressing proteorhodopsin, we successfully control the microrobot steering by illuminating our sample with green light patterns.
In Chapter 4 we design and use 3D microhelices from two different materials to investigate, through optical tweezers, Brownian dynamics and hydrodynamics of this kind of chiral particles. Through the statistical analysis of fluctuations in translational and rotational coordinates we study the roto-translational coupling element from the mobility matrix of the micro-helix. Besides, we conclude that a rotating helical propeller moves faster near a no-slip boundary. For the case of a deformable micro=helix, we find that hydrodynamic drag only depends on its elongation.
Finally, Chapter 5 presents the design for a microprobe to measure critical Casimir forces using holographic optical tweezers. We show a characterization experiment for a micro-cube with two handles, concluding that a third handle will improve the stability of a microprobe inside the sample.