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The increased flexibility in how commercial microscopes can be modified to add external features has opened the possibility of numerous new
types of experiments. In this talk I will present work on two such projects.



In a first study, the growth of condensed colloidal states from a fluid phase is explored in the presence of an external drive of the system by a controlled thermal
field and thermophoresis. A thermal gradient is formed in a fluid bath of colloids by adding and trapping with optical tweezers a high refractive index particle.
By absorbing a portion of the laser light from the tweezers, the particle acts as a point-like heat source and causes a temperature gradient in the mixture that is
highly localized in both space and time. This leads to the clustering and phase transition by thermophoresis of colloids from the bath around the trapped particle.
I will show that the steady-state size of the aggregates can be determined by the shape of the external field and the bulk volume fraction of the system and that condensed
structures with various order can be generated (crystalline, amorphous and icosahedral order). This novel technique to trigger phase transitions opens the way to probe the dynamics
of growth of phase transitions from highly out-of-equilibrium conditions and to observe the behaviour of interfaces in potentials that are inhomogeneous at the scale of the fluctuations
of the interface.



In the second project, I explore the strength of soft materials against puncture and penetration by sharp objects. These are properties of condensed matter relevant to various
situations spanning the design of strong gloves, to understanding animals bite mechanics or the invasion of filament-shaped yeasts breaking through human skin. Puncture and penetration
are described by an interplay between several mechanisms: the material elastic response, fracture, crack propagation, friction and adhesion. This variety of mechanisms raises the question
of which are mostly relevant to any particular situation, and my focus will be here on two types of micron-scale objects indenting soft polydimethyl siloxane (PDMS). First, nanoindentation
is used to measure the force acting on micropipettes as they are pushed into a PDMS block. I will show that, here, the critical force to puncture is highly dependent on the speed of the indenter
and that there is an optimal polymer crosslinker concentration to resist against puncture. Second, deformations of PDMS are measured as an invasive yeast, Candida albicans, grows in this
substrate in the shape of a filament with a tip continuously moving forward by cell wall creation. Using a dedicated "compression force microscopy" technique, the 3d the displacement of
fluorescent particles in the PDMS is monitored and highlights that it sustains deformations that are surprisingly smaller compared to the nanoindentation tests as well as other detailed
features of the growth.

Pour plus d'informations, merci de contacter Truzzolillo D.