Browsing by Author "Backholm, Matilda"

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Biomechanical study of roots
(2019-12-17) Sillanpää, Heikki
Perustieteiden korkeakoulu | Master's thesis
Biomekaniken bakom växters gravitropism
(2023-08-12) Westerholm, Sanna
Perustieteiden korkeakoulu | Bachelor's thesis
Designed inorganic porous nanovector with controlled release and MRI features for safe administration of doxorubicin
(2019-01-10) Näkki, Simo; Wang, Julie T.W.; Wu, Jianwei; Fan, Li; Rantanen, Jimi; Nissinen, Tuomo; Kettunen, Mikko I.; Backholm, Matilda; Ras, Robin H.A.; Al-Jamal, Khuloud T.; Lehto, Vesa Pekka; Xu, Wujun
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The inability of traditional chemotherapeutics to reach cancer tissue reduces the treatment efficacy and leads to adverse effects. A multifunctional nanovector was developed consisting of porous silicon, superparamagnetic iron oxide, calcium carbonate, doxorubicin and polyethylene glycol. The particles integrate magnetic properties with the capacity to retain drug molecules inside the pore matrix at neutral pH to facilitate drug delivery to tumor tissues. The MRI applicability and pH controlled drug release were examined in vitro together with in-depth material characterization. The in vivo biodistribution and compound safety were verified using A549 lung cancer bearing mice before proceeding to therapeutic experiments using CT26 cancer implanted mice. Loading doxorubicin into the porous nanoparticle negated the adverse side effects encountered after intravenous administration highlighting the particles’ excellent biocompatibility. Furthermore, the multifunctional nanovector induced 77% tumor reduction after intratumoral injection. The anti-tumor effect was comparable with that of free doxorubicin but with significantly alleviated unwanted effects. These results demonstrate that the developed porous silicon-based nanoparticles represent promising multifunctional drug delivery vectors for cancer monitoring and therapy.
Determining swimming kinematics of Artemia larvae through image analysis
(2023-11-24) Ansas, Lotta
Perustieteiden korkeakoulu | Bachelor's thesis
Droplet Friction on Superhydrophobic Surfaces Scales With Liquid-Solid Contact Fraction
(2024-09-17) Lepikko, Sakari; Turkki, Valtteri; Koskinen, Tomi; Raju, Ramesh; Jokinen, Ville; Kiseleva, Mariia S.; Rantataro, Samuel; Timonen, Jaakko V.I.; Backholm, Matilda; Tittonen, Ilkka; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
It is generally assumed that contact angle hysteresis of superhydrophobic surfaces scales with liquid–solid contact fraction, however, its experimental verification has been problematic due to the limited accuracy of contact angle and sliding angle goniometry. Advances in cantilever-based friction probes enable accurate droplet friction measurements down to the nanonewton regime, thus suiting much better for characterizing the wetting of superhydrophobic surfaces than contact angle hysteresis measurements. This work quantifies the relationship between droplet friction and liquid–solid contact fraction, through theory and experimental validation. Well-defined micropillar and microcone structures are used as model surfaces to provide a wide range of different liquid–solid contact fractions. Micropillars are known to be able to hold the water on top of them, and a theoretical analysis together with confocal laser scanning microscopy shows that despite the spiky nature of the microcones droplets do not sink into the conical structure either, rendering a diminishingly small liquid–solid contact fraction. Droplet friction characterization with a micropipette force sensor technique reveals a strong dependence of the droplet friction on the contact fraction, and the dependency is described with a simple physical equation, despite the nearly three-orders-of-magnitude difference in liquid–solid contact fraction between the sparsest cone surface and the densest pillar surface.
Droplet slipperiness despite surface heterogeneity at molecular scale
(2024-04) Lepikko, Sakari; Morais Jaques, Ygor; Junaid, Muhammad; Backholm, Matilda; Lahtinen, Jouko; Julin, Jaakko; Jokinen, Ville; Sajavaara, Timo; Sammalkorpi, Maria; Foster, Adam; Ras, Robin
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Friction determines whether liquid droplets slide off a solid surface or stick to it. Surface heterogeneity is generally acknowledged as the major cause of increased contact angle hysteresis and contact line friction of droplets. Here we challenge this long-standing premise for chemical heterogeneity at the molecular length scale. By tuning the coverage of self-assembled monolayers (SAMs), water contact angles change gradually from about 10° to 110° yet contact angle hysteresis and contact line friction are low for the low-coverage hydrophilic SAMs as well as high-coverage hydrophobic SAMs. Their slipperiness is not expected based on the substantial chemical heterogeneity of the SAMs featuring uncoated areas of the substrate well beyond the size of a water molecule as probed by metal reactants. According to molecular dynamics simulations, the low friction of both low- and high-coverage SAMs originates from the mobility of interfacial water molecules. These findings reveal a yet unknown and counterintuitive mechanism for slipperiness, opening new avenues for enhancing the mobility of droplets.
Experimental analysis of magnetic field-induced splitting of ferrofluid droplets
(2016-09-16) Ballesio, Alberto
Perustieteiden korkeakoulu | Master's thesis
Ferrofluid Microdroplet Splitting for Population-Based Microfluidics and Interfacial Tensiometry
(2020-07) Latikka, Mika; Backholm, Matilda; Baidya, Avijit; Ballesio, Alberto; Serve, Amandine; Beaune, Grégory; Timonen, Jaakko V.I.; Pradeep, Thalappil; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Ferrofluids exhibit a unique combination of liquid properties and strong magnetic response, which leads to a rich variety of interesting functional properties. Here, the magnetic-field-induced splitting of ferrofluid droplets immersed in an immiscible liquid is presented, and related fascinating dynamics and applications are discussed. A magnetic field created by a permanent magnet induces instability on a mother droplet, which divides into two daughter droplets in less than 0.1 s. During the splitting process, the droplet undergoes a Plateau–Rayleigh-like instability, which is investigated using high-speed imaging. The dynamics of the resulting satellite droplet formation is shown to depend on the roughness of the supporting surface. Further increasing the field results in additional splitting events and self-assembly of microdroplet populations, which can be magnetically actuated. The effects of magnetization and interfacial tension are systematically investigated by varying magnetic nanoparticles and surfactant concentrations, and a variety of outcomes from labyrinthine patterns to discrete droplets are observed. As the splitting process depends on interfacial tension, the droplet splitting can be used as a measure for interfacial tension as low as 0.1 mN m−1. Finally, a population-based digital microfluidics concept based on the self-assembled microdroplets is presented.
Force-Based Wetting Characterization of Stochastic Superhydrophobic Coatings at Nanonewton Sensitivity
(2021-10-21) Hokkanen, Matti J.; Backholm, Matilda; Vuckovac, Maja; Zhou, Quan; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Superhydrophobic coatings have extraordinary properties like self-cleaning and staying dry, and have recently appeared on industrial and consumer markets. The stochastic nature of the coating components and coating processes (e.g., spraying, painting) affects the uniformity of the water repellency across the coated substrate. The wetting properties of those coatings are typically quantified on macroscale using contact angle goniometry (CAG). Here, highly sensitive force-based methods, scanning droplet adhesion microscopy (SDAM), and micropipette force sensor (MFS), are used, to quantify the microscale heterogeneity in the wetting properties of stochastic superhydrophobic coatings with irregular surface topography that cannot be investigated by CAG. By mapping the wetting adhesion forces with SDAM and friction forces with MFS, it is demonstrated that even the best coatings on the market are prone to heterogeneities that induce stick–slip motion of droplets. Thus, owing to their high spatial and force resolution, the advantages of these techniques over CAG are demonstrated.
Free-swimming kinematics of Artemia
(2024-10-10) Rislakki, Ensio
Perustieteiden korkeakoulu | Bachelor's thesis
A huge variety of organisms inhabit the oceans, lakes and rivers of the earth. These forms of life differ vastly in shape and size, from the largest mammal, blue whale, with body length of 22-24 m to single bacteria in the size scale of 1-2 mum. This amounts to a 10^14-fold range in Reynolds number; the ratio of inertial and viscous forces. This is remarkably large compared to the 10^5-fold range in aerial locomotion. Furthermore, in aquatic locomotion, the Reynolds number is found to have values less than or equal to 1 for the smallest organisms, which never occurs in aerial locomotion. One would certainly expect the physics of the locomotion to be completely different in the extremes of this scale, and indeed, this turns out to be the case. The two extremes, micro- and macroscale, are relatively well understood, since in these regimes one can ignore the inertial or the viscous force, respectively. However, a particular challenge is proposed by the scale in between: the mesoscale. In this regime, characterized by a Reynolds number close to one, modeling swimming becomes exceedingly difficult. Unlike the microscale, where the linear Stokes equation applies, or the macroscale, governed by the quasi-linear Euler equation, locomotion at the mesoscale is described by the full-form Navier-Stokes equation, which is nonlinear and often numerically unstable. This range in Reynolds number is inhabited by the species Artemia salina, which was the subject of the experiments for this thesis. In this thesis, the strategies for swimming in mesoscale are analysed in great detail and some remarks are made on the scaling of the kinematics as the Artemia grow. Swimming kinematics are measured using an inverted microscope and an image analysis software. Much attention is paid to ensure that the Artemia swim undisturbed, allowing the locomotion to be observed as naturally as possible. Additionally, the concept of symmetry breaking area is introduced to assess the significance of non-reciprocal motion in swimmers at intermediate Reynolds numbers.
Light-triggered Time-Programmed Self-adhesion in Low-Hysteresis Hydrogels
(2024-01-12) Savolainen, Henri; Backholm, Matilda; Ikkala, Olli; Zhang, Hang
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Self-healing of hydrogels has attracted intensive research based on dynamically exchangeable bonds. However, this typically leads to decreased elasticity and increased energy dissipation during mechanical deformations, which is not desirable in, for example, soft robotic applications. Thus, there exists an unmet demand for low hysteresis, that is, resilient, materials to quickly recover the mechanical properties after damage. Here, we show low-hysteresis chemically cross-linked hydrogels with on-demand local light-triggered fast self-adhesion, with time-dependent adhesion strength controlled by the duration of irradiation. Low mechanical hysteresis is achieved by swelling of the loosely cross-linked poly(N-isopropylacrylamide) network. Rapid self-adhesion is followed by localized photothermal heating of embedded gold nanoparticles, causing collapse and promoting local entanglements of the thermoresponsive poly(N-isopropylacrylamide) chains. This provides 95.7% initial recovery of the original mechanical properties, while the interface undergoes gradual rehydration and disentanglement depending on the irradiation details, leading to a decrease of the adhesive strength to zero within 7 h. The concept can be combined with conventional slow self-healing that is allowed by additional clay nanoplatelets for long-standing healing. The application potential is demonstrated by oscillations, time-programmed release of the adhered object, and on-demand assembly of designed hydrogel shapes. The proposed mechanism with facile, efficient, and light-controlled temporal profiles that are time-programmed can be applied to soft robotics, biomedical applications, and flexible electronics.
Living droplets on superhydrophobic surfaces
(2025-01-02) Lamminmäki, Martta
Perustieteiden korkeakoulu | Bachelor's thesis
This thesis explores the behavior of small water droplets containing active live swimmers on a superhydrophobic surface. In previous studies with bacteria or nanoparticles it has been shown that the collective movement of the swimmers can result in shape fluctuations and spontaneous self-propulsion of the droplet. There are also models that predict a droplet's resonance oscillation frequency based on the droplet volume and liquid properties. On a superhydrophobic surface, the droplets obtain an almost spherical shape and any friction is minimized, allowing for minimally restricted movement. In the future, understanding the dynamics of small-scale swimmers could help in the design of synthetic motors that could have applications for example in medicine. The behavior of droplets on superhydrophobic surfaces is crucial for example for designing self-cleaning and water-repellent solar panel coatings. The swimmers used in experiments are mesoscale organisms of the species Artemia, also more commonly known as sea monkeys or brine shrimp. Artemia are easily cultivated in a salt water solution in the lab setting, and hatch in approximately 24 hours. The experimental setup is quite simple, the most important parts being the supherhydrophobic etched black silicon surface, a micropipette force sensor (MFS) and a high speed camera. A droplet containing a number of Artemia is set on the substrate, and the movement of the droplet is captured in the deflection of the pipette. Experiments are done for varying droplet volumes (2-12 $\mu L$), 2 different light intensities and with different numbers of Artemia (1-7) per drop. With image analysis, we calculate the droplet volume, diameter, and the deflection of the MFS as a function of time. Through an fft algorithm we then obtain the droplet oscillation frequency for each data point. The results show a clear inverse relationship between the droplet oscillation frequency and the droplet volume. This is as predicted by the models for droplet resonance frequency which fit the results quite well. Interestingly though, with a higher light intensity we record slightly higher frequencies. This could be explained by the Artemia reacting to the light with increasing swimming activity or speed. The data is not clear enough to make clear conclusions about the effect of the number of Artemia on the frequencies. We conclude that overall the discussed models fit our data well; It seems that the internal movement caused by the swimmers 'activates' the droplet to oscillate at a frequency close to its resonance frequency. However, the models do not account or explain the effect of the Artemia activity. More careful experiments are required to see if there is a pattern in the effect of Artemia number. The present experiments have not controlled well enough for variables such as the Artemia age, length and liquid salinity. For future experiments these variables should be more carefully recorded and monitored.
Long-term stability of aerophilic metallic surfaces underwater
(2023-12) Tesler, Alexander B.; Kolle, Stefan; Prado, Lucia H.; Thievessen, Ingo; Böhringer, David; Backholm, Matilda; Karunakaran, Bhuvaneshwari; Nurmi, Heikki A.; Latikka, Mika; Fischer, Lena; Stafslien, Shane; Cenev, Zoran M.; Timonen, Jaakko V.I.; Bruns, Mark; Mazare, Anca; Lohbauer, Ulrich; Virtanen, Sannakaisa; Fabry, Ben; Schmuki, Patrik; Ras, Robin H.A.; Aizenberg, Joanna; Goldmann, Wolfgang H.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Aerophilic surfaces immersed underwater trap films of air known as plastrons. Plastrons have typically been considered impractical for underwater engineering applications due to their metastable performance. Here, we describe aerophilic titanium alloy (Ti) surfaces with extended plastron lifetimes that are conserved for months underwater. Long-term stability is achieved by the formation of highly rough hierarchically structured surfaces via electrochemical anodization combined with a low-surface-energy coating produced by a fluorinated surfactant. Aerophilic Ti surfaces drastically reduce blood adhesion and, when submerged in water, prevent adhesion of bacteria and marine organisms such as barnacles and mussels. Overall, we demonstrate a general strategy to achieve the long-term stability of plastrons on aerophilic surfaces for previously unattainable underwater applications.
Micropipette force sensor measurements on swimming Artemia
(2024-07-05) Saastamoinen, Aaro
Perustieteiden korkeakoulu | Bachelor's thesis
The physics of systems of self-propelling objects, also known as active matter physics, provides a wide array of system sizes with interesting properties for examination. One highly interesting subset of these systems are flocks or swarms of individual organisms, often referred to as living matter. In these experiments, the focus was on waterlogged swimmers and in particular, the larvae of the genus Artemia. These mesoscale animals are particularly interesting due to their size during their life cycle spanning several orders of magnitude within the intermediate Reynolds numbers. Due to the properties of the fluid-governing Navier-Stokes equations, the intermediate Reynolds numbers give complicated solutions which are neither laminar nor turbulent and thus direct measurement is preferred. Fortunately, the Micropipette Force Sensor technique (MFS) provides an affordable and convenient method for force magnitude measurements at this scale. The MFS technique utilizes very thin-pulled glass capillaries and their spring-like properties. An Artemia is caught on a micropipette using suction from a syringe. The subject is then filmed for several swimming cycles and then released. The video is then analyzed. Due to high viscous forces, Artemia larvae exert considerable amounts of force in the opposite direction to their intended propulsion direction. As a force measurement starts with the subject already attached and frantically moving, there is no obvious way to determine how much force is exerted in each direction, other than measuring the zero position of the pipette when the Artemia is released. Since the swimming of Artemia is cyclic, it was thought that a pattern of their swimming appendages, their antennae, would give rise to a consistently detectable pattern at the zero force position. Two patterns were found, and the analysis showed potential clustering when comparing the zero force to the measured angle. The zero force angle didn't appear to change with the length of the Artemia. Supplementary angle measurements at maximum and minimum forces were done where especially the latter seemed to go down as the length of the Artemia increased. During the release experiments, it was found that an increase in propulsive force seemed to roughly correlate with the square of the length of the Artemia. Temperature was also monitored and at high external temperatures the Artemia were more likely to contract disease and fungus. This resulted in growth batches where the motor skills of the specimens would gradually diminish and ones where the hatching was unsuccessful.
Micropipette force sensors for in vivo force measurements on single cells and multicellular microorganisms
(2019-02-01) Backholm, Matilda; Bäumchen, Oliver
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Measuring forces from the piconewton to millinewton range is of great importance for the study of living systems from a biophysical perspective. The use of flexible micropipettes as highly sensitive force probes has become established in the biophysical community, advancing our understanding of cellular processes and microbial behavior. The micropipette force sensor (MFS) technique relies on measurement of the forces acting on a force-calibrated, hollow glass micropipette by optically detecting its deflections. The MFS technique covers a wide micro- and mesoscopic regime of detectable forces (tens of piconewtons to millinewtons) and sample sizes (micrometers to millimeters), does not require gluing of the sample to the cantilever, and allows simultaneous optical imaging of the sample throughout the experiment. Here, we provide a detailed protocol describing how to manufacture and calibrate the micropipettes, as well as how to successfully design, perform, and troubleshoot MFS experiments. We exemplify our approach using the model nematode Caenorhabditis elegans, but by following this protocol, a wide variety of living samples, ranging from single cells to multicellular aggregates and millimeter-sized organisms, can be studied in vivo, with a force resolution as low as 10 pN. A skilled (under)graduate student can master the technique in ~1–2 months. The whole protocol takes ~1–2 d to finish.
Oscillating Ferrofluid Droplet Microrheology of Liquid-Immersed Sessile Droplets
(2017-06-27) Backholm, Matilda; Vuckovac, Maja; Schreier, Jan; Latikka, Mika; Hummel, Michael; Linder, Markus B.; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The damped oscillations of liquid-immersed ferrofluid sessile droplets were studied with high-speed imaging experiments and analytical modeling to develop a novel microrheology technique. Droplet oscillations were induced with an external magnetic field, thereby avoiding transients in the resulting vibrational response of the droplet. By following the droplet relaxation with a high-speed camera, the frequency and relaxation time of the damped harmonic oscillations were measured. We extend upon existing analytical theories to describe our liquid-immersed sessile droplet system, and directly quantify the droplet relaxation with the viscosity of the internal and external fluid as well as the interfacial tension between these. The easily controllable magnetic droplets make our oscillating ferrofluid droplet technique a potential candidate for high-throughput microrheology and tensiometry in the future.
Probing surface wetting across multiple force, length and time scales
(2023-12) Daniel, Dan; Vuckovac, Maja; Backholm, Matilda; Latikka, Mika; Karyappa, Rahul; Koh, Xue Qi; Timonen, Jaakko V.I.; Tomczak, Nikodem; Ras, Robin H.A.
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Surface wetting is a multiscale phenomenon where properties at the macroscale are determined by features at much smaller length scales, such as nanoscale surface topographies. Traditionally, the wetting of surfaces is quantified by the macroscopic contact angle that a liquid droplet makes, but this approach suffers from various limitations. In recent years, several techniques have been developed to address these shortcomings, ranging from direct measurements of pinning forces using cantilever-based force probes to atomic force microscopy methods. In this review, we will discuss how these new techniques allow for the probing of surface wetting properties in far greater detail. Advances in surface characterization techniques will improve our understanding of surface wetting and facilitate the design of functional surfaces and materials, including for antifogging and antifouling applications.
Slippery and never wet
(2017-09-01) Backholm, Matilda; Timonen, Jaakko V.I.; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Superhydrophobic surfaces let water droplets roll off with low friction and falling droplets rebound, leaving the surfaces completely dry. Such extremely water repellent surfaces are found in nature on lotus leaves, the legs of water striders and feather coatings of birds, and portray a beautiful example of ingenious biological design. They provide an exciting research avenue for physicists and materials scientists aspiring to understand and mimic nature.
Toward vanishing droplet friction on repellent surfaces
(2024-04-15) Backholm, Matilda; Kärki, Tytti; Nurmi, Heikki; Vuckovac, Maja; Turkki, Valtteri; Lepikko, Sakari; Jokinen, Ville; Quéré, David; Timonen, Jaakko; Ras, Robin
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Superhydrophobic surfaces are often seen as frictionless materials, on which water is highly mobile. Understanding the nature of friction for such water-repellent systems is central to further minimize resistance to motion and energy loss in applications. For slowly moving drops, contact-line friction has been generally considered dominant on slippery superhydrophobic surfaces. Here, we show that this general rule applies only at very low speed. Using a micropipette force sensor in an oscillating mode, we measure the friction of water drops approaching or even equaling zero contact-line friction. We evidence that dissipation then mainly stems from the viscous shearing of the air film (plastron) trapped under the liquid. Because this force is velocity dependent, it can become a serious drag on surfaces that look highly slippery from quasi-static tests. The plastron thickness is found to be the key parameter that enables the control of this special friction, which is useful information for designing the next generation of ultraslippery water-repellent coatings.
Viscosity-enhanced droplet motion in sealed superhydrophobic capillaries
(2020-10) Vuckovac, Maja; Backholm, Matilda; Timonen, Jaakko V.I.; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
It is well known that an increased viscosity slows down fluid dynamics. Here we show that this intuitive rule is not general and can fail for liquids flowing in confined liquid-repellent systems. A gravity-driven, highly viscous glycerol droplet inside a sealed superhydrophobic capillary is moving more than 10 times faster than a water droplet with three-orders-of-magnitude lower viscosity. Using tracer particles, we show that the low-viscosity droplets are rapidly rotating internally, with flow velocities greatly exceeding the center-of-mass velocity. This is in stark contrast to the faster moving high-viscosity droplets with nearly vanishing internal flows. The anomalous viscosity-enhanced flow is caused by a viscosity-suppressed deformation of the droplet-air interface and a hydro- and aerodynamic coupling between the droplet and the air trapped within the micro/nanostructures (plastron). Our work demonstrates the unexpected role of the plastron in controlling fluid flow beyond the mere reduction in contact area and friction.